Negative stacking fault energy in FCC materials-Its implications

Recent atomistic simulations on medium entropy alloys uncovered the possibility of negative intrinsic stacking fault energies (SFEs), which suggest infinite stacking fault widths (SFWs). However, experimental measurements of SFWs in the same alloys have shown that SFWs are finite, which contradicts the classical derivations based on force balance. To address this contradiction, we develop an advanced treatment employing atomistic lattice and continuum theories that produce finite SFW solutions corresponding to negative SFEs. The idea is based on energy minimization, where the finite SFW corresponds to the first local minimum in the energy landscape. By exploring combinations of intrinsic and unstable fault energies, we identify regimes in which solutions for finite SFWs exist for thousands of hypothetical materials. Elastic moduli and lattice constants also impact the results, with lower moduli and smaller lattice constants expanding the negative stacking fault energy domain corresponding to finite SFWs. Additionally, the study has revealed a distribution of SFEs due to possible chemical heterogeneities within the alloy, resulting in variations in SFWs within the same material. The work underscores the capabilities of the theory for SFW and CRSS (Critical Resolved Shear Stress) determination for medium to high entropy alloys in agreement with experiments.