EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

With 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 presentation is aimed to provide quantitative insights into the nucleation of dislocations in the metastable HEAs and reveal the corresponding effects of interstitial alloying elements through nanoindentation experiments and statistical physical modeling. The results indicate that dislocation nucleation in a representative metastable non-equiatomic FeMnCoCr HEA is motivated by the thermally activated displacement of single principal atom, suggesting a dominant homogeneous mode of dislocation nucleation. As well, minor heterogeneous nucleation via monovacancy-atom exchange is possible based on a quantitative analysis for the potential effects of initial defects. The activation volume necessary for dislocation nucleation in the metastable HEA is increased upon doping 0.5 at. % Carbon and 1.0 at. % Nitrogen into the face-centered cubic structure. Statistical modeling and experimental nanoindentation results both suggest that interstitial Carbon and Nitrogen atoms are prone to facilitate the nucleation rate of Shockley partials under intense shears. But 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 inhibits the displacive phase transformation of the C-N co-doped metastable HEAs.