The effects of interstitials on the mechanical properties of single-phase f.c.c. high entropy alloys (HEAs) have been assessed based on a review of the literature. It is found that in nearly all ...studies, carbon increases the yield strength, in some cases by more than in traditional alloys. This suggests that carbon can be an excellent way to strengthen HEAs. This strength increase is related to the lattice expansion from the carbon. The effects on other mechanical behavior is mixed. Most studies show a slight reduction in ductility due to carbon, but a few show increases in ductility accompanying the yield strength increase. Similarly, some studies show little or modest increases in work-hardening rate (WHR) due to carbon, whereas a few show a substantial increase. These latter effects are due to changes in deformation mode. For both undoped and carbon doped CoCrFeMnNi, the room temperature ductility decreases slightly with decreasing grain size until ~2–5 µm, below which the ductility appears to decrease rapidly. The room temperature WHR also appears to decrease with decreasing grain size in both undoped and carbon-doped CoCrFeMnNi and in nitrogen-doped medium entropy alloy NiCoCr, and, at least for the undoped HEA, shows a sharp decrease at grain sizes <2 µm. Interestingly, carbon has been shown to almost double the Hall–Petch strengthening in CoCrFeMnNi, suggesting the segregation of carbon to the grain boundaries. There have been few studies on the effects of other interstitials such as boron, nitrogen and hydrogen. It is clear that more research is needed on interstitials both to understand their effects on mechanical properties and to optimize their use.
In this short review, we highlight instances where interstitials have been shown to substantially increase the yield strength and work-hardening rate (WHR) of f.c.c. alloys, particularly high entropy ...alloys, medium entropy alloys, TWIP steels and stainless steels. However, the common practice of describing interstitial strengthening in f.c.c. alloys using models that are used to explain substitutional strengthening appears to be neither appropriate nor accurate. Here we suggest, based on the literature, that the yield strength increase due to interstitials in f.c.c. alloys is more appropriately described by a linear dependence on the concentration: due to a paucity of experimental studies, the dependence of the yield strength and WHR on misfit parameters is currently unclear. Thus, the source of the strengthening remains unclear. A feature that has been observed in several f.c.c. alloys is that interstitial additions lead to a change from wavy to planar slip although the origin of this change, which may be related to changes in stacking fault energy as well as other factors, remains unclear. The paper concludes by outlining areas of future research, including the need to develop a new model for interstitial strengthening in f.c.c. alloys.
Tensile tests performed on a number of f.c.c. alloys containing several different concentrations of either C or N have revealed a σy∝c relationship, see Figure. Interstitials also increase the work-hardening rate. Interstitials can also change dislocation motion from wavy to planar slip. Figure. Yield stress increase versus at.% C in Fe40.4Ni11.3Mn34.8Al7.5Cr6 alloys. After Wang et al. Acta Materialia 120 (2016): 228-39. Display omitted
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The effects of adding boron or carbon on the microstructure and room-temperature mechanical properties of a new single-phase f.c.c. high entropy alloy Fe40.4Ni11.3Mn34.8Al7.5Cr6 are presented. ...Remarkably, 1.1at% carbon in solution not only increases the yield strength (by a factor of 2), but also increases the elongation to failure (from ~41% to 50%) and the work-hardening rate. Surprisingly, the latter increases with increasing strain up to a strain of 35%. When fine-grained (4.7µm), the C-doped HEA exhibits a yield strength of 557MPa.
•At least 1.1 at. % carbon can be dissolved in a FeNiMnAlCr HEA.•The carbon solute produces substantial increase in both strength and ductility.•The C-doped HEAs in mechanical properties are better than most advanced steels.
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The effects of cold rolling followed by annealing on the mechanical properties and dislocation substructure evolution of undoped and 1.1 at. % carbon-doped Fe40.4Ni11.3Mn34.8Al7.5Cr6 high entropy ...alloys (HEAs) have been investigated. X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atom probe tomography (APT) were employed to characterize the microstructures. The as-cast HEAs were coarse-grained and single phase f.c.c., whereas the thermo-mechanical treatment caused recrystallization (to fine grain sizes) and precipitation (a B2 phase for the undoped HEA; and a B2 phase, and M23C6 and M7C3 carbides for the C-doped HEA). Carbon, which was found to have segregated to the grain boundaries using APT, retarded recrystallization. The reduction in grain size resulted in a sharp increase in strength, while the precipitation, which produced only a small increase in strength, probably accounted for the small decrease in ductility for both undoped and C-doped HEAs. For both undoped and C-doped HEAs, the smaller grain-sized material initially exhibited higher strain hardening than the coarse-grained material but showed a much lower strain hardening at large tensile strains. Wavy slip in the undoped HEAs and planar slip in C-doped HEAs were found at the early stages of deformation irrespective of grain size. At higher strains, dislocation cell structures formed in the 19 μm grain-sized undoped HEA, while microbands formed in the 23 μm grain-sized C-doped HEA. In contrast, localized dislocation clusters were found in both HEAs at the finest grain sizes (5 μm). The inhibition of grain subdivision by the grain boundaries and precipitates lead to the transformation from regular dislocation configurations consisting of dislocation-cells and microbands to irregular dislocation configurations consisting of localized dislocation clusters, which further account for the decrease in ductility. Investigation of the formation mechanism and strain hardening of dislocation cells and microbands could benefit future structural material design.
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Multi-principle component alloys (MPCAs) differ from traditional alloys in that they consist of four or more elements or components each with concentrations of 5–35 at. %. Since the first eutectic ...multi-principle component alloy (MPCA) was produced in 2008, there has been a growing number of papers on developing eutectic MPCAs as potential structural materials. Eutectic MPCAs can show high ambient temperature yield strengths that increase with decreasing interlamellar spacing, λ, according to either λ−1/2 or λ−1, similar to that observed in pearlitic steels, with a tradeoff between this increased strength and reduced tensile ductility. Ambient temperature tensile ductility has been observed in eutectic MPCAs only when one phase is f.c.c. and when the harder second phase is itself deformable. The yield strength in eutectic MPCAs has been shown to decrease with increasing temperature, and, limited data suggest that, this is related to the softening of the harder phase. Annealing of as-cast eutectic MPCAs, which are not typically at equilibrium, can produce precipitation of fine particles that further increase the strength, and which often reduce the ductility. Both thermo-mechanical processing and nitriding can increase the strengths of eutectic MPCAs by transforming the lamellar eutectic into equi-axed grains and producing fine AlN particles (in aluminum-containing MPCAs), respectively. The properties of eutectic MPCAs can largely be explained by models used for traditional alloys. While a number of different elements have been used to produce eutectic MPCAs, the design of eutectic MPCAs for structural applications should avoid the use of expensive elements like cobalt and niobium, which have often been used.
Room temperature yield strengths have been shown to increase with decreasing f.c.c. interlamellar spacing, λ, in lamellar eutectic FeNiMnAl alloys (shown right) according to either λ 1 or λ 1/2, with a resulting trade-off between increased strength and reduced tensile ductility. Display omitted
•The microstructures and mechanical properties of eutectic/eutectoid multi-principle component alloys (MPCAs) are critically reviewed.•The yield strength and interlamellar spacing, λ, of eutectic/eutectoid MPCAs obey a Hall-Petch-type relationship with either a λ−1 or λ-1/2 relationship.•Thermo-mechanical treatments transform the lamellar eutectic into equi-axed two-phase grain structures, leading to either increases or decreases in strength.•Traditional models which relate strength and ductility to the lamellae spacing of the hard and soft phases in mild steels can explain the trade-off between increased yield strength and reduced tensile ductility in eutectic MCPAs.
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A systematic study of the effects of up to 1.1 at. % carbon on the mechanical properties and evolution of the dislocation substructure in a series of a high entropy alloys (HEA) based on ...Fe40.4Ni11.3Mn34.8Al7.5Cr6 is presented. Transmission electron microscopy (TEM), synchrotron X-ray diffraction (XRD) and atom probe tomography (APT) were used to show that all the alloys are single-phase f.c.c. random solid solutions. The lattice constant, determined from synchrotron XRD measurements, increases linearly with increasing carbon concentration, which leads to a linear relationship between the yield strength and the carbon concentration. The dislocation substructures, as determined by a TEM, show a transition from wavy slip to planar slip and, at higher strains, and from cell-forming structure (dislocations cells, cell blocks and dense dislocation walls) to non-cell forming structure (Taylor lattice, microbands and domain boundaries) with the addition of carbon, features related to the increase in lattice friction stress. The stacking fault energy (measured via weak-beam imaging of the separation of dislocation partials) decreases with increasing carbon content, which also contributes to the transition from wavy slip to planar slip. The formation of non-cell forming structure induced by carbon leads to a high degree of strain hardening and a substantial increase in the ultimate tensile strength. The consequent postponement of necking due to the high strain hardening, along with the plasticity accommodation arising from the formation of microbands and domain boundaries, result in an increase of ductility due to the carbon addition.
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The microstructures and mechanical properties of both undoped and carbon-doped (1.26at%) f.c.c./B2 Fe36Ni18Mn33Al13 multi-component alloys have been investigated in the as-cast, annealed and ...recrystallized states. A lamellar structure is present in the undoped alloy, while a tetragonal martensite with irregular shape is present in the carbon-doped alloy. B2-structured precipitates form upon annealing in both alloys, whose size and volume fraction increases with increasing annealing time. Lamellar coarsening also occurs during annealing of the undoped alloy. Two different thermo-mechanical treatments were applied to the carbon-doped alloy in order to decrease the grain size and disperse the martensite, which produced a significant increase in strength. The changes in yield strength are discussed in terms of the underlying strengthening mechanisms, i.e., phase boundary strengthening, grain boundary strengthening, interstitial strengthening, and precipitation strengthening. The carbon addition results in a sharp increase in ductility of the as-cast alloy, a feature ascribed to microband formation during the tensile test arising from the increase of lattice friction stress. Similar to some single-phase f.c.c. alloys, microband-induced plasticity (MBIP) effect is found to present in a two-phase multicomponent alloy in this study.
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Constant-load creep tests were performed at −10°C at various compressive stresses from 0.05 to 0.75 MPa on specimens taken every 10 m along a firn core extracted at Summit, Greenland in June 2017. ...The microstructures before and after creep testing were examined using both X-ray microtomography (micro-CT) and optical images from thin sections. An Andrade-like equation was used to describe the primary creep behavior and yielded the time exponent k of 0.17–0.76. The onset of secondary creep occurred at strains of ~0.5–3% but was sometimes not observed at all in shallow firn specimens and at stresses ⩽0.43 MPa even for strain up to 32%. For the 50–80 m firn crept at stresses ⩾0.55 MPa, secondary creep occurred at strains of 2.6 ± 0.28%, and the stress exponent, n, in Glen's law, was found to range from 4.1 to 4.6, similar to those observed for fully dense ice. Micro-CT observations of crept specimens showed that in most cases, the specific surface area, the total porosity and the structure model index decreased, while the structure thickness increased with increasing density. These microstructural characteristics are consistent with the densification of the firn. Optical images from thin sections showed that recrystallization occurred in some specimens that had undergone secondary creep.
The effect of aging on the microstructure and mechanical behavior of an alumina-forming austenitic stainless steel, Fe–20Cr–30Ni–2Nb–5Al (at%) has been investigated. The alloy was fully solutionized ...after a 1250°C, 24h heat treatment, and the precipitation of B2 and Laves phases was studied after aging at 800°C for up to 1325h. While after 24h the Laves phase precipitates in the matrix were 205nm in diameter, they showed a further increase in diameter of only 50nm even after aging for 1325h. In contrast, the B2 precipitates in the matrix grew at a faster rate: after first being observed after aging for 24h at an average diameter of 194nm, they more than doubled in size from 330 to 734nm as the aging time increased from 240h to 1325h. Both the Laves and B2 precipitates in the grain boundaries grew at a faster rate and were larger than matrix precipitates. The grain boundary coverage at 2.4h (Laves 192nm, NiAl 126nm) was 56% with Laves phase initially making up the bulk of the precipitates, but after 2.4h Laves phase and B2 precipitates alternated on the grain boundaries and total coverage reached 93% after 1325h. An increase in the volume fraction of precipitates in the alloy was accompanied by an increase in the yield strength from 205MPa after the solutionizing treatment up to 383MPa after aging at 800°C for 1325h. After aging for 1325h, even with extensive intermetallic grain boundary coverage, the alloy showed a room temperature elongation of 19%.
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10.
The Onset of Recrystallization in Polar Firn Ogunmolasuyi, Ayobami; Murdza, Andrii; Baker, Ian
Geophysical research letters,
16 December 2023, Volume:
50, Issue:
23
Journal Article
Peer reviewed
Open access
Constraining the onset of dynamic recrystallization (DRX) and its effects on the mechanical properties of firn is crucial for firn densification modeling. To that end, samples from a depth of 13 m in ...a Summit, Greenland (72°35′N, 38°25′W) firn core were subjected to creep tests at −14°C and 0.21 MPa compressive stress to strains of 7%, 12%, 18%, and 29%. Microstructural analyses using thin‐section imaging and microcomputed x‐ray tomography (micro‐CT) revealed smaller grain sizes, reduced specific surface area and connectivity, and increased density in relation to reduced porosity as the strain increases. These results show that DRX occurs in firn under creep, with strain‐induced boundary migration (SIBM) and nucleation and growth starting at ∼7%. DRX leads to elongated grains, reduced grain size, and the development of a preferred crystallographic orientation, indicating that DRX occurs by both SIBM and nucleation and growth.
Plain Language Summary
Firn is multi‐year snow that undergoes densification due to the load from the snow overburden and from sintering. Understanding firn densification is important for interpreting ice core records, predicting ice sheet mass balance and elevation changes, and studying climate change effects. Previous densification models focused on accumulation rate and temperature, overlooking the role of recrystallization. To address this gap, compression tests were performed on Greenland firn samples from a depth of 13 m. The deformation resulted in reduced median grain size, preferred crystallographic orientation, and increased density. Our findings indicate that dynamic recrystallization starts when the firn is subjected to a strain of about 7% through boundary migration of old grains, around which new stress‐free grains also start to form.
Key Points
Dynamic Recrystallization occurs in firn through strain‐induced boundary migration, and nucleation and growth
Average grain size in firn decreases under constant temperature and compressive stress in firn
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