Deformation faulting in a metastable CoCrNiW complex concentrated alloy: A case of negative intrinsic stacking fault energy?

Microscopic crystalline defects are of fundamental importance in unraveling the plastic deformation response and thereby tailoring the macroscopic load-bearing performances of metallic alloys. Especially in the microstructural metastability engineering context of complex concentrated alloys (CCAs), while profuse interest has been focused on phase and twin boundaries as well as their interactions with glissile dislocations, stacking faults, as another essential planar defects, have remained comparatively less explored and unutilized. In the present work, by investigating a metastable CoCrNiW CCA via the combination of in-situ synchrotron X-ray diffraction and in -situ electron channeling contrast imaging (ECCI), we show that stacking faults formation can also operate as the predominant deformation micro-mechanism, accommodating plastic strain while enabling macroscopic strain hardening. Through the examination of relative phase stability by thermodynamic modeling, we reveal that this sort of deformation faulting response is largely correlated with a negative intrinsic stacking fault energy. The corresponding physical revelation is explored in greater details regarding thermodynamics, structure, and mechanics, followed by in-situ experimental verification of the stacking fault activities. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.