This paper describes the main findings from an experimental investigation into local and overall
strength and fracture behavior of a microstructurally flexible, quadruplex, high entropy alloy
(HEA), Fe42Mn28Co10Cr15Si5 (in at%). The alloy consists of metastable face-centered cubic
austenite (g), stable hexagonal epsilon martensite (ε), stable body-centered cubic ferrite (a), and
stable tetragonal sigma (σ) phases. The overall behavior of the alloy in compression features a
great deal of plasticity and strain hardening before fracture. While the contents of diffusion
created a and σ phases remain constant during deformation, the fraction of ε increases at the
expanse of g due to the diffusionless strain induced γ→ε phase transformation. High-throughput
nanoindentation mapping is used to assess the mechanical hardness of individual phases
contributing to the plasticity and hardening of the alloy. Increasing the fraction of the dislocated
ε phase during deformation due to the transformation is found to act as a secondary source of
hardening because g and ε exhibit similar hardness at a given strain level. While these two phases
exhibit moderate hardening during plasticity, significant softening is observed in σ owing to the
phase fragmentation. While the phase transformation mechanism facilitates accommodation of
the plasticity, the primary source of strain hardening in the alloy is the refinement of the structure
during the transformation inducing a dynamic Hall-Petch-type barrier effect. Results pertaining
to the evolution of microstructure and local behavior of the alloy under compression are
presented and discussed clarifying the origins of strain hardening. While good under
compression, the alloy poorly behaves under tension. Fracture surfaces after tension feature
brittle micromechanisms of fracture. Such behavior is attributed to the presence of the brittle σ
phase.
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New Insights into the Microstructural Changes During the Processing of Dual-Phase Steels from Multiresolution Spherical Indentation Stress–Strain Protocols
In this study, recently established multiresolution spherical indentation stress–strain protocols have been employed to derive new insights into the microstructural changes that occur during the processing of dual-phase (DP) steels. This is accomplished by utilizing indenter tips of different radii such that the mechanical responses can be evaluated both at the macroscale (reflecting the bulk properties of the sample) and at the microscale (reflecting the properties of the constituent phases). More specifically, nine different thermo-mechanical processing conditions involving different combinations of intercritical annealing temperatures and bake hardening after different amounts of cold work were studied. In addition to demonstrating the tremendous benefits of the indentation protocols for evaluating the variations within each sample and between the samples at different material length scales in a high throughput manner, the measurements provided several new insights into the microstructural changes occurring in the alloys during their processing. In particular, the indentation measurements indicated that the strength of the martensite phase reduces by about 37% when quenched from 810 °C compared to being quenched from 750 °C, while the strength of the ferrite phase remains about the same. In addition, during the 10% thickness reduction and bake hardening steps, the strength of the martensite phase shows a small decrease due to tempering, while the strength of the ferrite increases by about 50% by static aging.
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- Award ID(s):
- 1761406
- NSF-PAR ID:
- 10147980
- Date Published:
- Journal Name:
- Metals
- Volume:
- 10
- Issue:
- 1
- ISSN:
- 2075-4701
- Page Range / eLocation ID:
- 18
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Austenitic stainless steels are used in power generation components subjected to elevated temperatures over long service lives. Replacing these components can involve lengthy lead times and deteriorate the robustness of the energy infrastructure. Wire arc directed energy deposition (WA-DED) has the potential to enable rapid manufacturing of replacement parts, but the long-term stability of microstructures and mechanical properties produced by WA-DED is not well understood. In this work, we explore the influence of aging at 650°C for 1000 h on the formation of embrittling phases, such as sigma (σ), in the commercially available austenitic stainless steel wire feedstocks 316L, 316LSi, 316H and 16-8-2. All WA-DED samples formed secondary phases at grain boundaries (likely σ, possibly other phases as well), but these phases caused negligible changes in tensile properties in 316L, 316LSi and 316H. Samples of 16-8-2 formed significant amounts of ferrite and/or martensite after aging, which increased tensile strength but reduced ductility when tested at room temperature. This work demonstrates the need to design feedstock compositions that are stable with respect to ferrite and/or martensite formation, in addition to phases typically associated with embrittlement, to ensure microstructure and mechanical property stability for high-temperature applications with long service lives.
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Abstract A gas metal-directed energy deposition process was used to fabricate builds using two commercial weld fillers used in power generation applications, 16-8-2 and 316H. Microstructure stability and mechanical properties were investigated through room-temperature and elevated temperature tensile testing and creep testing at 650°C, 750°C, and 825°C. 16-8-2 exhibited reduced austenite stability which resulted in athermal martensite formation after aging at 650°C for 1000 h and strain-induced martensite formation during room-temperature tensile testing. 316H exhibited relatively higher austenite stability due to increased alloying content, resulting in no athermal martensite or strain-induced martensite. Due to lower austenite stability, ferrite formed during creep at 650°C in 16-8-2, which resulted in reduced creep life and lower creep ductility compared to 316H. At 750°C and 825°C, when ferrite is no longer thermodynamically stable, 16-8-2 exhibited longer creep life and similar creep ductility as 316H. The formation of ferrite in 16-8-2 appears to have a greater impact on creep performance than the formation of embrittling topologically close-packed phases like the σ phase, as 316H exhibited superior creep performance while predicted to form 14 vol.% σ phase at 650°C.
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Quenching and partitioning (Q&P) processing of third-generation advanced high strength steels generates multiphase microstructures containing metastable retained austenite. Deformation-induced martensitic transformation of retained austenite improves strength and ductility by increasing instantaneous strain hardening rates. This paper explores the influence of martensitic transformation and strain hardening on tensile performance. Tensile tests were performed on steels with nominally similar compositions and microstructures (11.3 to 12.6 vol. pct retained austenite and 16.7 to 23.4 vol. pct ferrite) at 980 and 1180 MPa ultimate tensile strength levels. For each steel, tensile performance was generally consistent along different orientations in the sheet relative to the rolling direction, but a greater amount of austenite transformation occurred during uniform elongation along the rolling direction. Neither the amount of retained austenite prior to straining nor the total amount of retained austenite transformed during straining could be directly correlated to tensile performance. It is proposed that stability of retained austenite, rather than austenite volume fraction, greatly influences strain hardening rate, and thus controls strength and ductility. If true, this suggests that tailoring austenite stability is critical for optimizing the forming response and crash performance of quenched and partitioned grades.more » « less
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The objective of this study is to evaluate the radiation induced microstructural and mechanical differences influenced by alloying elements including phosphorus, chromium, and nitrogen and crystal orientation in iron-based binary alloys. Fe-4.5at%P, Fe-9.5at%Cr, and Fe-2.3at%N binary model alloys were irradiated with 4.4 MeV Fe++ ions at 370 °C to 8.5 displacements per atom (DPA). Transmission electron microscopy (TEM) characterization including brightfield scanning electron microscopy (BFSTEM), diffraction, and TEM in situ irradiation, energy dispersive spectroscopy (EDS) compositional analysis, and nanoindentation were used to evaluate the radiation induced microstructural evolution and mechanical responses in these model alloys. Microstructure is of particular interest in irradiated nuclear structural materials because it plays an integral role in the mechanical integrity of these materials. Radiation induced defects present obstacles to dislocation motion and thus lead to hardening and embrittlement. P is highly undersized and forms a strong covalent bond with Fe which progresses to an Fe3P beta phase in BCC iron when the solubility limit is reached. The covalent nature of the bonding as well as the smaller atomic volume of P leads to enhanced radiation induced defect nucleation, phosphorus segregation, and radiation induced precipitation. The high density of defects in the Fe-P alloy contributed to enhanced hardening of the irradiated Fe-P alloy in comparison to the Fe-Cr and Fe-N alloys. The density of these defects and depth of the ion irradiated damaged layer and thus the mechanical response is also heavily dependent on orientation and is made evident by nanoindentation and indentation cross section BFSTEM imaging.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3390/met10010018
