| 1. |
- Leicht, Alexander, 1987, et al.
(författare)
-
Effect of part thickness on the microstructure and tensile properties of 316L parts produced by laser powder bed fusion
- 2021
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Ingår i: Advances in Industrial and Manufacturing Engineering. - : Elsevier BV. - 2666-9129. ; 2
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Tidskriftsartikel (refereegranskat)abstract
- Additive manufacturing provides a unique possibility to manufacture parts with advanced design and thin-walled structures. To explore thin-wall capacity, laser powder bed fusion was employed to fabricate 316L samples with different section thicknesses. A detailed microstructure characterization was then carried out, and tensile properties were assessed. It was found that reducing the part thickness did not affect the microstructure but did reduce the tensile properties. Samples with 1 mm thickness exhibited the lowest yield strength of 457 ± 11 MPa and elongation to fracture of 49 ± 20%, while the tensile properties improved when the sample thickness was increased to 3 mm. The 3 mm thick samples generated tensile properties comparable to those of standard dimensions. The results emphasize that part thickness must be considered when assessing mechanical properties and must be adjusted when performing design optimization and simulations of samples produced with laser powder bed fusion.
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| 2. |
- Leicht, Alexander, 1987, et al.
(författare)
-
Effect of process parameters on the microstructure, tensile strength and productivity of 316L parts produced by laser powder bed fusion
- 2020
-
Ingår i: Materials Characterization. - : Elsevier BV. - 1044-5803. ; 159:110016
-
Tidskriftsartikel (refereegranskat)abstract
- The influence of process parameters during laser-powder bed fusion on the microstructure, tensile strength and build time of 316L parts is studied. By increasing both, scan speed and hatch distance, an improved productivity while maintaining acceptable properties were achieved. The samples were evaluated using volumetric energy density. The microstructure characterization showed that all produced samples had elongated grains aligned with the building direction independent on energy density that was used, whereas the grain size was strongly affected by the different energy densities. A high energy density resulted in larger grains with a very strong 〈101〉 crystallographic orientation while the lower energy generated small grains with a random crystallographic orientation. This indicates how both, crystallographic orientation and grain size, can be tailored by altering the energy density. Even if a large variance in microstructure was observed, the tensile test results only showed about 5% difference in yield strength between standard process parameters and the samples produced with the lowest energy density but with a 20% faster build time. Due to the decreased energy density, the amount of defects was increased which had a negative effect on the elongation. The samples produced with the lowest energy density had 32% elongation compared to 45% for the samples produced with standard process parameters. This study clearly indicates that the build time can be improved while maintaining good mechanical properties by adjusting the process parameters.
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| 3. |
- Rashidi, Masoud, 1987, et al.
(författare)
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Mechanistic insights into the transformation processes in Z-phase strengthened 12% Cr steels
- 2018
-
Ingår i: Materials and Design. - : Elsevier BV. - 1873-4197 .- 0264-1275. ; 158, s. 237-247
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Tidskriftsartikel (refereegranskat)abstract
- Compositionally complex Z-phase strengthened 12% Chromium steels are considered as potentially viable materials for components used in highly-demanding environments in steam power plants, operating at a target temperature of 650 °C. To date, however, the transformation processes of various phases into the desired precipitate, i.e., Z-phase CrTaN, are not fully understood. In this research, we first designed and produced three different alloys and then studied the microstructure in the as-tempered and aged conditions (for up to 10,000 h at 650 °C) using advanced electron microscopy, X-ray diffraction, and atom probe tomography. We report on the evolution of the densely distributed MX (Ta(C, N)) and M2X ((Cr, Ta)2N) precipitates into blade-like and bulky Z-phase, respectively. The blade-like precipitates benefit from a smaller size compared to the bulky ones, providing precipitation hardening for creep resistance. We discuss an interactive role of carbon and nitrogen content in the formation of the Z-phase. Our findings pave the way towards designing new alloys with improved properties to serve in harsh environments at 650 °C.
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| 4. |
- Esmaily, Mohsen, 1987, et al.
(författare)
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Microstructural characterization and formation of α' martensite phase in Ti-6Al-4V alloy butt joints produced by friction stir and gas tungsten arc welding processes
- 2013
-
Ingår i: Materials and Design. - : Elsevier BV. - 0261-3069 .- 1873-4197 .- 0264-1275. ; 47, s. 143-150
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Tidskriftsartikel (refereegranskat)abstract
- The obtained microstructures of a Ti-6Al-4V alloy welded by Gas Tungsten Arc Welding (GTAW) and Friction Stir Welding (FSW) were investigated and evaluated quantitatively. In the GTAW method, the effect of current was examined so that the samples were subjected to various currents between 90 and 120. A. In the FSW process, samples were welded by different rotational speeds (450-850. rpm). Non-destructive tests including Visual and Radiography Tests (VT and RT) were used to identify defect-free samples. The microstructural studies by electron microscopes revealed formation of different phases in the weld area of the samples welded via mentioned methods. The recorded peak temperatures in the weld regions compared favorably with the expectations about the evolved microstructures. A bi-modal microstructure was just obtained in the FSWed sample with a peak temperature below β transus temperature (T<995°C). α' martensite phase, which is an acicular and strengthening phase in this alloy, was only observed in FSWed specimens.
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| 5. |
- Fredi, Giulia, et al.
(författare)
-
Graphitic microstructure and performance of carbon fibre Li-ion structural battery electrodes
- 2018
-
Ingår i: Multifunctional Materials. - : IOP Publishing. - 2399-7532. ; 1:1
-
Tidskriftsartikel (refereegranskat)abstract
- Carbon fibres (CFs), originally made for use in structural composites, have also been demonstrated as high capacity Li-ion battery negative electrodes. Consequently, CFs can be used as structural electrodes; simultaneously carrying mechanical load and storing electrical energy in multifunctional structural batteries. To date, all CF microstructural designs have been generated to realise a targeted mechanical property, e.g. high strength or stiffness, based on a profound understanding of the relationship between the graphitic microstructure and the mechanical performance. Here we further advance this understanding by linking CF microstructure to the lithium insertion mechanism and the resulting electrochemical capacity. Different PAN-based CFs ranging from intermediate-to highmodulus types with distinct differences in microstructure are characterised in detail by SEM and HRTEMand electrochemical methods. Furthermore, the mechanism of Li-ion intercalation during charge/discharge is studied by in situ confocal Raman spectroscopy on individual CFs. RamanGband analysis reveals a Li-ion intercalation mechanism in the high-modulus fibre reminiscent of that in crystalline graphite. Also, the combination of a relatively low capacity of the high-modulus CFs (ca. 150 mAh g-1) is shown to be due to that the formation of a staged structure is frustrated by an obstructive turbostratic disorder. In contrast, intermediate-modulus CFs, which have significantly higher capacities (ca. 300 mAh g-1), have Raman spectra indicating a Li-ion insertion mechanism closer to that of partly disordered carbons. Based on these findings, CFs with improved multifunctional performance can be realised by tailoring the graphitic order and crystallite sizes.
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| 6. |
- Gruber, Hans, 1983, et al.
(författare)
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The effect of boron and zirconium on the microcracking susceptibility of IN-738LC derivatives in laser powder bed fusion
- 2022
-
Ingår i: Applied Surface Science. - : Elsevier BV. - 0169-4332. ; 573
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Tidskriftsartikel (refereegranskat)abstract
- The effect of boron (<0.01 to 0.03 wt%) and zirconium (<0.01 to 0.07 wt%) on the microcracking susceptibility of the γ’-strengthened Ni-base superalloy IN-738LC during laser powder bed fusion (LPBF) was studied using custom designed powder grades. It was found that both elements have a strong effect on the microcracking susceptibility, the microcracks are located at high angle grain boundaries based on EBSD measurements and crack density increases with the content of both elements. High crack density in the material with high boron and zirconium content corresponds to a large fraction of intergranular decohesion facets exhibiting a dendritic morphology on the fracture surface, typical for solidification cracking. Investigation of the fracture surface chemistry by X-ray photoelectron spectroscopy (XPS) indicates that considerable amounts of B and Zr are present in oxide state. Auger electron spectroscopy (AES) confirms that both elements are segregated to the intergranular decohesion facets on the fracture surface. Thin layers of B- and Zr-containing oxide on the microcrack surfaces were indicated by atom probe tomography (APT) as well. Hence, it is suggested that the cracking susceptibility of the studied alloying system is caused by formation of B- and Zr-containing oxide at high-angle grain boundaries during solidification.
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| 7. |
- Liu, Fang, 1975, et al.
(författare)
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A new 12% chromium steel strengthened by Z-phase precipitates
- 2016
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Ingår i: Scripta Materialia. - : Elsevier BV. - 1359-6462. ; 113, s. 93-96
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Tidskriftsartikel (refereegranskat)abstract
- In order to increase the corrosion resistance and simultaneously maintain the creep resistance of 9–12% Cr steels at 650 °C, a new alloy design concept was proposed, using thermodynamically stable Z-phase (CrTaN) precipi- tates to strengthen the steel. A new trial Z-phase strengthened 12% Cr steel was produced and creep tested. The steel exhibited good long-term creep resistance. Dense nano-sized Z-phase precipitates were formed at an early stage, and coarsened slowly. They remained small after more than 10,000 h.
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| 8. |
- Liu, Fang, 1975, et al.
(författare)
-
Initial study of the microstructure of carbon fibres acting as negative electrodes in structural battery composites
- 2016
-
Ingår i: Procedings of 17th European Conference on Composite Materials, ECCM17.
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Konferensbidrag (refereegranskat)abstract
- Structural composite batteries are a novel type of multifunctional devices, which have a great potential to remarkably reduce the mass of electric vehicles, and thus increase their energy efficiency. In these batteries, carbon fibres (Carbon fibres) play dual roles: reinforcements (as in CARBON FIBRE composites) and negative electrodes (as in batteries). However, the relationship between the microstructure and the electrochemical property of the Carbon fibres is not well understood. In this study, the microstructure of two Carbon fibres, M60J and IMS65, were studied by using scanning electron microscopy and transmission electron microscopy. Detailed microstructural features were revealed, and correlated to the electrochemical properties of the Carbon fibres. The more disordered microstructure, and rather large pores are the reasons for the better electrochemical properties of IMS65 compared to M60J.
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| 9. |
- Liu, Fang, 1975, et al.
(författare)
-
Microstructure of Z-Phase Strengthened Martensitic Steels: Meeting the 650°C Challenge
- 2017
-
Ingår i: Materials Science Forum. - 1662-9752 .- 0255-5476. ; 879, s. 1147-1152
-
Konferensbidrag (refereegranskat)abstract
- We studied three series of Z-phase strengthened steels using scanning electron microscopy, transmission electron microscopy, and atom probe tomography to reveal the detailed microstructure of these steels. In particular, the phase transformation from M(C,N) to Z-phase (CrMN) was studied. Carbon content in the steels is the governing factor in this transformation. The impact toughness of some test alloys was rather low. This is attributed to the formation of a continuous W-rich film along prior austenite grain boundaries. Cu and C addition to the test alloys changed Laves phase morphology to discrete precipitates and improved toughness dramatically. BN particles were found in some steels. Formation of BN is directly linked to the B concentration in the steels.
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| 10. |
- Marchese, Giulio, et al.
(författare)
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The role of texturing and microstructure evolution on the tensile behavior of heat-treated Inconel 625 produced via laser powder bed fusion
- 2020
-
Ingår i: Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing. - : Elsevier BV. - 0921-5093. ; 769
-
Tidskriftsartikel (refereegranskat)abstract
- Inconel 625 (IN625) alloy has high-temperature strength coupled with high oxidation and corrosion resistance. Additionally, due to its excellent weldability, IN625 can be processed by laser powder bed fusion (LPBF) additive manufacturing (AM) process allowing the production of complex shapes. However, post-AM heat treatment is necessary to develop the desired microstructure and mechanical properties to meet industrial needs. This work is focused on the influence of different heat treatment processes, namely stress relieving, recrystallization annealing and solution annealing on the microstructure and tensile properties of LPBF IN625 alloy. Investigation of the crystallographic texture by electron backscattered diffraction indicated that heat treatments at 1080 °C and 1150 °C tend to eliminate anisotropy in the material by the recrystallization and grain growth resulting in the formation of equiaxed grains. Tensile properties of heat-treated LPBF IN625 alloy built along different orientations revealed higher tensile properties than the minimum recommended values of wrought IN625 alloy in the annealed and solution annealed states.
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| 11. |
- Parizia, Simone, et al.
(författare)
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Effect of heat treatment on microstructure and oxidation properties of Inconel 625 processed by LPBF
- 2020
-
Ingår i: Journal of Alloys and Compounds. - : Elsevier BV. - 0925-8388. ; 846
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents a study of the microstructure evolution due to oxidation exposure of Inconel 625 (IN625) alloy produced by Laser Powder Bed Fusion (LPBF). IN625 is a nickel-based superalloy characterized by good mechanical properties, excellent oxidation, and corrosion resistance from cryogenic temperatures up to 980 °C, allowing its wide use in various harsh environments. In order to enable the application of LPBF IN625 components at high temperatures, the oxidation properties and microstructure of as-built and post-heat treated LPBF IN625 alloy must be carefully investigated. For this reason, an extensive characterization of the oxidation behavior of the alloy in the as-built condition and after solution treatment was performed. For both these conditions, the oxidation treatments were performed at 900 °C up to 96 h. The characterization was performed using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and scratch test analysis. The characteristics of the oxide layer and formed phases were investigated. The as-built IN625 state presented greater oxidation resistance compared to the solutionized IN625 one. The latter condition showed a defected oxide layer with the presence of Nb and Ni oxides inside the Cr oxide layer.
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| 12. |
- Parizia, Simone, et al.
(författare)
-
High temperature oxidation behavior of DMLs produced inconel 625
- 2020
-
Ingår i: Euro PM 2018 Congress and Exhibition.
-
Konferensbidrag (refereegranskat)abstract
- This work presents a study of the high temperature oxidation behavior of Inconel 625 (IN625) processed by direct metal laser sintering (DMLS). IN625 is a Nickel-based superalloy characterized by outstanding high temperature oxidation and corrosion resistance. However, taking into account that DMLS produced IN625 has significantly different microstructure and surface properties than the conventional produced IN625, it is important to evaluate the potential change in the oxidation behavior of this alloy, not reported in literature yet. In this study a complete characterization of the oxidation behavior of DMLS fabricated IN625, in as-built and solutionized (1150°C/2h) conditions, was performed at 900°C for 8, 24, 48, 72 and 96 hours in air. Oxidized samples were characterized utilizing advanced microscopy and surface analysis techniques (SEM+EDX, XPS). Results indicate that oxide formed is mainly composed of Cr2O3. Oxidation stabilizes after around 24 hours, indicating good corrosion resistance of the DMLS produced IN625.
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| 13. |
- Rashidi, Masoud, 1987, et al.
(författare)
-
Atom probe study of MX to Z-phase transformation in Z-phase strengthened 12%Cr steels
- 2017
-
Ingår i: Microscopy and Microanalysis. - 1435-8115 .- 1431-9276. ; 23, s. 360-365
-
Tidskriftsartikel (refereegranskat)abstract
- In creep resistant Z-phase strengthened 12% Cr steels, MX to Z-phase transformation plays an important role in achieving a fine distribution of Z-phase precipitates for creep strengthening. Atom probe tomography was employed to investigate the phase transformation in a Nb-based Z-phase strengthened trial steel. Using iso-concentration surfaces with different concentration values, and subtracting the matrix contribution enabled us to reveal the core-shell structure of the transient precipitates between MX and Z-phase. It was shown that Z-phase forms by diffusion of Cr into NbN upon ageing, and Z-phase has a composition corresponding to Cr1+xNb1-xN with x=0.08.
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| 14. |
- Rashidi, Masoud, 1987, et al.
(författare)
-
Core-Shell Structure of Intermediate Precipitates in a Nb-Based Z-Phase Strengthened 12% Cr Steel
- 2017
-
Ingår i: Microscopy and Microanalysis. - 1435-8115 .- 1431-9276. ; 23:2, s. 360-365
-
Tidskriftsartikel (refereegranskat)abstract
- In creep resistant Z-phase strengthened 12% Cr steels, MX (M=Nb, Ta, or V, and X=C and/or N) to Z-phase (CrMN, M=Ta, Nb, or V) transformation plays an important role in achieving a fine distribution of Z-phase precipitates for creep strengthening. Atom probe tomography was employed to investigate the phase transformation in a Nb-based Z-phase strengthened trial steel. Using iso-concentration surfaces with different concentration values, and subtracting the matrix contribution enabled us to reveal the core-shell structure of the transient precipitates between MX and Z-phase. It was shown that Z-phase forms by diffusion of Cr into NbN upon ageing, and Z-phase has a composition corresponding to Cr1+xNb1-xN with x=0.08.
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| 15. |
- Rashidi, Masoud, 1987
(författare)
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Development of a new generation of 12% Cr steels: Z-phase strengthened steels
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Fossil-fuel fired steam power plants provide more than 60% of the electricity generated worldwide, and account for about one third of the global CO2 emissions. The thermal efficiency of the steam power plants is limited by the maximum allowed steam temperature and pressure, which in turn are determined by the long-term corrosion and creep resistance of economically viable materials. Today’s best martensitic steels contain 9% Cr and can be used at ~ 600°C/300 bar. All attempts so far to reach 650°C with 11-12% Cr steels have failed, and the reason is the formation of a complex nitride, Z-phase, after a few years of service. In this thesis it is shown that Z-phase can be used as strengthening rather than weakening phase to develop a new generation of martensitic steels. These contain 12% Cr for better corrosion resistance and densely distributed fine Z-phase precipitates for creep resistance. A high Cr content together with Ta and N additions were used to stimulate the formation of a fine distribution of Z-phase in two trial steels. Atom probe tomography, transmission electron microscopy, and scanning electron microscopy were employed for a detailed characterization of the microstructure. In the first trial steel, 12Cr7CoTa-uLC, the C content was limited to 0.005 wt.%, which resulted in a fast transformation from TaN to CrTaN Z-phase, and subsequently a fine distribution of Z-phase was achieved. In the second trial steel a higher C content of 0.06 wt.% was used, which resulted in a slower phase transformation from Ta(C,N) to CrTaN Z-phase. In the 12Cr7CoTa-uLC trial steel Laves phase formed continuously at the prior austenite grain boundaries, which gave poor impact toughness. In the second trial steel, 12Cr3CoTa-HC, the addition of Cu and higher C content enhanced the distribution of Laves phase, and equiaxed Laves phase particles of a few hundred nanometer in size formed resulting in improved toughness.
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| 16. |
- Rashidi, Masoud, 1987
(författare)
-
Development of a new generation of creep resistant 12% chromium steels: Microstructure of Z-phase strengthened steels
- 2017
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Fossil-fuel fired steam power plants provide more than 60% of the electricity generated worldwide, and account for about one third of the global CO2 emissions. Increasing the steam temperature and pressure leads to a higher thermal efficiency of the power plants and thus lower emissions. The efficiency is limited by the long-term corrosion and creep resistance of economically viable materials used in the critical components of such a power plant, 9–12% Cr steels. Increasing the Cr content from 9% to 11–12% in the best commercially available steels provides sufficient corrosion resistance for an increase from the current maximum steam temperature of 620°C to 650°C for future power plants. However, after a few years of service, formation of coarse Z-phase (Cr(Nb, V)N) precipitates at the expense of a fine distribution of VN precipitates degrades the precipitation hardening and accordingly creep resistance of the steels.In this work a new family of 12% chromium steels is studied, where Ta or Nb is used instead of V to strengthen the steel by forming a dense distribution of Z-phase rather than VN. Z-phase does not nucleate directly as Z-phase, instead it forms through a gradual transformation of existing MX and M2N precipitates. The former leads to the formation of Z-phase with a blade-like morphology and the latter promotes large bulky Z-phase precipitates. As a result of the MX to Z-phase transformation, creep strength comparable to commercially available 9% Cr steels can be achieved. Investigation on the Z-phase precipitates based on Ta or Nb showed that Ta-based Z-phase benefits from a denser distribution and a slower coarsening rate, and thus is recommended for alloy design.Carbon is found to play the most critical role in the precipitation processes of Z-phase strengthened steels. An ultra-low C content and an optimal balance between Ta and N in a model alloy lead to the formation of a fine distribution of TaN in the as-tempered condition, which are transformed to blade-like Z-phase after short-term ageing. Such a low C content leads to very little formation of M23C6 at grain boundaries, which allows for the formation of a continuous film of Laves-phase there and a low impact toughness. Although, the addition of C results in precipitation of Ta(C, N), and hence a slower phase transformation to Z-phase, a low but not ultra-low carbon content is preferred in the new Z-phase strengthened steels.
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| 17. |
- Rashidi, Masoud, 1987, et al.
(författare)
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Experimental and theoretical investigation of precipitate coarsening rate in Z-phase strengthened steels
- 2018
-
Ingår i: Materialia. - : Elsevier B.V.. - 2589-1529. ; 4, s. 247-254
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Tidskriftsartikel (refereegranskat)abstract
- Two Z-phase strengthened 12% Cr steels were investigated: they are similar in composition, however one steel contains Nb and the other contains Ta. Z-phase precipitates (CrMN, M = Nb or Ta) provide precipitation hardening for creep resistance at 650 °C in these steels. Experimental data based on the transmission electron microscopy investigation of the size evolution of Z-phase precipitates during isothermal ageing at 650 °C show that the Ta-based Z-phase benefits from a five times smaller coarsening constant compared to the Nb-based Z-phase. Theoretical calculations show that this is attributed to the smaller diffusivity of Ta compared to Nb in the steel matrix. Besides, comparing the size of the Ta-based Z-phase precipitates in the gauge and head portion of a crept specimen, it is shown that Z-phase coarsens faster under stress.
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| 18. |
- Rashidi, Masoud, 1987, et al.
(författare)
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Microstructure and mechanical properties of two Z-phase strengthened 12%Cr martensitic steels: the effects of Cu and C
- 2017
-
Ingår i: Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing. - : Elsevier BV. - 0921-5093. ; 694, s. 57-65
-
Tidskriftsartikel (refereegranskat)abstract
- Z-phase strengthened 12% Cr steels are designed to combine good corrosion and creep resistance for applications in fossil fuel power plants with steam temperatures up to 650 °C. Two trial Z-phase strengthened steels were investigated, Z-steel with ultra-low C content, and ZCuC-steel with relatively high C content and Cu addition. The Z-steel has better creep strength; however, the alloy has low impact toughness due to the formation of continuous Laves-phase films at grain boundaries. Atom probe tomography, transmission electron microscopy, and scanning electron microscopy were employed to study the effects of C and Cu on the microstructure of the two steels in the as-tempered condition, and after ageing for different times. The Z-steel shows a fast transformation from TaN to Z-phase. The relatively high C content in the ZCuC-steel resulted in the formation of two categories of MX: Ta(C, N) and TaN. The phase transformation from Ta(C, N) to Z-phase is slower compared to that from TaN to Z-phase. In addition, precipitation of M23C6 and Cu particles in the ZCuC-steel led to easier nucleation of Laves-phase, and hence a much improved toughness.
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| 19. |
- Rashidi, Masoud, 1987, et al.
(författare)
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Microstructure characterization of two Z-phase strengthened 12%chromium steels
- 2014
-
Ingår i: 10th Liège Conference on Materials for Advanced Power Engineering. September 14th - 17th, 2014..
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Konferensbidrag (refereegranskat)abstract
- The microstructure of two Z-phase strengthened steels was characterized in different heat treatment conditions,as-tempered and after 1000 hours ageing at 650°C. A high Cr content together with Ta and N additions wereused to stimulate the formation of a fine distribution of Z-phase as strengthening particles in both alloys. Atomprobe tomography, transmission electron microscopy, and scanning electron microscopy were employed for adetailed characterization of the microstructure. The A1 steel contained very little carbon, which resulted in a fasttransformation from MX to Z-phase. In the A2 steel that contained a higher carbon content and an addition ofCu, the MX to Z-phase transformation was slower. The Cu addition to the A2 steel improved Laves-phasedistribution and morphology.
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| 20. |
- Rashidi, Masoud, 1987, et al.
(författare)
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Tantalum and niobium based Z-phase in a Z-phase strengthened 12% Cr steel
- 2016
-
Ingår i: Advances in Materials Technology for Fossil Power Plants - Proceedings from the 8th International Conference on Advances in Materials Technology for Fossil Power Plants; Albufeira, Portugal; 11-14 October 2016. ; , s. 1058-1066
-
Konferensbidrag (refereegranskat)abstract
- Higher steam temperature in steam power plants increases their thermal efficiency. Thus there is a strong demand for new materials with better creep and corrosion resistance at higher temperatures, while retaining the thermal flexibility of martensitic steels. Z-phase strengthened 12% Cr steels have been developed to meet the 923 K (650 degrees C) challenge in these power plants. Ta, Nb, or V forms Z-phase together with Cr and N. A new trial steel was produced based on combining Ta and Nb to form Z-phase. It was shown that Z-phase was formed with a composition corresponding to Crl+x(Nb,Ta)(l+x)N. The Nb/Ta ratio in Z-phase precipitates was higher than that in MX precipitates. Z-phase precipitates based on Ta and Nb were coarser than precipitates in a similar trial steel based on Ta alone.
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| 21. |
- Riabov, Dmitri, 1990, et al.
(författare)
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Effect of atomization on surface oxide composition in 316L stainless steel powders for additive manufacturing
- 2020
-
Ingår i: Surface and Interface Analysis. - : Wiley. - 1096-9918 .- 0142-2421. ; 52:11, s. 694-706
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Tidskriftsartikel (refereegranskat)abstract
- The initial oxide state of powder is essential to the robust additive manufacturing of metal components using powder bed fusion processes. However, the variation of the powder surface oxide composition as a function of the atomizing medium is not clear. This work summarizes a detailed surface characterization of three 316L powders, produced using water atomization (WA), vacuum melting inert gas atomization (VIGA), and nitrogen atomization (GA). X‐ray photoelectron spectroscopy (XPS) and scanning electron microscopy analyses were combined to characterize the surface state of the powders. The results showed that the surface oxides consisted of a thin (~4 nm) iron oxide (Fe2O3) layer with particulate oxide phases rich in Cr, Mn, and Si, with a varying composition. XPS analysis combined with depth‐profiling showed that the VIGA powder had the lowest surface coverage of particulate compounds, followed by the GA powder, whereas the WA powder had the largest fraction of particulate surface oxides. The composition of the oxides was evaluated based on the XPS analysis of the oxide standards. Effects of Ar sputtering on the peak positions of the oxide standards were evaluated with the aim of providing an accurate analysis of the oxide characteristics at different etch depths.
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| 22. |
- Riabov, Dmitri, 1990, et al.
(författare)
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Effect of the powder feedstock on the oxide dispersion strengthening of 316L stainless steel produced by laser powder bed fusion
- 2020
-
Ingår i: Materials Characterization. - : Elsevier BV. - 1044-5803. ; 169
-
Tidskriftsartikel (refereegranskat)abstract
- In this study, the concept of enhancing the in-situ oxide precipitation in laser powder-bed fusion processed parts is investigated using powder produced by water and gas atomization. By using water-atomized 316L powder, compared to gas-atomized powder, more oxide precipitates were introduced into the microstructure with the intent to enhance the strength of the material, as an alternative path to oxide dispersion strengthened materials. The results showed that oxide precipitation was homogenous, with higher-number densities of oxides in the sample built using the water-atomized powder. The oxides were observed to be amorphous and enriched in Si and Cr. The average size of the oxides was ~56 nm. After an annealing heat-treatment at 900 °C, the oxides were observed to remain within the microstructure with only minor changes in size and composition. Mechanical testing at room temperature and at elevated temperature did not show any increase in strength relative to the sample built using gas-atomized powder. However, it was shown that the use of water atomized powder in the L-PBF process provides a viable method of introducing and tailoring the number of oxide particles within a built component relative to a conventional gas atomized powder.
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| 23. |
- Shaikh, Abdul Shaafi, 1989, et al.
(författare)
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On as-built microstructure and necessity of solution treatment in additively manufactured Inconel 939
- 2023
-
Ingår i: Powder Metallurgy. - : Informa UK Limited. - 0032-5899 .- 1743-2901. ; 66:1, s. 3-11
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Tidskriftsartikel (refereegranskat)abstract
- Increased adoption of additively manufactured superalloys has led to the consideration of revised heat treatment approaches for these materials. The rapid cooling during additive manufacturing processes has been seen to suppress gamma prime (γ′) precipitation, which has raised the possibilities for omitting the high-temperature solution treatment step that usually precedes ageing heat treatment for these alloys. In this work, the as-built microstructure of a high gamma prime fraction superalloy Inconel 939 is presented, where the absence of any γ′ precipitation is notable. However, transmission electron microscopy shows the presence of nano-sized Eta (η) phase. It is shown that the omission of solution treatment leads to the growth of the deleterious η phase upon ageing, which results in embrittlement in tensile loading. It is concluded that at least for this particular alloy the solution treatment plays a critical role in the establishment of the required microstructure and hence cannot be omitted from the heat treatment.
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