Interstitial effects on the incipient plasticity and dislocation behavior of a metastable high-entropy alloy: Nanoindentation experiments and statistical modeling

Owing to the partial dislocation assisted twinning and/or displacive transformation upon stress loading, metastable high-entropy alloys (HEAs) and their interstitial variants have shown excellent strength-plasticity synergy. However, the fundamental mechanisms of the dislocation nucleation and the onset of plasticity in these emerging materials remain unclear. The present study is to provide quantitative insights into the dislocation nucleation in the metastable HEAs and reveal the corresponding effects of interstitial alloying elements through combined instrumented nanoindentation experiments and statistical physical modeling. The results indicate that dislocation nucleation in a representative metastable non-equiatomic FeMnCoCr HEA is triggered by the thermally activated displacement of single principal atom, suggesting a dominant homogeneous mode of dislocation nucleation according to a continuum mechanical description for neonatal dislocation loop energy. Also, minor heterogeneous nucleation via monovacancy-atom exchange is possible according to a quantitative analysis for the potential effects of pre-existing defects. The activation volume necessary for dislocation nucleation in the metastable HEA is increased upon dissolving 0.5 at.% C plus 1.0 at.% N into the face-centered cubic structure. Statistical modeling and experimental nanoindentation results suggest that interstitial C and N atoms are prone to facilitate the nucleation rate of Shockley partials under intense shears. Yet, the significant drag effect by interstitial atoms can trap those neonatal mobile partials and reduce their mean free path before exhaustion. Thus, the width of stacking faults (SFs) formed by slip of such partials is severely constrained, which hinders the generation of SFs on multiple atomic layers and further inhibits the displacive phase transformation during deformation of the C-N co-doped metastable HEAs.