| 1. |
- Mahade, Satyapal, 1987-, et al.
(författare)
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Incorporation of graphene nano platelets in suspension plasma sprayed alumina coatings for improved tribological properties
- 2021
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Ingår i: Applied Surface Science. - : Elsevier BV. - 0169-4332 .- 1873-5584. ; 570
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Tidskriftsartikel (refereegranskat)abstract
- Graphene possesses high fracture toughness and excellent lubrication properties, which can be exploited to enhance tribological performance of coating systems utilized to combat wear. In this work, suspension plasma spray (SPS) process was employed to deposit a composite, graphene nano-platelets (GNP) incorporated alumina coating. For comparison, monolithic alumina was also deposited utilizing identical spray conditions. The as-deposited coatings were characterized in detail for their microstructure, porosity content, hardness, fracture toughness and phase composition. Raman analysis of the as-deposited composite coating confirmed retention of GNP. The composite coating also showed good microstructural integrity, comparable porosity, higher fracture toughness and similar alumina phase composition as the monolithic alumina coating. The as-deposited coatings were subjected to dry sliding wear tests. The GNP incorporated composite coating showed lower CoF and lower specific wear rate than the pure alumina coating. Additionally, the counter surface also showed a lower wear rate in case of the composite coating. Post-wear analysis performed by SEM/EDS showed differences in the coating wear track and in the ball wear track of monolithic and composite coatings. Furthermore, Raman analysis in the wear track of composite coating confirmed the presence of GNP. The micro-indentation and wear test results indicate that the presence of GNP in the composite coating aided in improving fracture toughness, lowering CoF and specific wear rate compared to the monolithic coating. Results from this work demonstrated retention of GNP in an SPS processed coating, which can be further exploited to design superior wear-resistant coatings.
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| 2. |
- Dimitrios, Nikas, et al.
(författare)
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Effect of annealing on microstructure in railway wheel steel
- 2022
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Ingår i: 42Nd Riso International Symposium On Materials Science. - : Institute of Physics Publishing (IOPP). ; 1249
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Konferensbidrag (refereegranskat)abstract
- Railway wheels are commonly made from medium carbon steels (similar to 0.55 wt.% C), heat treated to a near pearlitic microstructure with 5-10% pro-eutectoid ferrite. During the operation of freight trains, where block brakes are used, high thermal loads occur together with the high contact stresses, which combined can affect the mechanical properties of the material. In this study, the effects of annealing on local microstructure and mechanical properties in pearlitic railway wheel steel were investigated using electron microscopy and micro-hardness. It is found that after annealing at 650 degrees C, the room temperature hardness reduces about 25%, accompanied by significant spheroidization of cementite in the pearlitic colonies, though the size and the orientation gradients of the pearlitic colonies have not changed much. The relationship between the microstructural changes and the mechanical properties are discussed.
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| 3. |
- Krakhmalev, Pavel, 1973-, et al.
(författare)
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Influence of heat treatment under hot isostatic pressing (HIP) on microstructure of intermetallic-reinforced tool steel manufactured by laser powder bed fusion
- 2020
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Ingår i: Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing. - : Elsevier BV. - 0921-5093 .- 1873-4936. ; 772
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Tidskriftsartikel (refereegranskat)abstract
- Microstructure and properties of as-built laser powder bed fusion (LPBF) steels differ from the conventional ones, and they may contain some porosity and lack of fusion. Therefore, post-treatments, including hot isostatic pressing (HIP), are used to density the material, and tailor the properties of the final product. Usually, HIP is performed as an operation separate from heat treatment. In the present investigation a new approach was used, in which the whole cycle of the heat treatment was carried out in HIP under pressure, and the influence of HIP on microstructure of an advanced stainless maraging tool steel manufactured by LPBF was investigated. For a comparison, a conventional steel grade of the same chemical composition, after a heat treatment at the same temperature-time conditions, was also characterized. The microstructure of the steel was investigated by means of advanced microscopy and atom probe tomography. The influence of the manufacturing route, heat treatment and HIP on microstructure, austenitic phase fraction and size distribution of precipitates was investigated, and the role of high pressure in stabilization of austenite in the microstructure was discussed. It was concluded that since HIP influences phase transformations, a fundamental understanding of the influence of HIP on microstructure is nececcary, and development of new post processing regimes guaranteeing the best performance of the material is required.
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| 4. |
- Xu, Jinghao, et al.
(författare)
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On the strengthening and embrittlement mechanisms of an additively manufactured Nickel-base superalloy
- 2020
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Ingår i: Materialia. - : Elsevier BV. - 2589-1529. ; 10
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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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| 5. |
- Hanning, Fabian, et al.
(författare)
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The Effect of Grain Size on theSusceptibility Towards Strain Age Crackingof Wrought Haynes® 282®
- 2020
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Ingår i: SPS 2020. - Amsterdam : IOS Press. - 9781643681467 - 9781643681474 ; 13, s. 407-416
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The effect of grain size on the suceptibility towards strain age cracking (SAC) has been investigated for Haynes® 282® in the tempeature range of 750 to 950°C after isothermal exposure up to 1800s. Grain growth was induced by heattreating the material at 1150°C for 2h, resulting in a fourfold increase in grain size. Hardness was significanlty reduced after heat treatment as compared to millannealed material. Large grain size resulted in intergranular fracture over a widertemperature range than small grain size material. Ductility was lowest at 850°C, while lower values were observed to be correlated to increased grain size. The rapid formation of grain boundary carbide networks in Haynes® 282® is found to be notable to compensate for higher local stresses on grain boundaries due to incresedgrain size.
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| 6. |
- Qin, Xiao, 1993, et al.
(författare)
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Microstructure and texture evolutions in FeCrAl cladding tube during pilger processing
- 2023
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Ingår i: Journal of Materials Research and Technology. - 2238-7854. ; 25, s. 5506-5519
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Tidskriftsartikel (refereegranskat)abstract
- The microstructure of FeCrAl cladding tubes depends on the fabricating process history. In this study, the microstructural characteristics of wrought FeCrAl alloys during industrial pilger processing into thin-walled tubes were investigated. The hot extruded tube showed ∼100 μm equiaxed grains with weak α∗-fiber in {h11}<1/h12> texture, while pilger rolling process change the microstructure to fragmented and elongated grains along the rolling direction. The pilgered textures could be predicted with the VPSC model. The inter-pass annealing at 800–850 °C for 1 h results in recovery and recrystallization of the ferric matrix and restoration of ductility. The final finished tube shows fine recrystallized grains (∼11 μm) with dominant γ-fiber in three dimensions. Pilger rolling enhanced α-fiber while annealing reduced α-fiber and enhanced γ-fiber. Microstructural evolution in the Laves precipitates followed the sequence of faceted needle-like → spherical → faceted ellipsoidal. Thermomechanical processing resulted in cladding tubes with an area fraction of ∼5% and a number density of 5 × 10−11 m−2 in Laves precipitates, which is half that of the first-pilgered tube. Laves precipitates pin the grain boundaries to control the microstructure and prevent grain coarsening.
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| 7. |
- Shoja, Siamak, 1980
(författare)
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Microstructure and plastic deformation of textured CVD alumina coatings
- 2020
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- It is known that the wear performance of α-alumina coatings produced by chemical vapor deposition (CVD) is significantly influenced by the type and degree of texture. However, the main reasons behind this behavior are not fully understood. This thesis contains studies of two related topics for increasing the understanding of α-alumina coatings. The first topic concerns the microstructure and texture development of CVD α-Al2O3 coatings, and the second topic concerns calculations and analysis of the Schmid factors ( m ) for coatings with different textures. By combining different analysis methods (such as XRD, SEM, FIB/SEM, TKD, TEM, STEM, XEDS), and theoretical and experimental Schmid factor analysis by MATLAB and EBSD, the microstructure and plastic deformation of α-alumina coatings were investigated. The microstructures of three different CVD α-Al2O3 layers deposited onto a Ti(C,N,O) bonding layer were studied. Grain boundary diffusion of heavy elements from the substrate to the bonding layer/α-Al2O3 interface was observed. This may be the cause of a disturbance in the early growth of α-Al2O3. Additionally, it was found that the number of interfacial pores at the bonding layer/α-Al2O3 interface increased by introducing the H2S gas. The H2S gas also promoted an earlier development of the (0001) texture. The orientation of the grains was developed to the desired texture both as a gradual change over several grains and as an abrupt transformation from one grain to another. The probability of plastic deformation in different wear zones on the rake face of a cutting tool was investigated theoretically and experimentally by analyzing Schmid factors for textured α-Al2O3 coatings. Schmid factor diagrams were constructed using MATLAB/MTEX and used to extract frequency distributions for different slip systems and textures. The results were compared with lateral distribution maps of Schmid factors obtained from experimental coatings. It was observed that basal slip is most easily activated in the transition zone, followed by prismatic slip systems 1 and 2 in coatings with an (0001)-texture. The homogeneous plastic deformation behavior observed in this coating is also connected to mostly high Schmid factors in the m -value distribution. The differences between the m -value distributions for the three slip systems are not that pronounced in the (01-12) and (11-20) textures, and the distributions are relatively wide. The low wear rate and more homogeneous deformation of the coating with (0001) texture compared to the other coating textures may be the result of the high plasticity, offered by the easy activation of basal slip and prismatic 1 slip, and the low spread of Schmid factor values at the transition zone. In conclusion, the results presented in this thesis form a knowledge platform that can be used to understand the microstructure and wear mechanisms of textured CVD α-alumina coatings.
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| 8. |
- al-Karawi, Hassan, 1993, et al.
(författare)
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The efficiency of HFMI treatment and TIG remelting for extending the fatigue life of existing welded structures
- 2021
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Ingår i: Steel Construction. - : Wiley. - 1867-0520 .- 1867-0539. ; 14:2, s. 95-106
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Tidskriftsartikel (refereegranskat)abstract
- Different post‐weld treatment methods have been developed to enhance the fatigue strength of welded steel structures and extend the service lives of their components. High‐frequency mechanical impact (HFMI) treatment and tungsten inert gas (TIG) remelting are two methods that have attracted considerable interest in recent decades. This paper presents the results of a study of fatigue life extension for pre‐fatigued welded steel details which can be achieved using HFMI treatment and TIG remelting. More than 250 fatigue test results were collected – including different details such as butt welds, longitudinal attachments, transverse attachments and cover plate attachments. HFMI treatment was found to extend the life considerably when the specimens treated were free from cracks or when existing cracks were < 2.25 mm deep. TIG remelting could extend fatigue lives even with cracks > 4 mm deep. In comparison to TIG remelting, HFMI treatment results in a longer fatigue life extension for pre‐fatigued details, provided existing cracks are < 2.25 mm deep. Regarding TIG remelting, the depth of possible remaining cracks was found to be a substantial parameter when assessing the degree of life extension.
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| 9. |
- Fazi, Andrea, 1992
(författare)
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Cold spray Cr-coated Optimized ZIRLO claddings: an option for accident tolerant fuels
- 2021
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Accident tolerant fuel development has started with the aim of providing nuclear fuels able to endure severe accident conditions. Research in this field has also sparked a wave of material renewal in the nuclear industry that had being delayed for the last few decades. Climate change is an ever-growing public concern, and policies about greenhouse gas emissions are becoming more stringent both at the national and international level. Nuclear energy produces very low carbon emissions and the successful development of new accident tolerant materials might play a role in making this technology a viable solution to this global issue. Cr-coated zirconium claddings are one of the most promising candidates as near-term response to the need for accident tolerant materials. These coatings can be produced via a range of different technologies, but the two main designs being currently developed are obtained with physical vapour deposition or cold spray (CS) deposition. In cold spray, high pressure gas is fed through a nozzle together with Cr powder. The Cr particles are accelerated up to 1200 m/s and directed to the substrate, in this case cladding tubes made of Optimized ZIRLO™ alloy. The resulting Cr-coated zirconium cladding is the subject of this work. As-fabricated samples and autoclave tested material are characterized with atom probe tomography and a range of electron microscopy techniques. The scope of the investigation is to evaluate the performances of Cr-coated claddings under operating conditions. Additionally, the nature of the adhesion in cold spray coating and the effects of this deposition method on the substrate are explored. A 10-20 nm thick intermixed bonding region was observed at the Cr/Zr interface of the as-fabricated cladding. This region is deemed to play an important role in the strong adhesion of CS coatings. When exposed to operating conditions, ZrCr2-Laves phase was found to nucleate in the intermixed bonding region. CS deposition involves severe plastic deformation of both coating and substrate. As a result, a 1-2 μm thick nanocrystalline layer can be found in the substrate adjacent to the Cr/Zr interface. After autoclave exposure, a Zr-Cr-Fe phase was discovered precipitating in this nanocrystalline layer at the Zr grain boundaries.
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| 10. |
- Ganvir, Ashish, 1991-, et al.
(författare)
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Novel utilization of liquid feedstock in high velocity air fuel (HVAF) spraying to deposit solid lubricant reinforced wear resistant coatings
- 2021
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Ingår i: Journal of Materials Processing Technology. - : Elsevier BV. - 0924-0136 .- 1873-4774. ; 295
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Tidskriftsartikel (refereegranskat)abstract
- The ability to axially inject liquid feedstock has encouraged the thermal spray research community to explore this concept to deposit coatings for various next generation functional applications. The current study explores the utilization of liquid feedstock in high velocity air fuel (HVAF) spraying to deposit solid lubricant reinforced wear resistant coatings for the first time. The study successfully demonstrates the use of a powder-suspension hybrid processing approach to incorporate a solid lubricant Boron Nitride (as suspension) in a wear resistant Cr3C2–NiCr (as powder) cermet matrix. Coatings were characterized using Scanning Electron Microscopy and Raman Spectroscopy to analyze their microstructure and phase constitution. The results show that the tribological performance of the hexagonal boron nitride (hBN)-incorporated composite coating was significantly better than the traditional powder-derived Cr3C2–NiCr coating. Such hBN-incorporated composite coatings are needed to improve the mechanical properties and enhance the overall tribological performance of metallic components used in various applications, especially at high temperature such as cylinder bore, pistons, deformation tools, etc. The limitations of liquid based lubricants at high temperature motivates the use of hBN reinforced composite coatings as it can form a protective solid lubrication tribo-film. The study concludes that the emerging HVAF technology can accommodate liquid feedstock and be successfully utilized to deposit hybrid powder-suspension composite coatings to create multi length scale microstructures which can be attractive for combining different tribological attributes in the same coatings system.
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