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Träfflista för sökning "WFRF:(Peng Ru Lin 1960 ) "

Sökning: WFRF:(Peng Ru Lin 1960 )

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1.
  • Zhang, Pimin, 1990-, et al. (författare)
  • Effects of surface finish on the initial oxidation of HVAF-sprayed NiCoCrAlY coatings
  • 2019
  • Ingår i: Surface & Coatings Technology. - Elsevier : Elsevier BV. - 0257-8972 .- 1879-3347. ; 364, s. 43-56
  • Tidskriftsartikel (refereegranskat)abstract
    • Oxide scale formed on HVAF-sprayed NiCoCrAlY coatings and the effect of surface treatment were investigated by a multi-approach study combining photo-stimulated luminescence, microstructural observation and mass gain. The initial oxidationbehaviour of as-sprayed, polished and shot-peened coatings at 1000 °C is studied. Both polished and shot-peened coatings exhibited superior performance due to rapid formation of α-Al2O3 fully covering the coating and suppressing the growth of transient alumina, assisted by a high density of α-Al2O3 nuclei on surface treatment induced defects. Moreover, the fast development of a two-layer alumina scale consisting of an inward-grown inner α-Al2O3 layer and an outer layer transformed from outward-grown transient alumina resulted in a higher oxide growth rate of the as-sprayed coating.
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2.
  • Zhang, Pimin, 1990-, et al. (författare)
  • Isothermal oxidation behavior of HVAF-sprayed NiCoCrAlY coatings : Effect of surface treatment
  • 2017
  • Ingår i: Proceedings of the International Thermal Spray Conference & Exposition (ITSC 2017). - New York : Curran Associates, Inc. - 9781510858220 ; , s. 456-461
  • Konferensbidrag (refereegranskat)abstract
    • NiCoCrAlY coatings are widely used as bond coats for ceramic thermal barrier coatings (TBCs) andoxidation and corrosion protective overlay coatings in industrial gas turbines. High temperature oxidation behaviour of NiCoCrAlYs has a great influence on the coating performance and lifetime of TBCs. A promising route to decrease the oxidation rate of such coatings is post-coating surface modification which can facilitate formation of a uniform alumina scale with a considerably slower growth rate compared to the as-sprayed coatings. In this work, the effect of surface treatment by means of shot peening and laser surface melting (LSM) on the oxidation resistance of high velocity air-fuel (HVAF) sprayed NiCoCrAlY coatings was studied. Isothermal oxidation was carried out at 1000⁰C for 1000h. Results showed that the rough surface of as-sprayed HVAF sprayed coatings was significantly changed after shot peening and LSM treatment, with a compact and smooth appearance. After the exposure, the oxide scales formed on surface-treated NiCoCrAlY coatings showed different morphology and growth rate compared to those formed on as-sprayed coating surface. The oxidation behaviour of surface treated HVAF-sprayed NiCoCrAlY coatings were revealed and discussed.
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5.
  • Chen, Zhe, 1987-, et al. (författare)
  • Effect of Machining Parameters on Cutting Force and Surface Integrity when High-Speed Turning AD730™ with PCBN Tools
  • 2019
  • Ingår i: The International Journal of Advanced Manufacturing Technology. - : Springer London. - 0268-3768 .- 1433-3015. ; 100:9-12, s. 2601-2615
  • Tidskriftsartikel (refereegranskat)abstract
    • The novel wrought nickel-based superalloy, AD 730™, is a good candidate material for turbine disc applications at high temperatures beyond 650 °C. The present study focuses on the machining performance of this newly developed alloy under high-speed turning conditions with advanced PCBN tools. Meanwhile, the machined surface integrity as influenced by cutting speed and feed rate was also investigated. The surface integrity was thoroughly characterized in terms of surface roughness and morphology, machining-induced plastic deformation, white layer formation, and residual stresses. It has been found that the cutting speed and feed rate had a strong effect on the cutting forces and resultant surface integrity. The cutting forces required when machining the alloy were gradually reduced with increasing cutting speed, while at 250 m/min and above, the flank tool wear became stronger which led to increased thrust force and feed force. A higher feed rate, on the other hand, always resulted in higher cutting forces. Increasing the cutting speed and feed rate in general deteriorated the surface integrity. High cutting speeds within the range of 200–250 m/min and a low feed rate of 0.1 mm/rev are preferable in order to implement more cost-effective machining without largely reducing the surface quality achieved. The formation of tensile residual stresses on the machined AD 730™, however, could be of a concern where good fatigue resistance is critical.
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6.
  • Chen, Z., et al. (författare)
  • Plastic Deformation and Residual Stress in High Speed Turning of AD730™ Nickel-based Superalloy with PCBN and WC Tools
  • 2018
  • Ingår i: Procedia CIRP. - : Elsevier BV. - 2212-8271. ; 71, s. 440-445, s. 440-445
  • Konferensbidrag (refereegranskat)abstract
    • A higher gas turbine efficiency can be achieved by increasing the operating temperature in hot sections. AD730™ is a recently-developed wrought/cast nickel-based superalloy which can maintain excellent mechanical properties above 700. However, machining of AD730™ could be a difficult task like other nickel-based superalloys. Therefore, studies are needed with respect to the machinability of this new alloy. In this paper, high-speed turning was performed on AD730™ using polycrystalline cubic boron nitride (PCBN) tools and coated tungsten carbide (WC) tools at varied cutting speeds. The surface integrity was assessed in two important aspects, i.e., surface and sub-surface plastic deformation and residual stresses. The PCBN tools generally showed better performance compared with the WC tools since it led to reduced machining time without largely compromising the surface integrity achieved. The optimal cutting speed was identified in the range of 200-250 m/min when using the PCBN tools, which gives rise to a good combination of machining efficiency and surface integrity. The further increase of the cutting speed to 300 m/min resulted in severe and deep plastic deformation. Meanwhile, a continuous white layer was formed at the machined surface. When turning with the WC tools, the increased cutting speed from 80 m/min to 100 m/min showed very little effect with respect to the plastic deformation on the machined surface. It was found that tensile residual stresses were developed on all machined surfaces no matter when the PCBN or WC tools were used, and the surface tension was generally increased with increasing cutting speed. The tensile layer might need to be modified by e.g., post-machining surface treatments such as shot peening, if taking good fatigue performance into consideration.
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7.
  • Cui, Luqing, et al. (författare)
  • A new approach for determining GND and SSD densities based on indentation size effect : An application to additive-manufactured Hastelloy X
  • 2022
  • Ingår i: Journal of Materials Science & Technology. - : Elsevier. - 1005-0302. ; 96, s. 295-307
  • Tidskriftsartikel (refereegranskat)abstract
    • Dislocation plays a crucial role in controlling the strength and plasticity of bulk materials. However, determining the densities of geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs) is one of the classical problems in material research for several decades. Here, we proposed a new approach based on indentation size effect (ISE) and strengthening theories. This approach was performed on a laser powder bed fused (L-PBF) Hastelloy X (HX), and the results were verified by the Hough-based EBSD and modified Williamson–Hall (m-WH) methods. Furthermore, to better understand the new approach and essential mechanisms, an in-depth investigation of the microstructure was conducted. The distribution of dislocations shows a clear grain orientation-dependent: low density in large <101> preferentially orientated grains while high density in fine <001> orientated grains. The increment of strengthening in L-PBF HX is attributed to a huge amount of edge-GNDs. Planar slip is the main operative deformation mechanism during indentation tests, and the slip step patterns depend mostly on grain orientations and stacking fault energy. This study provides quantitative results of GND and SSD density for L-PBF HX, which constructs a firm basis for future quantitative work on other metals with different crystal structures.
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8.
  • Cui, Luqing, et al. (författare)
  • Cyclic Response of Additive Manufactured 316L Stainless Steel : The Role of Cell Structures
  • 2021
  • Ingår i: Scripta Materialia. - : Elsevier. - 1359-6462 .- 1872-8456. ; 205
  • Tidskriftsartikel (refereegranskat)abstract
    • We report the effect of cell structures on the fatigue behavior of additively manufactured (AM) 316L stainless steel (316LSS). Compared with the cell-free samples, the fatigue process of fully cellular samples only consists of steady and overload stages, without an initial softening stage. Moreover, the fully cellular sample possesses higher strength, lower cyclic softening rate and longer lifetime. Microscopic analyses show no difference in grain orientations, dimensions, and shapes. However, the fully cellular samples show planar dislocation structures, whereas the cell-free samples display wavy dislocation structures. The existence of cell structures promotes the activation of planar slip, delays strain localization, and ultimately enhances the fatigue performance of AM 316LSS.
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9.
  • Cui, Luqing, et al. (författare)
  • Dependence of microstructures on fatigue performance of polycrystals : A comparative study of conventional and additively manufactured 316L stainless steel
  • 2022
  • Ingår i: International journal of plasticity. - : Elsevier. - 0749-6419 .- 1879-2154. ; 149
  • Tidskriftsartikel (refereegranskat)abstract
    • The fatigue properties and microstructural evolution of 316 L stainless steel (316LSS) manufactured by laser powder bed fusion (L-PBF) were systematically studied and compared with its wrought counterpart. The as-built L-PBF 316LSS shows a pronounced heterogeneity, not only structurally but also chemically, with a unique microstructure of highly serrated grain boundaries, bimodal grain structure, nano-precipitates, solidification cell structures, and chemical segregations. The microindentation test showed that the hardness of the as-built L-PBF 316LSS reached 2.589 GPa, which was about 1.6 times higher than that of the wrought solution annealed counterpart, and the sparser slip steps around indentations revealed its greater dislocation storage capability. The S-N curves indicated that the fatigue resistance of the as-built L-PBF 316LSS was significantly better than that of the wrought solution annealed samples, and this was ascribed to its unique microstructural characteristics, especially the pre-existing high-density dislocations and chemical microsegregation within cellular solidification features. Furthermore, the enhanced planar slip in L-PBF 316LSS by its unique microstructure, especially the formation of deformation twins, delays the strain localization and restrains slip band generation, thereby significantly inhibiting crack initiation, and contributing greatly to the fatigue performance. The unique cell structure appears to be more effective in improving the low-cycle fatigue performance of L-PBF 316LSS due to the enhanced ductility.
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10.
  • Cui, Luqing, et al. (författare)
  • Low Cycle Fatigue Behavior and Microstructural Evolution of Nickel-based Superalloy M951G at Elevated Temperatures
  • 2020
  • Ingår i: Materials Characterization. - : Elsevier. - 1044-5803 .- 1873-4189. ; 163
  • Tidskriftsartikel (refereegranskat)abstract
    • Low cycle fatigue (LCF) tests of the newly developed nickel-based superalloy M951G have been conducted at 900 and 1000 °C under different total strain amplitudes. Results show that the fatigue properties, fracture mechanisms as well as coarsening of γ′ precipitates are dependent on testing temperatures and strain amplitudes. Fatigue life and cyclic stress response under the same total strain amplitude at 1000 °C are lower than that at 900 °C, which is due to the degradation of microstructures, shearing of γ′ precipitates by dislocations and serious oxidation. Fracture modes change from intergranular cracking to the mixed mode cracking as the strain amplitude increases. At low strain amplitudes, M951G alloy fails in the form of intergranular cracking owing to the oxidation of surface carbides and the relatively low deformation rate. At higher strain amplitudes, the strain localization in grain interior, the distribution of broken carbides and eutectics as well as the relatively higher strain rate are the main reasons for the formation of transgranular microcracks. Ultimately, the effects of fatigue conditions on coarsening of cubic γ′ precipitates are also analyzed from the aspect of γ′ volume fraction, fatigue life and flow stress difference between the γ/γ′ interfaces.
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11.
  • Cui, Luqing, et al. (författare)
  • Revealing Relationships between Microstructure and Hardening Nature of Additively Manufactured 316L Stainless Steel
  • 2021
  • Ingår i: Materials & Design. - : Elsevier. - 0261-3069 .- 0264-1275. ; 198
  • Tidskriftsartikel (refereegranskat)abstract
    • Relationships between microstructures and hardening nature of laser powder bed fused (L-PBF) 316 L stainless steel have been studied. Using integrated experimental efforts and calculations, the evolution of microstructure entities such as dislocation density, organization, cellular structure and recrystallization behaviors were characterized as a function of heat treatments. Furthermore, the evolution of dislocation-type, namely the geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs), and their impacts on the hardness variation during annealing treatments for L-PBF alloy were experimentally investigated. The GND and SSD densities were statistically measured utilizing the Hough-based EBSD method and Taylor's hardening model. With the progress of recovery, the GNDs migrate from cellular walls to more energetically-favourable regions, resulting in the higher concentration of GNDs along subgrain boundaries. The SSD density decreases faster than the GND density during heat treatments, because the SSD density is more sensitive to the release of thermal distortions formed in printing. In all annealing conditions, the dislocations contribute to more than 50% of the hardness, and over 85.8% of the total dislocations are GNDs, while changes of other strengthening mechanism contributions are negligible, which draws a conclusion that the hardness of the present L-PBF alloy is governed predominantly by GNDs.
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12.
  • Cui, Luqing, et al. (författare)
  • Superior low cycle fatigue property from cell structures in additively manufactured 316L stainless steel
  • 2022
  • Ingår i: Journal of Materials Science & Technology. - Amsterdam, Netherlands : Elsevier. - 1005-0302. ; 111, s. 268-278
  • Tidskriftsartikel (refereegranskat)abstract
    • We have investigated the low cycle fatigue (LCF) properties and the extent of strengthening in a dense additively manufactured stainless steel containing different volume fractions of cell structures but having all other microstructure characteristics the same. The samples were produced by laser powder bed fusion (L-PBF), and the concentration of cell structures was varied systematically by varying the annealing treatments. Load-controlled fatigue experiments performed on samples with a high fraction of cell structures reveal an up to 23 times increase in fatigue life compared to an essentially cell-free sample of the same grain configuration. Multiscale electron microscopy characterizations reveal that the cell structures serve as the soft barriers to the dislocation propagation and the partials are the main carrier for cyclic loading. The cell structures, stabilized by the segregated atoms and misorientation between the adjacent cells, are retained during the entire plastic deformation, hence, can continuously interact with dislocations, promote the formation of nanotwins, and provide massive 3D network obstacles to the dislocation motion. The compositional micro-segregation caused by the cellular solidification features serves as another non-negligible strengthening mechanism to dislocation motion. Specifically, the cell structures with a high density of dislocation debris also appear to act as dislocation nucleation sites, very much like coherent twin boundaries. This work indicates the potential of additive manufacturing to design energy absorbent alloys with high performance by tailoring the microstructure through the printing process.
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13.
  • Deng, Dunyong, 1989- (författare)
  • Additively Manufactured Inconel 718 : Microstructures and Mechanical Properties
  • 2018
  • Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Additive manufacturing (AM), also known as 3D printing, has gained significant interest in aerospace, energy, automotive and medical industries due to its capabilities of manufacturing components that are either prohibitively costly or impossible to manufacture by conventional processes. Among the various additive manufacturing processes for metallic components, electron beam melting (EBM) and selective laser melting (SLM) are two of the most widely used powder bed based processes, and have shown great potential for manufacturing high-end critical components, such as turbine blades and customized medical implants. The futures of the EBM and SLM are doubtlessly promising, but to fully realize their potentials there are still many challenges to overcome.Inconel 718 (IN718) is a nickel-base superalloy and has impressive combination of good mechanical properties and low cost. Though IN718 is being mostly used as a turbine disk material now, the initial introduction of IN718 was to overcome the poor weldability of superalloys in 1960s, since sluggish precipitation of strengthening phases λ’/λ’’ enables good resistance to strain-age cracking during welding or post weld heat treatment. Given the similarity between AM and welding processes, IN718 has been widely applied to the metallic AM field to facilitate the understandings of process-microstructure-property relationships.The work presented in this licentiate thesis aims to better understand microstructures and mechanical properties EBM and SLM IN718, which have not been systematically investigated. Microstructures of EBM and SLM IN718 have been characterized with scanning electron microscopy (SEM), transmission electron microscopy (TEM) and correlated with the process conditions. Monotonic mechanical properties (e.g., Vickers microhardness and tensile properties) have also been measured and rationalized with regards to the microstructure evolutions before and after heat treatments.For EBM IN718, the results show the microstructure is not homogeneous but dependant on the location in the components, and the anisotropic mechanical properties are probably attributed to alignment of porosities rather than texture. Post heat treatment can slightly increase the mechanical strength compared to the as-manufactured condition but does not alter the anisotropy. SLM IN718 shows significantly different microstructure and mechanical properties to EBM IN718. The as-manufactured SLM IN718 has very fine dendritic microstructure and Laves phases in the interdendrites, and is “work-hardened” by the residual strains and dislocations present in the material. Mechanical properties are different between horizontally and vertically built samples, and heat treatment can minimize this difference. Results from this licentiate thesis provide the basis for the further research on the cyclic mechanical properties of EBM and SLM IN718, which would be the focus of following phase of the Ph.D. research.
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14.
  • Deng, Dunyong, 1989-, et al. (författare)
  • High Temperature Mechanical Integrity of Selective Laser Melted Alloy 718 Evaluated by Slow Strain Rate Tests
  • 2021
  • Ingår i: International journal of plasticity. - : Elsevier. - 0749-6419 .- 1879-2154. ; 140
  • Tidskriftsartikel (refereegranskat)abstract
    • Strain rate dependent deformation behaviours of selective laser melted Alloy 718 (IN718) are systematically studied at 550 and 650 °C by slow strain rate testing, with a forged counterpart as a reference. Selective laser melted IN718 shows significant susceptibility to intergranular cavitation, resulting in ductility degradation with decreasing strain rate. Detailed fractography and cross section inspections are employed to identify the damage mechanisms. Creep rates are also estimated and compared with the conventional counterparts. The possible critical factors for the inferiority of time dependent damage resistance of selective laser melted IN718 are discussed.
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16.
  • Deng, Dunyong, et al. (författare)
  • Microstructure and Anisotropic Mechanical Properties of EBM Manufactued Inconel 718 and Effects of Post Heat Treatment
  • 2017
  • Ingår i: Materials Science & Engineering. - : Elsevier. - 0921-5093 .- 1873-4936. ; 693, s. 151-163
  • Tidskriftsartikel (refereegranskat)abstract
    • Materials manufactured with electron beam melting (EBM) have different microstructures and properties to those manufactured using conventional manufacturing methods. A detailed study of the microstructures and mechanical properties of Inconel 718 manufactured with EBM was performed in both as-manufactured and heat-treated conditions. Different scanning strategies resulted in different microstructures: contour scanning led to heterogeneous grain morphologies and weak texture, while hatch scanning resulted in predominantly columnar grains and strong 〈001〉 building direction texture. Precipitates in the as-manufactured condition included γ′, γ″, δ  , TiN and NbC, among which considerable amounts of γ″ yielded relatively high hardness and strength. Strong texture, directionally aligned pores and columnar grains can lead to anisotropic mechanical properties when loaded in different directions. Heat treatments increased the strength and led to different δ precipitation behaviours depending on the solution temperatures, but did not remove the anisotropy. Ductility seemed to be not significantly affected by heat treatment, but instead by the NbC and defects inherited from manufacturing. The study thereby might provide the potential processing windows to tailor the microstructure and mechanical properties of EBM IN718.
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17.
  • Deng, Dunyong, 1989- (författare)
  • On the Microstructures and Anisotropic Mechanical Behaviours of Additively Manufactured IN718
  • 2019
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Additive manufacturing (AM), also known as 3D printing, offers great design flexibility for manufacturing components with complex geometries, and has attracted significant interest in the aero and energy industries in the past decades. Among the commercial AM processes, selective laser melting (SLM) and electron beam melting (EBM) are the two most widely used ones for metallic materials. Inconel 718 (IN718) is a nickel-base superalloy and has impressive combination of good mechanical properties, weldability and low cost. Due to its excellent weldability, IN718 has been intensively applied in the AM filed, to gain more understanding of the AM processes and fully realize AM’s potentials.The study objects in the present thesis include both EBM and SLM IN718. The solidification conditions in EBM and SLM are very different and are different to that of conventional cast, leading to unique microstructures mechanical properties. Therefore, this thesis aims to gain better understanding of the microstructures and anisotropic mechanical behaviours of both EBM and SLM IN718, by detailed characterizations and by comparisons with the forged counterpart.The as-built microstructure of EBM IN718 is spatially dependent: the periphery (contour) region has a mixture of equiaxed and columnar grains, while the bulk (hatch) region has columnar grains elongated along the building direction; the last solidified region close to the top sample surface shows segregation and Laves phases, otherwise the rest of the whole sample is well homogenized. Differently, the as-built microstructure of SLM IN718 is spatially homogeneous: the grains is rather equiaxed and with subgrain cell structures. These microstructures also respond differently to the standard heat treatment routines for the conventional counterparts.Anisotropic mechanical properties are evident in the room temperature tensile tests and high temperature dwell-fatigue tests. The anisotropic tensile properties of EBM IN718 at room temperature are more likely due to the directional alignment of porosities along the building direction rather than the strong crysiii tallographic texture of ⟨100⟩ _ building direction. While for SLM IN718, the anisotropy is more likely attributed to the different extents of ‘work-hardening’ or dislocations accumulated between the horizontally and vertically built specimens. The anisotropy mechanisms in dwell-fatigue crack propagations at 550 ◦C for EBM and SLM IN718 are identical: higher effective stress intensity factor when intergranular cracking path is perpendicular to the loading direction, but lower effective stress intensity factor when intergranular cracking path is parallel to or slightly deviated from the loading direction.The 2160s dwell-fatigue cracking behaviours at 550 ◦C are of significant interest for AM IN718, of which test condition is similar to that of real service for IN718 disk in turbine engine. Generally, after conventional or short-term heat treatments, EBM IN718 shows better dwell-fatigue cracking resistance than SLM IN718. The damage mechanism is different for EBM and SLM IN718: the intergranular cracking in EBM IN718 is due to environmentally assisted grain boundary attack, while creep damage is active for SLM IN718. The considerably ‘deformed’ microstructure, specifically the subgrain cell structures in SLM IN718 resulted from the manufacturing process, is believed to activate creep damage even at a low temperature of 550 ◦C. And for SLM IN718, heat treatment routine must be carefully established to alter the ‘deformed’ microstructure for better time dependent cracking resistance at elevated temperature.
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18.
  • Jonnalagadda, Krisha Praveen, 1988-, et al. (författare)
  • Comparison of Damage Evolution During Thermal Cycling in a High Purity Nano and Conventional Thermal Barrier Coating
  • 2017
  • Ingår i: Surface & Coatings Technology. - : Elsevier. - 0257-8972 .- 1879-3347. ; 332, s. 47-56
  • Tidskriftsartikel (refereegranskat)abstract
    • Thermal barrier coatings (TBCs), consisting of a ceramic top coat and a metallic bond coat, offer resistance against high temperature degradation of turbine components. Cyclic oxidation of the bond coat, thermal stresses due to their thermal mismatches during cyclic operations, and sintering of the top coat are considered to be the common ways by which thermal barrier coatings fail. To reduce sintering, a nano structured high purity yttria stabilized zirconia (YSZ) was developed. The focus of this work is to compare the damage development of such high purity nano YSZ TBC during thermal cycling with a conventional YSZ TBC. Thermal cyclic fatigue (TCF) tests were conducted on both the TBC systems between 100 °C and 1100 °C with a 1 h hold time at 1100 °C. TCF test results showed that conventional YSZ TBC exhibited much higher life compared to the high purity nano YSZ TBC. The difference in the lifetime is explained by the use of microstructural investigations, crack length measurements along the cross-section and the difference in the elastic modulus. Furthermore, stress intensity factors were calculated in order to understand the difference(s) in the damage development between the two TBC systems.
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20.
  • Jonnalagadda, Krishna Praveen, 1988-, et al. (författare)
  • Failure of Multilayer Suspension Plasma Sprayed Thermal Barrier Coatings in the Presence of Na2SO4 and NaCl at 900 °C
  • 2019
  • Ingår i: Journal of thermal spray technology (Print). - : Springer Science and Business Media LLC. - 1059-9630 .- 1544-1016. ; 28:1-2, s. 212-222
  • Tidskriftsartikel (refereegranskat)abstract
    • The current investigation focuses on understanding the influence of a columnar microstructure and a sealing layer on the corrosion behavior of suspension plasma sprayed thermal barrier coatings (TBCs). Two different TBC systems were studied in this work. First is a double layer made of a composite of gadolinium zirconate + yttria stabilized zirconia (YSZ) deposited on top of YSZ. Second is a triple layer made of dense gadolinium zirconate deposited on top of gadolinium zirconate + YSZ over YSZ. Cyclic corrosion tests were conducted between 25 and 900 °C with an exposure time of 8 h at 900 °C. 75 wt.% Na2SO4 + 25 wt.% NaCl were used as the corrosive salts at a concentration of 6 mg/cm2. Scanning electron microscopy analysis of the samples’ cross sections showed that severe bond coat degradation had taken place for both the TBC systems, and the extent of bond coat degradation was relatively higher in the triple-layer system. It is believed that the sealing layer in the triple-layer system reduced the number of infiltration channels for the molten salts which resulted in overflowing of the salts to the sample edges and caused damage to develop relatively more from the edge.
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21.
  • Jonnalagadda, Krishna Praveen, 1988-, et al. (författare)
  • Fatigue life prediction of thermal barrier coatings using a simplified crack growth model
  • 2019
  • Ingår i: Journal of the European Ceramic Society. - : Elsevier BV. - 0955-2219 .- 1873-619X. ; 39:5, s. 1869-1876
  • Tidskriftsartikel (refereegranskat)abstract
    • Models that can predict the life of thermal barrier coatings (TBCs) during thermal cycling fatigue (TCF) tests are highly desirable. The present work focuses on developing and validating a simplified model based on the relation between the energy release rate and the TCF cycles to failure. The model accounts for stresses due to thermal mismatch, influence of sintering, and the growth of TGO (alumina and other non-protective oxides). The experimental investigation of TBCs included; 1) TCF tests at maximum temperatures of 1050 °C, 1100 °C, 1150 °C and a minimum temperature of 100 °C with 1 h and 5 h (1100 °C) hold times. 2) Isothermal oxidation tests at 900, 1000 and 1100 °C for times up to 8000 h. The model was calibrated and validated with the experimental results. It has been shown that the model is able to predict the TCF life and effect of hold time with good accuracy.
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22.
  • Jonnalagadda, Krishna Praveen, et al. (författare)
  • Hot corrosion behavior of multi-layer suspension plasma sprayed Gd2Zr2O7/YSZ thermal barrier coatings
  • 2017
  • Ingår i: InterCeram: International Ceramic Review. - Düsseldorf : Expert Fachmedien GmbH. - 0020-5214 .- 2523-8957. ; 66:5, s. 180-184
  • Tidskriftsartikel (refereegranskat)abstract
    • This study investigates the corrosion resistance of double layer Gd2Zr2O7/YSZ, triple layer dense Gd2Zr2O7 / Gd2Zr2O7/YSZ and a reference single layer YSZ coating with a similar overall top coat thickness of 300-320 ÎŒm. All the coatings were manufactured by suspension plasma spraying (SPS), resulting in a columnar structure. Corrosion tests were conducted at 900°C for 8 hours using vanadium pentoxide and sodium sulphate as corrosive salts at a concentration of 4 mg/cm2. SEM investigations after the corrosion tests show that Gd2Zr2O7 coatings exhibited lower corrosion resistance than the reference material, YSZ. Reaction between the corrosive salts and Gd2Zr2O7 results in the formation of gadolinium vanadate ( GdVO4) along the top surface and between the columns. While the stresses due to phase transformation of zirconia can be relieved to some extent by realigning of the columns in the top coat, it is believed that GdVO4 formation between the columns, along with low fracture toughness of Gd2Zr2O7 had resulted in lower corrosion resistance. Furthermore, the presence of a relatively dense layer of Gd2Zr2O7 on the top, as a preventive layer for salt infiltration, did not improve the corrosion resistance.
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23.
  • Jonnalagadda, Krishna Praveen, 1988-, et al. (författare)
  • Hot gas corrosion and its influence on the thermal cycling performance of suspension plasma spray TBCs
  • 2019
  • Ingår i: Proceedings of ASME Turbo Expo 2019. - New York, NY : American Society of Mechanical Engineers. - 9780791858677
  • Konferensbidrag (refereegranskat)abstract
    • Thermal barrier coatings (TBCs) manufactured with suspension plasma spray (SPS) are promising candidates for use in gas turbines due to their high strain tolerance during thermal cyclic fatigue (TCF). However, corrosion often occurs alongside thermal fatigue and coating durability under these conditions is highly desirable. The current study focuses on understanding the corrosion behavior and its influence on the thermal cyclic fatigue life of SPS TBCs. Corrosion tests were conducted at 780 OC using a mixed-gas (1SO2-0.1CO-20CO2-N2(bal.) in vol. %) for 168h. They were later thermally cycled between 100-1100 ⁰C with a 1h hold time at 1100 ⁰C. Corrosion test results indicated that the damage predominantly started from the edges and a milder damage was observed at the center. Nickel sulfide was observed on top of the top coat and also in the columnar gaps of the top coat. Chromium oxides were observed inside the top coat columnar gaps but close to the bond coat/top coat interface. They were believed to reduce the strain tolerance of SPS TBCs to an extent and also amplify the thermal mismatch stresses during TCF tests. This, together with a fast growth of alumina during the TCF, resulted in a significant drop in the TCF life compared to the standard TCF tests.
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24.
  • Jonnalagadda, Krishna Praveen, 1988-, et al. (författare)
  • Influence of Top Coat and Bond Coat Pre-Oxidation on the Corrosion Resistance of Thermal Barrier Coatings in the Presence of SO2
  • 2018
  • Ingår i: PROCEEDINGS OF THE ASME TURBO EXPO: TURBOMACHINERY TECHNICAL CONFERENCE AND EXPOSITION, 2018, VOL 6. - : AMER SOC MECHANICAL ENGINEERS. - 9780791851128
  • Konferensbidrag (refereegranskat)abstract
    • Thermal barrier coatings (TBCs) degradation due to corrosion is one of the commonly observed failure types in land-based gas turbines due to the usage of low grade fuels. Sulfur in its gaseous form, as SO2, can attack the TBC system and result in the degradation of both the coating and the turbine component. The present study aims to understand the difference in the corrosion induced damage caused by SO2 gas mixture in different coating architectures. Corrosion tests were conducted at 780 degrees C in a tube furnace for a period of 168h. The inlet test gas had a composition of 1SO(2)-0.1CO-20CO(2)-N-2 (bal.) in vol. %. The coating architectures consisted of 1) an overlay coating, 2) a single-side bond coat TBC, 3) an all-side bond coat TBC, 4) an all-side bond coat TBC subjected to pre-oxidation prior to the corrosion tests. The results from the corrosion tests showed that the damage was the most severe for the overlay followed by single-side bond coat TBC. Between the other two systems, the TBC subjected to pre-oxidation had relatively lower corrosion damage. The corrosion damage started from the edges for the overlay and single-side bond coat TBC and as well as through the penetration of the gas through the coating. For the coatings with bond coat on all sides, the edge damage appeared to be considerably reduced and the damage is predominantly through the gas infiltration.
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25.
  • Jonnalagadda, Krishna Praveen, 1988- (författare)
  • Thermal Barrier Coatings : Failure Mechanisms and Life Prediction
  • 2019
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Thermal barrier coatings (TBCs) use in the hot sections of gas turbine engine enables them to run at higher temperatures, and as a consequence, achieve higher thermal efficiency. For full operational exploitation of TBCs, understanding their failure and knowing the service life is essential. The broad objective of the current research is to study the failure mechanisms of new TBC materials and deposition techniques during corrosion and thermal cycling and to develop life models capable of predicting the final failure during thermal cycling.Yttria-stabilized zirconia (YSZ) has constraints such as limited operation temperature, despite being the current industry standard. Pyrochlores of A2B2O7 type have been suggested as a potential replacement for YSZ and were studied in this work. Additionally, improvements to the conventional YSZ in the form of nanostructured YSZ were also explored. The requirement for the new deposition process comes from the fact that the existing low-cost deposition processes, like atmospheric plasma spray (APS), generally exhibit lower strain tolerance. A relatively new technique, suspension plasma spray (SPS), known to be promising with better strain tolerance, has been studied in this work.At the gas turbine operating conditions, TBCs degrade and eventually fail. Common failure observed in gas turbines can be due to corrosion, thermal mismatch between the ceramic and the metallic layers, and bond coat oxidation during thermal cycling. SPS and APS TBCs were subjected to different test conditions to understand their corrosion behavior. A study on the multi-layered SPS TBCs in the presence of V2O5+Na2SO4 showed that YSZ based SPS coatings were less susceptible to corrosion damage compared to Gd2Zr2O7 SPS TBCs. A study on the influence of a sealing layer in multi-layered SPS TBCs in the presence of Na2SO4+NaCl showed that the sealing layer is ineffective if the material used for sealing is inert to the molten salts. A new study on the influence of corrosion, caused by a mixed-gas atmosphere, on the thermal cycling fatigue life of SPS TBCs was conducted. Results showed that corrosive products grew inside the top coat close to the bond coat/top coat interface along with accelerated growth of alumina. These, together, reduced the TCF life of corrosion exposed samples significantly. Finally, a study on the influence of salt concentration and temperature on a thin (dense) and a thick (porous) coating showed that thick and porous coatings have lower corrosion resistance than the thin and dense coatings. Additionally, a combination of low temperature and high salt concentration was observed to cause more damage.Thermal cycling studies were done with the objective of understanding the failure mechanisms and developing a life model. A life model based on fracture mechanics approach has been developed by taking into account different crack growth paths during thermal cycling, sintering of the top coat, oxidation of the bond coat and the thermal mismatch stresses. Validation of such a life model by comparing to the experimental results showed that the model could predict the TCF life reasonably well at temperatures of 1100 °C or below. At higher temperatures, the accuracy of the model became worse. As a further development, a simplified crack growth model was established. This simplified model was shown to be capable of predicting the TCF life as well as the effect of hold times with good accuracy.
  •  
26.
  • Jonnalagadda, Krishna Praveen, 1988-, et al. (författare)
  • Thermal barrier coatings : Life model development and validation
  • 2019
  • Ingår i: Surface & Coatings Technology. - : Elsevier BV. - 0257-8972 .- 1879-3347. ; 362, s. 293-301
  • Tidskriftsartikel (refereegranskat)abstract
    • The failure of thermal barrier coatings (TBCs) during thermal cyclic fatigue (TCF) tests depends mainly on the thermal mismatch between the coating and the substrate, the thermally grown oxides (TGO) at the top coat-bond coat interface, and the sintering of the top coat. Understanding the interplay between these factors is essential for developing a life model. The present work focuses on further development of a previously established fracture mechanics based life model and its validation by comparing with the experimental results. The life model makes use of a Paris' law type equation to estimate the cycles to failure based on micro-crack growth. The fitting parameters for the Paris' law were obtained from the experimentally measured crack lengths after the interruption of TCF tests at different cycles. An alternative approach to obtain the fitting parameters through video monitoring was also discussed. It is shown that regardless of the approach to obtain the fitting parameters, the life model in its current form is able to predict the TCF life at different temperatures with reasonable accuracy. However, at very high temperatures (1150 °C) the predictive capabilities of the model appeared to be poor.
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27.
  • Krakhmalev, Pavel, 1973-, et al. (författare)
  • Microstructure, solidification texture, and thermal stability of 316 L stainless steel manufactured by laser powder bed fusion
  • 2018
  • Ingår i: Metals. - : MDPI AG. - 2075-4701. ; 8:8, s. 1-18
  • Tidskriftsartikel (refereegranskat)abstract
    • This article overviews the scientific results of the microstructural features observed in 316 L stainless steel manufactured by the laser powder bed fusion (LPBF) method obtained by the authors, and discusses the results with respect to the recently published literature. Microscopic features of the LPBF microstructure, i.e., epitaxial nucleation, cellular structure, microsegregation, porosity, competitive colony growth, and solidification texture, were experimentally studied by scanning and transmission electron microscopy, diffraction methods, and atom probe tomography. The influence of laser power and laser scanning speed on the microstructure was discussed in the perspective of governing the microstructure by controlling the process parameters. It was shown that the three-dimensional (3D) zig-zag solidification texture observed in the LPBF 316 L was related to the laser scanning strategy. The thermal stability of the microstructure was investigated under isothermal annealing conditions. It was shown that the cells formed at solidification started to disappear at about 800 °C, and that this process leads to a substantial decrease in hardness. Colony boundaries, nevertheless, were quite stable, and no significant grain growth was observed after heat treatment at 1050 °C. The observed experimental results are discussed with respect to the fundamental knowledge of the solidification processes, and compared with the existing literature data.
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28.
  • Lundberg, Mattias, 1985-, et al. (författare)
  • Effective X-ray Elastic Constant of Cast Iron
  • 2018
  • Ingår i: Journal of Materials Science. - : Springer Science and Business Media LLC. - 0022-2461 .- 1573-4803. ; 53:4, s. 2766-2773
  • Tidskriftsartikel (refereegranskat)abstract
    • X-ray diffraction is a non-destructive method used for strain measurements in crystalline materials. Conversion of strain to stress can be achieved using the X-ray elastic constants (XEC), s1 and ½s2. The sin2ψ method was used during in situ loading to determine XEC for flake, vermicular, and spherical graphite iron. A fully pearlitic steel was used as reference. Uniaxial testing was conducted on the cast iron to create a homogeneous strain field, as well as four-point bending in both tension and compression due to the tension/compression asymmetry. The commonly used XEC value ½s2 = 5.81 × 10−6 MPa−1 is theoretically derived from an α-Fe single crystal. When investigating materials that contain ferrite, such as polycrystalline cast iron, this value is not accurate. Determination of an effective XEC for polycrystalline cast iron yields a better correlation between the measured microstrains and the properties observed on a macroscopic scale. The need for an effective XEC is evident, especially when it comes to model validation of, for example, casting simulations. Effective XEC values have been determined for flake, vermicular, and spherical graphite iron. The determined value is lower than the theoretical value.
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29.
  • Lundberg, Mattias, 1985- (författare)
  • Residual stresses, fatigue and deformation in cast iron
  • 2018
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • The complex geometry of cylinder heads in heavy-duty diesel engines makes grey iron or compact graphite iron a preferred material choice due to its price, castability, thermal conductivity and damping capacity. Today’s strict emission laws have increased the demands on engine performance and engine efficiency. This means that material properties such as fatigue resistance need to be improved. Shot peening is often used to improve the fatigue resistance of components and the benefits of shot peening are associated with the induced compressive surface stresses and surface hardening. How different shot peening parameters can affect fatigue strength of grey and compact graphite iron has been investigated within the project underlying this thesis. To do this, X-ray diffraction (XRD) was utilized for residual stress measurements, scanning electron microscopy (SEM) for microstructural characterizations and mechanical fatigue testing for mechanical quantifications. The ultimate aim of this work has been to increase the fatigue resistance of cast iron by residual stress optimization.XRD measurements and SEM examinations revealed that the shot peening parameters shot size and peening intensity significantly influence residual stresses and surface deformation. Residual stress profiles, similar to the one general considered to improve the fatigue strength in steels, were obtained for both grey and compact graphite iron. Uniaxial push-pull fatigue testing on grey iron with these shot peening parameters reduced the fatigue strength with 15–20 %. The negative effect is likely related to surface damage associated with over peening and relatively high subsurface tensile residual stresses. With very gentle shot peening parameters, the uniaxial fatigue strength were unaltered from the base material but when subjected to bending fatigue an increase in fatigue strength were observed. An alternative way to increase the fatigue strength was to conduct a 30 min annealing heat treatment at 285 XC which increased the fatigue strength by almost 10 % in uniaxial loading. The improvement could be an effect of favourable precipitates forming during the annealing, which could hinder dislocation movement during fatigue.Measuring residual stresses using XRD and the sin2 -method demands accurate X-ray elastic constants (XEC) for meticulous stress analysis. The XEC referred to as 1~2s2 should therefore always be calibrated for the specific material used. The experiments conducted revealed that the XEC value is independent of the testing method used in this work. A small correction from the theoretical value should be applied when the material contains small amounts of residual stresses. The amount of residual stresses has a great impact on the XEC and thus on the stress analysis. Concluding that proper analysis of residual stresses in cast iron is not straight forward.
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30.
  • Pant, Prabhat, 1990-, et al. (författare)
  • Mapping of residual stresses in as-built Inconel 718 fabricated by laser powder bed fusion : A neutron diffraction study of build orientation influence on residual stresses
  • 2020
  • Ingår i: Additive Manufacturing. - : Elsevier B.V.. - 2214-8604 .- 2214-7810. ; 36
  • Tidskriftsartikel (refereegranskat)abstract
    • Manufacturing of functional (ready to use) parts with the powder bed fusion method has seen an increase in recent times in the field of aerospace and in the medical sector. Residual stresses (RS) induced due to the process itself can lead to defects like cracks and delamination in the part leading to the inferior quality of the part. These RS are one of the main reasons preventing the process from being adopted widely. The powder bed methods have several processing parameters that can be optimized for improving the quality of the component, among which, build orientation is one. In this current study, influence of the build orientation on the residual stress distribution for the Ni-based super-alloy Inconel 718 fabricated by laser-based powder bed fusion method is studied by non- destructive technique of neutron diffraction at selected cross-sections. Further, RS generated in the entire part was predicted using a simplified layer by layer approach using a finite element (FE) based thermo-mechanical numerical model. From the experiment, the part printed in horizontal orientation has shown the least amount of stress in all three directions and a general tendency of compressive RS at the center of the part and tensile RS near the surface was observed in all the samples. The build with vertical orientation has shown the highest amount of RS in both compression and tension. Simplified simulations results are in good agreement with the experimental value of the stresses. © 2020 The Authors
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31.
  • Pant, Prabhat, 1990- (författare)
  • Residual Stress Distributions in Additively Manufactured Parts : Effect of Build Orientation
  • 2020
  • Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Additive manufacturing (AM) of parts using a layer by layer approach has seen a rapid increase in application for production of net shape or near-net shape complex parts, especially in the field of aerospace, automotive, etc. Due to the superiority of manufacturing complex shapes with ease in comparison to the conventional methods, interest in these kinds of processes has increased. Among various methods in AM, laser powder bed fusion (LPBF) is one of the most widely used techniques to produce metallic components.As in all manufacturing processes, residual stress (RS) generation during manufacturing is a relevant issue for the AM process. RS in AM are generated due to a high thermal gradient between subsequent layers. The impact of residual stresses can be significant for the mechanical integrity of the built parts and understanding the generation of RS and the effect of AM process parameters is therefore important for a broader implementation of AM techniques. The work presented in this licentiate thesis aims to investigate the influence of build orientation on the RS distribution in AM parts. For this purpose, L-shaped Inconel 718 parts were printed by LPBF in three different orientations, 0°, 45°, and 90°, respectively. Inconel 718 was selected because it is a superalloy widely used for making gas turbine components. In addition, IN718 has in general good weldability which renders it a good material for additive manufacturing.Residual stress distributions in the parts removed from the build plate were measured using neutron diffraction technique. A simple finite element model was developed to predict the residual stresses and the effect of RS relaxation due to the separation of the parts and build plate. The trend of residual stress distribution predicted was in good agreement with experimental results. In general, compressive RS at the part center and tensile RS near the surface were found. However, while the part printed in 0° orientation had the least amount of RS in all three principal directions of part, the part built in 90° orientation possessed the highest amount of RS in both compression and tension. The study has shown that residual stress distributions in the parts are strongly dependent on the building process. Further, it has shown that the relaxation of RS associated with the removal of the parts from the build plate after printing has a great impact on the final distribution of residual stress in the parts. These results can be used as guidelines for choosing the orientations of the part during printing.
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32.
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33.
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34.
  • Sun, Xiaoyu, et al. (författare)
  • Impeding the γ' depletion during the interdiffusion between bond coatings and superalloys via introduction of tantalum in bond coatings
  • 2023
  • Ingår i: Materials & design. - : Elsevier. - 0264-1275 .- 1873-4197. ; 227
  • Tidskriftsartikel (refereegranskat)abstract
    • The use of aluminiferous coatings profoundly improves the service life of superalloys but leads to microstructural degradation of superalloys and thus loss of mechanical properties. In this study, we mod- ified MCrAlY coatings by adding Ta to reduce the interdiffusion effect on substrate alloys. This strategy was verified by 2000 h/1100 °C oxidation tests in two Ta-containing MCrAlY-IN792 systems. The system with 3.3 wt% Ta MCrAlY exhibits an outstanding resistance to c0 depletion in the substrate and compa- rable oxidation property in comparison with a reference system of Ta-free MCrAlY-IN792. Increasing Ta to 7.4 wt% results in reduced oxidation resistance. Thermodynamic simulations revealed the phase- transformation mechanism induced by initial interdiffusion, uncovering the cause of c0 depletion in the substrate and the mechanism behind improving resistance to c0 depletion by Ta addition.
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35.
  • Sun, Xiaoyu (författare)
  • Performance of High-temperature Coatings : Oxidation and Interdiffusion
  • 2023
  • Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • The use of aluminiferous coatings profoundly improves the service life of superalloys but leads to the microstructural degradation of superalloys and thus the loss of mechanical properties. To solve this trade-off, two strategies were employed in this research.At first, we modified MCrAlY coatings by inducing Ta to reduce the interdiffusion effect on substrate alloys. This strategy was verified by 2000 h/1100 °C oxidation tests in two Ta-containing MCrAlY-IN792 systems. The system with 3.3 wt.% Ta MCrAlY displays an outstanding resistance to γ′ depletion in the substrate and comparable oxidation property in comparison with a reference system of Ta-free MCrAlY-IN792. Increasing Ta to 7.4 wt.% results in reduced oxidation resistance. Thermodynamic simulations revealed the phase-transformation mechanism induced by initial interdiffusion, uncovering the cause of γ′ depletion in the substrate and the mechanism behind improving resistance to γ′ depletion by Ta addition.In addition, we developed novel Y-doped AlCoCrFeNi high-entropy alloys by tuning Al/Cr ratio ACR. After a long-term isothermal test in air at 1100 °C up to 1000 h, the higher ACR alloy displayed a stronger oxidation resistance at the early oxidation stage, whereas a contrary result could be detected in the later stage. The microstructural analysis confirmed that the fast growth of spinel dominated the early oxidation process, leading to higher oxidation rate of the lower ACR alloys. The later stage was governed by the growth of Al2O3. Lager size Al2O3 gain formed on the lower ACR alloy impeded the inward diffusion of oxygen and thus reduced the oxidation rate, which was further verified by our thermodynamic calculations.
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36.
  • Xu, Jinghao, 1992- (författare)
  • Alloy Design and Characterization of γ′ Strengthened Nickel-based Superalloys for Additive Manufacturing
  • 2021
  • Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Nickel-based superalloys, an alloy system bases on nickel as the matrix element with the addition of up to 10 more alloying elements including chromium, aluminum, cobalt, tungsten, molybdenum, titanium, and so on. Through the development and improvement of nickel-based superalloys in the past century, they are well proved to show excellent performance at the elevated service temperature. Owing to the combination of extraordinary high-temperature mechanical properties, such as monotonic and cyclic deformation resistance, fatigue crack propagation resistance; and high-temperature chemical properties, such as corrosion and oxidation resistance, phase stability, nickel-based superalloys are widely used in the critical hot-section components in aerospace and energy generation industries.The success of nickel-based superalloy systems attributes to both the well-tailored microstructures with the assistance of carefully doped alloying elements, and the intently developed manufacturing processes. The microstructure of the modern nickel-based superalloys consists of a two-phase configuration: the intermetallic precipitates (Ni,Co)3(Al,Ti,Ta) known as γ′ phase dispersed into the austenite γ matrix, which is firstly introduced in the 1940s.  The recently developed additive manufacturing (AM) techniques, acting as the disruptive manufacturing process, offers a new avenue for producing the nickel-based superalloy components with complicated geometries. However, γ′ strengthened nickel-based superalloys always suffer from the micro-cracking during the AM process, which is barely eliminated by the process optimization.On this basis, the new compositions of γ′ strengthened nickel-based superalloy adapted to the AM process are of great interest and significance. This study sought to design novel γ′ strengthened nickel-based superalloys readily for AM process with limited cracking susceptibility, based on the understanding of the cracking mechanisms. A two-parameter model is developed to predict the additive manufacturability for any given composition of a nickel-based superalloy. One materials index is derived from the comparison of the deformation-resistant capacity between dendritic and interdendritic regions, while another index is derived from the difference of heat resistant capacity of these two spaces. By plotting the additive manufacturability diagram, the superalloys family can be categorized into the easy-to-weld, fairly-weldable, and non-weldable regime with the good agreement of the existed knowledge. To design a novel superalloy, a Cr-Co-Mo-W-Al-Ti-Ta-Nb-Fe-Ni alloy family is proposed containing 921,600 composition recipes in total. Through the examination of additive manufacturability, undesired phase formation propensity, and the precipitation fraction, one composition of superalloy, MAD542, out of the 921,600 candidates is selected.Validation of additive manufacturability of MAD542 is carried out by laser powder bed fusion (LPBF). By optimizing the LPBF process parameters, the crack-free MAD542 part is achieved. In addition, the MAD542 superalloy shows great resistance to the post-processing treatment-induced cracking. During the post-processing treatment, extensive annealing twins are promoted to achieve the recrystallization microstructure, ensuring the rapid reduction of stored energy. After ageing treatment, up to 60-65% volume fraction of γ′ precipitates are developed, indicating the huge potential of γ′ formation. Examined by the high-temperature slow strain rate tensile and constant loading creep testing, the MAD542 superalloy shows superior strength than the LPBF processed and hot isostatic pressed plus heat-treated IN738LC superalloy. While the low ductility of MAD542 is existed, which is expected to be improved by modifying the post-processing treatment scenarios and by the adjusting building direction in the following stages of the Ph.D. research.MAD542 superalloy so far shows both good additive manufacturability and mechanical potentials. Additionally, the results in this study will contribute to a novel paradigm for alloy design and encourage more γ′-strengthened nickel-based superalloys tailored for AM processes in the future.
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37.
  • Xu, Jinghao, 1992-, et al. (författare)
  • Effect of heat treatment on the microstructure characteristics and microhardness of a novel γ′ nickel-based superalloy by laser powder bed fusion
  • 2021
  • Ingår i: Results in Materials. - : Elsevier BV. - 2590-048X. ; 12
  • Tidskriftsartikel (refereegranskat)abstract
    • The fabrication of gamma prime (γ′) strengthened nickel-based superalloys by additive manufacturing (AM) techniques is of huge interest from the industrial and research community owing to their excellent high-temperature properties. The effect of post-AM-processing heat treatment on the microstructural characteristics and microhardness response of a laser powder bed fused (LPBF) γ′ strengthened nickel-based superalloy, MAD542, is systematically investigated. Post-processing heat treatment shows the significant importance of tailoring the γ′ morphology. With insufficient solutioning duration time, coarse γ′ formed in the interdendritic region heterogeneously, due to the lack of chemical composition homogenization. The cooling rate from the super-solvus solutioning plays an important role in controlling the γ′ size and morphology. Spherical γ′ is formed during the air cooling while irregularly shaped γ′ formed during the furnace cooling. The following aging heat treatment further tunes the γ′ morphology and γ channel width. After two-step aging, cuboidal γ′ is developed in the air-cooled sample, while in contrast, bi-modally distributed γ′ is developed in the furnace cooled sample with fine spherical γ′ embedded in the wide γ channel between coarse irregular shaped secondary γ′. More than 90% of the grains recrystallized during solutioning treatment at the super-solvus temperature for 30 min. The rapid recrystallization kinetics are attributed to the formation of annealing twins which significantly reduced the stored energy. Microhardness responses from different heat-treated conditions were examined.
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38.
  • Xu, Jinghao, 1992-, et al. (författare)
  • Effect of post-processes on the microstructure and mechanical properties of laser powder bed fused IN718 superalloy
  • 2021
  • Ingår i: Additive Manufacturing. - : Elsevier B.V.. - 2214-8604 .- 2214-7810. ; 48
  • Tidskriftsartikel (refereegranskat)abstract
    • The post-processing on the additively manufactured component is of huge interest as the key to tailor the microstructure to obtain certain mechanical properties. In this present study, the effects of hot isostatic pressing, as well as heat treatment on the microstructure, phase configuration and mechanical properties of laser powder bed fused (LPBF) IN718 superalloy were systematically investigated. Three different post-processes were studied such as hot isostatic pressing (HIP), heat treatment (HT), and HIP followed by HT (HIP+HT). The HIP process effectively eliminated the Laves phase remained in the as-built microstructure and brought uniformly distributed super fine γ″ precipitates in nano-meter size. In the heat-treated microstructure, larger γ″ precipitates were promoted directly from the as-built material. In comparison the HIP+HT process caused a moderate growth of γ″. In the latter two cases, the developed γ″ significantly strengthened the material. Yield strength of IN718 was increased from 738 MPa in as-built condition to 1015 MPa and 1184 MPa after HT and HIP+HT, respectively. On the contrary the ductility in the as-built IN718 condition was reduced by more than 40% after HT and HIP+HT. This can be compared to an increase in the ductility by almost 30% when subjected the as-built specimens to only HIPping. Finally, the correlation between microstructure evolution and mechanical properties is discussed in detail. © 2021 The Authors
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39.
  • Xu, Jinghao, et al. (författare)
  • On the strengthening and embrittlement mechanisms of an additively manufactured Nickel-base superalloy
  • 2020
  • Ingår i: Materialia. - : Elsevier BV. - 2589-1529. ; 10
  • Tidskriftsartikel (refereegranskat)abstract
    • The γ′ phase strengthened Nickel-base superalloy is one of the most significant dual-phase alloy systems for high-temperature engineering applications. The tensile properties of laser powder-bed-fused IN738LC superalloy in the as-built state have been shown to have both good strength and ductility compared with its post-thermal treated state. A microstructural hierarchy composed of weak texture, sub-micron cellular structures and dislocation cellular walls was promoted in the as-built sample. After post-thermal treatment, the secondary phase γ′ precipitated with various size and fraction depending on heat treatment process. For room-temperature tensile tests, the dominated deformation mechanism is planar slip of dislocations in the as-built sample while dislocations bypassing the precipitates via Orowan looping in the γ′ strengthened samples. The extraordinary strengthening effect due to the dislocation substructure in the as-built sample provides an addition of 372 MPa in yield strength. The results of our calculation are in agreement with experimental yield strength for all the three different conditions investigated. Strikingly, the γ′ strengthened samples have higher work hardening rate than as-built sample but encounter premature failure. Experimental evidence shows that the embrittlement mechanism in the γ′ strengthened samples is caused by the high dislocation hardening of the grain interior region, which reduces the ability to accommodate further plastic strain and leads to premature intergranular cracking. On the basis of these results, the strengthening micromechanism and double-edge effect of strength and ductility of Nickel-base superalloy is discussed in detail.
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40.
  • Xu, Jinghao, et al. (författare)
  • Short-term creep behavior of an additive manufactured non-weldable Nickel-base superalloy evaluated by slow strain rate testing
  • 2019
  • Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 179, s. 142-157
  • Tidskriftsartikel (refereegranskat)abstract
    • Additive manufacturing (AM) of high γ′ strengthened Nickel-base superalloys, such as IN738LC, is of high interest for applications in hot section components for gas turbines. The creep property acts as the critical indicator of component performance under load at elevated temperature. However, it has been widely suggested that the suitable service condition of AM processed IN738LC is not yet fully clear. In order to evaluate the short-term creep behavior, slow strain rate tensile (SSRT) tests were performed. IN738LC bars were built by laser powder-bed-fusion (L-PBF) and then subjected to hot isostatic pressing (HIP) followed by the standard two-step heat treatment. The samples were subjected to SSRT testing at 850 °C under strain rates of 1 × 10−5/s, 1 × 10−6/s, and 1 × 10−7/s. In this research, the underlying creep deformation mechanism of AM processed IN738LC is investigated using the serial sectioning technique, electron backscatter diffraction (EBSD), transmission electron microscopy (TEM). On the creep mechanism of AM polycrystalline IN738LC, grain boundary sliding is predominant. However, due to the interlock feature of grain boundaries in AM processed IN738LC, the grain structure retains its integrity after deformation. The dislocation motion acts as the major accommodation process of grain boundary sliding. Dislocations bypass the γ′ precipitates by Orowan looping and wavy slip. The rearrangement of screw dislocations is responsible for the formation of subgrains within the grain interior. This research elucidates the short-creep behavior of AM processed IN738LC. It also shed new light on the creep deformation mechanism of additive manufactured γ′ strengthened polycrystalline Nickel-base superalloys.
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41.
  • Yu, Cheng-Han, 1992-, et al. (författare)
  • Anisotropic behaviours of LPBF Hastelloy X under slow strain rate tensile testing at elevated temperature
  • 2022
  • Ingår i: Materials Science & Engineering. - : ELSEVIER SCIENCE SA. - 0921-5093 .- 1873-4936. ; 844
  • Tidskriftsartikel (refereegranskat)abstract
    • To improve the understanding of high temperature mechanical behaviours of LPBF Ni-based superalloys, this work investigates the influence of an elongated grain structure and characteristic crystallographic texture on the anisotropic tensile behaviours in LPBF Hastelloy X (HX) at 700 °C. Two types of loading conditions have been examined to analyse the anisotropy related to the building direction (BD), including the vertical loading (loading direction//BD) and the horizontal loading (loading direction ⊥ BD). To probe the short-term creep behaviours, slow strain rate tensile testing (SSRT) has been applied to address the strain rate dependent inelastic strain accumulation. In-situ time-of-flight neutron diffraction upon loading was performed to track the anisotropic lattice strain evolution in the elastic region and the texture evolution in the plastic region. Combined with the post microstructure and fracture analysis, the anisotropic mechanical behaviours are well correlated with the different microstructural responses between vertical and horizontal loading and the different strain rates. A better creep performance is expected in the vertical direction with the consideration of the better ductility and the higher level of texture evolution.
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42.
  • Yu, Cheng-Han, 1992-, et al. (författare)
  • Anisotropic Deformation and Fracture Mechanisms of an Additively Manufactured Ni-Based Superalloy
  • 2020
  • Ingår i: Superalloys 2020. - Cham : Springer International Publishing. ; , s. 1003-1013
  • Konferensbidrag (refereegranskat)abstract
    • This study investigates the anisotropic mechanical and microstructural behavior of the laser powder bed fusionLaser powder bed fusion (LPBF) manufactured Ni-based superalloy Hastelloy X (HX) by using slow strain rate (10−5 and 10−6s−1) tensile testing (SSRT) at 700 °C. LPBF HX typically exhibits an elongated grain structure along the building direction (BD) and the texture analysis from the combination of neutron diffractionNeutron diffraction and EBSD discloses a major texture component <011> and a minor texture component <001> along BD, and a texture component <001> in the other two sample directions perpendicular to BD. Two types of tests have been performed, the horizontal tests where the loading direction (LD) is applied perpendicular to BD, and the vertical tests where LD is applied parallel to BD. The vertical tests exhibit lower strength but better ductility, which is explained by the texture effect and the elongated grain structure. A comparison of the mechanical behavior to the wrought HX shows that LPBF HX has better yield strength due to the high dislocation density as proved by TEM images. Creep voids are observed at grain boundaries in SSRT for both directions and are responsible for the poor ductility of the horizontal tests. The vertical ductility in SSRT maintains the same level as the reference tensile test at the strain rate of 10−3s−1, due to the extra deformation capacity contributed by the discovered deformation twinningDeformation twinning and lattice rotation. The deformation twinningDeformation twinning, which is only observed in the vertical tests and has not been found in the conventionally manufactured HX, is beneficial to maintain the ductility but does not strengthen the material.
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43.
  • Yu, Cheng-Han, 1992- (författare)
  • Anisotropic mechanical behaviors and microstructural evolution of thin-walled additively manufactured metals
  • 2020
  • Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Additive manufacturing (AM), also known as 3D printing, is a concept and method of a manufacturing process that builds a three-dimensional object layer-by-layer. Opposite to the conventional subtractive manufacturing, it conquers various limitations on component design freedom and raises interest in various fields, including aerospace, automotive and medical applications. This thesis studies the mechanical behavior of thin-walled component manufactured by a common AM technique, laser powder bed fusion (LPBF). The studied material is Hastelloy X, which is a Ni-based superalloy, and it is in connection to a component repair application in gas turbines. The influence of microstructure on the deformation mechanisms at elevated temperatures is systematically investigated. This study aims for a fundamental and universal study that can apply to different material grades with FCC crystallographic structure.It is common to find elongated grain and subgrain structure caused by the directional laser energy input in the LPBF process, which is related to the different printing parameters and brands of equipment. This thesis will start with the study of scan rotation effect on stainless steel 316L in an EOS M290 equipment. The statistic texture analysis by using neutron diffraction reveals a clear transition when different level of scan rotation is applied. Scan rotation of 67° is a standard printing parameter with intention to lower anisotropy, yet, the elongated grain and cell structure is still found in the as-built microstructure. Therefore, the anisotropic mechanical behavior study is carried out on the sample printed with scan rotation of 67° in this thesis.Thin-walled effects in LPBF are investigated by studying a group of plate-like HX specimens, with different nominal thicknesses from 4mm down to 1mm, and a reference group of rod-like sample with a diameter of 18mm. A texture similar to Goss texture is found in rod-like sample, and it becomes <011>//BD fiber texture in the 4mm specimen, then it turns to be <001> fiber texture along the transverse direction (TD) in the 1mm specimen. Tensile tests with the strain rate of 10−3 s−1 have been applied to the plate-like specimens from room temperature up to 700 ℃. A degradation of strength is shown when the sample becomes thinner, which is assumed to be due to the overestimated load bearing cross-section since the as-built surface is rough. A cross-section calibration method is proposed by reducing the surface roughness, and a selection of proper roughness parameters is demonstrated with the consideration of the calculated Taylor’s factor and the residual stress. The large thermal gradient during the LPBF process induces high dislocation density and strengthens the material, hence, the LPBF HX exhibits better yield strength than conventionally manufactured, wrought HX, but the work hardening capacity and ductility are sacrificed at the same time.Two types of loading condition reveal the anisotropic mechanical behavior, where the vertical and horizontal tests refer to the loading direction being on the BD and TD respectively. The vertical tests exhibit lower strength but better ductility that is related to the larger lattice rotation observed from the samples with different deformation level. Meanwhile, the elongated grain structure and grain boundary embrittlement are responsible for the low horizontal ductility. A ductile to brittle transition is traced at 700 ℃, so a further study with two different slow strain rates, 10−5 s−1 and 10−6 s−1, are carried out at 700 ℃. Creep damage is shown in the slow strain rates testing. Deformation twinning is found only in the vertical tests where it forms mostly in the twin favorable <111> oriented grain along the LD. The large lattice rotation and the deformation twinning make the vertical ductility remain high level under the slow strain rates. The slow strain rate tensile testing lightens the understanding of creep behavior in LPBF Ni-based superalloys.In summary, this thesis uncovers the tensile behavior of LPBF HX with different variations, including geometry-dependence, temperature-dependence, crystallographic texture-dependence and strain rate-dependence. The generated knowledge will be beneficial to the future study of different mechanical behavior such as fatigue and creep, and it will also enable a more robust design for LPBF applications.
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44.
  • Yu, Cheng-Han, 1992- (författare)
  • Anisotropic Mechanical Behaviours and Thin-wall Effects of Additively Manufactured Austenitic Alloys
  • 2022
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Additive manufacturing (AM), also known as 3D printing, is a general concept of building a three-dimensional object layer-by-layer. AM breaks through the manufacturing limitations in conventionally subtractive manufacturing, leading to a great design freedom of components with complex geometries. The potential of integrating AM into existing manufacturing process with additional functionality raises interest in various fields, such as aerospace, automotive and medical applications. To ensure robust AM applications, this PhD project has carried out investigations on the mechanical behaviours of AM components with respect to the characteristic microstructure and the geometrical effects. The investigated materials include Hastelloy X (HX, a solid-solution strengthened Ni-based superalloy) and stainless steel 316L (SS 316L, a solid-solution strengthened austenitic stainless steel) manufactured by laser powder bed fusion (LPBF). The high temperature tensile behaviours, short-term creep resistance and low cycle fatigue performance have been examined. The aim of this thesis is to conduct a fundamental studies that can be applied to different material grades with single phase face-centred cubic (FCC) crystallographic structure. LPBF HX shows a great potential for the burner tip repair application in gas turbines. Due to the complex geometry of the burner and the requirement of high temperature mechanical performance, the tensile properties have been systematically examined. Multiple testing variables have been applied, including the specimen geometry, the elevated temperature, the strain rate and the loading direction (LD). Combined with the prior and post microstructural analysis, the deformation and fracture mechanisms have been investigated. For the thin-walled specimens, a clear texture transition is found when it comes to the thinner specimen, and it leads to the lower yield strength as a result. In addition, as the high surface roughness of the LPBF as-built specimen can cause inaccuracy of the yield strength determination due to the overestimated loading cross section, especially for the thin-walled specimen, a calibration method based on the crystallographic texture results has been proposed. Meanwhile, anisotropic tensile behaviours are observed at all the testing conditions due to the elongated grain structure and the characteristic texture along the building direction (BD). At elevated temperatures, the grain boundary embrittlement takes place at 700 °C that leads to the ductility loss in the horizontal loading (LD ⊥ BD). Slow strain rate tensile testing (SSRT) has been performed to probe the short-term creep resistance at 700 °C, since it is a useful tool to address the strain rate dependent in elastic strain accumulation. Surprisingly, the ductility of the vertical loading (LD // BD) remains at a high level not only at 700 °C but also at SSRT condition, and the high ductility results from the evident texture evolution and crystallographic orientation dependent deformation twinning. The good ductility of the vertical loading indicates a better creep performance compared to the horizontal loading. In-situ and ex-situ neutron diffraction measurements upon loading have also been applied for a full-length investigation on the anisotropic tensile behaviours. Thin-wall effects on strain-controlled low cycle fatigue (LCF) behaviours of LPBF SS 316L have been investigated by using the tubular fatigue specimens with different wall thicknesses. The comparison between the machined and as-built surface conditions have been drawn. The fully reversed LCF tests were successfully performed without the buckling problem in thin-walled structures owing to the tubular geometry. The surface roughness and the distinct microstructure at the surface region lead to the inferior fatigue strain-life, especially with the low applied strain range. The combined effects have been quantified by estimating the fatigue notch factor, Kf . The LCF tests have also been performed on the regular cylindrical specimens and compared to the wrought SS 316L. A comparable fatigue strain-life is found between the LPBF and the wrought SS 316L. Yet, the secondary hardening caused by strain-induced martensitic phase transformation is only observed in the wrought SS 316L, while continuous cyclic softening is shown in the LPBF SS 316L. In addition, as high level of residual stress (RS) is commonly found in the as-built specimen, the effect of stress relief heat treatment (600 °C /4 hours) on the LCF behaviours has been examined. A great reduction of RS is found after the heat treatment, and higher responding stresses are shown in the stress relieved specimen, which indicates a better fatigue stress-life. In summary, the deformation and fracture mechanisms of LPBF HX and SS 316L under different loading conditions have been systematically investigated. Via increasing deeper knowledge of LPBF material behaviours, the LPBF applications can be expanded to a greater extent. 
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45.
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46.
  • Yu, Cheng-Han, 1992-, et al. (författare)
  • Thin-wall Effects and Anisotropic Deformation Mechanisms of an Additively Manufactured Ni-based Superalloy
  • 2020
  • Ingår i: Additive Manufacturing. - : Elsevier. - 2214-8604 .- 2214-7810. ; 36
  • Tidskriftsartikel (refereegranskat)abstract
    • Laser powder bed fusion (LPBF) of Ni-based superalloys shows great potential for high temperature applications, for example, as a burner repair application for gas turbines where the thin-walled structure is important. It motivates this work to investigate the evolution of microstructure and the anisotropic mechanical behavior when plate-like specimens are built with a thickness from 4 mm down to 1 mm. By performing texture analysis using neutron diffraction, a clear transition in fiber texture from <011> to <001> is indicated when the specimen becomes thinner. The residual stress shows no thickness dependence, and at the subsurface the residual stress reaches the same level as the yield strength. Due to the rough as-built surface, a roughness compensation method for mechanical properties of thin-walled structures is outlined and demonstrated. Tensile tests from room temperature up to 700 °C have been carried out. Anisotropic mechanical behavior is found at all temperatures, which is strongly related to the anisotropic texture evolution. Stronger texture evolution and grain rotations are discovered when the tensile loading is applied along the building direction. The mechanical behavior has been compared to a wrought material, where the high dislocation density and the subgrain structure of the LPBF material result in a higher yield strength. Combining the statistical texture analysis by neutron diffraction with mechanical testing, EBSD grain orientation mapping and the investigation of dislocation structures using transmission electron microscopy, this work illustrates the significance of texture for the thin-wall effect and anisotropic mechanical behavior of LPBF materials.
  •  
47.
  • Zhang, Pimin, 1990-, et al. (författare)
  • Failure Mechanism of MCrAlY Coating at the Coating-Substrate Interface under Type I Hot Corrosion
  • 2019
  • Ingår i: Materials and corrosion - Werkstoffe und Korrosion. - : Wiley-VCH Verlagsgesellschaft. - 0947-5117 .- 1521-4176. ; 70:9, s. 1593-1600
  • Tidskriftsartikel (refereegranskat)abstract
    • MCrAlY coatings are widely used to provide protection of hot component in modern gas turbine engines against high‐temperature oxidation and hot corrosion. Coating‐substrate interface, where the substrate is only partially covered by the coatings, is vulnerable to the hot corrosion attack. The accelerated degradation at the coating‐substrate interface can cause fast spallation of the coating, leading to the early failure of the gas turbine components. In this paper, MCrAlY powder was deposited on IN792 disks by high‐velocity oxygen‐fuel spraying. The hot corrosion behavior of the coated sample was investigated using (0.8Na, 0.2K)2SO 4 salt deposition at 900°C in lab air. Results showed a minor attack in the coating center, however, an accelerated corrosion attack at the coating‐substrate interface. The fast growth of corrosion products from substrate caused large local volume expansions at the coating‐substrate interface, resulting in an early coating spallation.
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48.
  •  
49.
  • Zhang, Pimin, 1990-, et al. (författare)
  • Investigation of Element Effect on High-Temperature Oxidation of HVOF NiCoCrAlX Coatings
  • 2018
  • Ingår i: Coatings. - : M D P I AG. - 2079-6412. ; 8:4, s. 129-145
  • Tidskriftsartikel (refereegranskat)abstract
    • MCrAlX (M: Ni or Co or both, X: minor elements) coatings have been used widely to protect hot components in gas turbines against oxidation and heat corrosion at high temperatures. Understanding the influence of the X-elements on oxidation behavior is important in the design of durable MCrAlX coatings. In this study, NiCoCrAlX coatings doped with Y + Ru and Ce, respectively, were deposited on an Inconel-792 substrate using high velocity oxygen fuel (HVOF). The samples were subjected to isothermal oxidation tests in laboratory air at 900, 1000, and 1100 °C and a cyclic oxidation test between 100 and 1100 °C with a 1-h dwell time at 1100 °C. It was observed that the coating with Ce showed a much higher oxidation rate than the coating with Y + Ru under both isothermal and cyclic oxidation tests. In addition, the Y + Ru-doped coating showed significantly lower β phase depletion due to interdiffusion between the coating and the substrate, resulting from the addition of Ru. Simulation results using a moving phase boundary model and an established oxidation-diffusion model showed that Ru stabilized β grains, which reduced β-depletion of the coating due to substrate interdiffusion. This paper, combining experiment and simulation results, presents a comprehensive study of the influence of Ce and Ru on oxidation behavior, including an investigation of the microstructure evolution in the coating surface and the coating-substrate interface influenced by oxidation time.
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50.
  • Zhang, Pimin, 1990- (författare)
  • Oxidation behaviour of MCrAlX coatings : effect of surface treatment and an Al-activity based life criterion
  • 2018
  • Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • MCrAlY coatings (M=Ni and/or Co) have been widely used for the protection of superalloy components against oxidation and hot corrosion in the hot sections of gas turbines. The drive to improve engine combustion efficiency while reducing emissions by increasing the operation temperature brings a big challenge for coating design. As a result, the need for improvement of MCrAlY coatings for better oxidation resistance is essential.Formation of a stable, dense, continuous, and slow-growing α-Al2O3 layer, on the MCrAlY coating surface, is the key to oxidation protection, since the protective α-Al2O3 scale offers superior oxidation resistance due to its lower oxygen-diffusion rate as compared with other oxides. The ability of a MCrAlY coating to form and maintain such a protective scale depends on the coating composition and microstructure, and can be improved through optimization of deposition parameters, modification of coating surface conditions, and so on. Part of this thesis work focuses on studying the effect of post-deposition surface treatments on the oxidation behavior of MCrAlX coatings (X can be yttrium and/or other minor alloying elements). The aim is to gain fundamental understanding of alumina scale evolution during oxidation which is important for achieving improved oxidation resistance of MCrAlX coatings. Oxide scale formed on coatings at initial oxidation stage and the effect of surface treatment were investigated by a multi-approach study combining photo-stimulated luminescence, microstructural observation and weight gain. Results showed that both mechanically polished and shot-peened coatings exhibited superior performance due to rapid formation of α-Al2O3 fully covering the coating and suppressing growth of transient alumina, assisted by the high density of α-Al2O3 nuclei on surface treatment induced defects. The early development of a two-layer alumina scale, consisting of an inward-grown inner α-Al2O3 layer and an outer layer transformed from outward-grown transient alumina, resulted in a higher oxide growth rate of the as-sprayed coating. The positive effect of the surface treatments on retarding oxide scale growth and suppressing formation of spinel was also observed in oxidation test up to 1000 hrs.As the oxidation proceeds to the close-to-end stage, a reliable criterion to estimate the capability of coating to form α-Al2O3 is of great importance to accurately evaluate coating lifetime, which is the aim of the other part of the thesis work. Survey of published results on a number of binary Ni-Al and ternary Ni-Cr-Al, Ni-Al-Si systems shows that the empirical Al-concentration based criterion is inadequate to properly predict the formation of a continuous α-Al2O3 scale. On the other hand, correlating the corresponding Al-activity data, calculated from measured chemical compositions using the Thermo-Calc software, to the experimental oxidation results has revealed a temperature dependent, critical Al-activity value for forming continuous α-Al2O3 scale. To validate the criterion, long-term oxidation tests were performed on five MCrAlX coatings with varying compositions and the implementation of the Al-activity based criterion on these coatings successfully predicted α-Al2O3 formation, showing a good agreement with experiment results.
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