![]() Therefore, we introduce two intrinsic quantities: unstable martensite fault energy (UMFE) and unstable twin fault energy (UTFE). Previous experimental studies found that the formation of deformation twins is associated with the partial dislocation glide on every plane ( 29– 31). A more in-depth understanding of the deformation-induced martensite/twin formation process and their energy barrier is thus critical. ISFE is the energy change after the stacking fault formation, but it does not necessarily correlate with the energy barrier for the martensite or twin formation process. Moreover, recent studies have shown that the experiments tend to overestimate the ISFE in concentrated alloys ( 24), while the density functional theory (DFT) calculations can give negative ISFE values for TRIP HEAs ( 27, 28), making the ISFE criterion impractical for rigorous alloy design and discovery. For example, transmission electron microscopy (TEM) experiments found that Co 10Cr 10Fe 40Mn 40 has a low ISFE of 13 ± 4 mJ/m 2, but this HEA is a TWIP alloy ( 26). However, this rule does not generalize to other alloys such as HEAs. In austenitic steels, TRIP prevails over TWIP when ISFE is lower than 20 mJ/m 2, and TWIP is found in alloys when the ISFE lies between 20 and 40 mJ/m 2 ( 25). The ISFE is the excess energy associated with the formation of the ISF by the dissociation of a lattice dislocation a 0/2 into two a 0/6 partial dislocations ( a 0 is the lattice constant) ( 24). The competing deformation mechanisms in the face-centered cubic (fcc) phase, such as dislocation glide, twinning, and martensitic transformation, are usually determined by the intrinsic stacking fault energy (ISFE) ( 14, 21). ![]() ![]() Nonetheless, the effective design of HEAs with desired microstructure and deformation mechanisms is still a formidable challenge. In terms of the mechanical properties, the combination of “HEA effects,” such as the severe lattice distortion and solid solution strengthening, and the metastable engineering leads to HEAs with high strength and excellent ductility ( 14). The concept of HEAs offers a vast composition space and brings a previously undiscovered path for developing advanced materials with promising properties, such as excellent corrosion resistance ( 12), high strength ( 11), and biocompatibility ( 13). Recently, metastability engineering has also been used in developing high-entropy alloys (HEAs)( 3, 4) that contain multiple principal elements ( 7– 11). During deformation, martensite/twin formation provides alternative pathways for partial dislocations to glide, and the newly formed phase/twin boundary reduces the dislocation mean free path, leading to the dynamic Hall-Petch effect ( 6). Metastability engineering ( 3, 4) has been demonstrated to be an effective strategy to overcome the strength-ductility trade-off in ferrous alloys ( 5) by introducing interface hardening from a dual-phase structure and transformation- or twinning-induced plasticity (TRIP/TWIP). However, most strengthening mechanisms, such as precipitation and solid solution hardening, are detrimental to ductility ( 1, 2). The UMFE/UTFE criterion provides an effective paradigm for developing metastable alloys with TRIP/TWIP for an enhanced strength-ductility synergy.ĭeveloping alloys with a combination of high strength and ductility is the paramount goal in structural materials engineering. Among the studied HEAs and steels, the traditional ISFE criterion fails in more than half of the cases, while the UMFE/UTFE criterion accurately predicts the deformation mechanisms in all cases. We propose unstable fault energies as the more effective design metric and attribute the deformation mechanism of metastable face-centered cubic alloys to unstable martensite fault energy (UMFE)/unstable twin fault energy (UTFE) rather than ISFE. Here, we demonstrate a strategy for designing metastable HEAs and validate its effectiveness by discovering seven alloys with experimentally observed metastability for TRIP/TWIP. Originated from the development of traditional alloys, the intrinsic stacking fault energy (ISFE) has been applied to tailor TRIP/TWIP in high-entropy alloys (HEAs) but with limited quantitative success. Metastable alloys with transformation-/twinning-induced plasticity (TRIP/TWIP) can overcome the strength-ductility trade-off in structural materials.
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