Abstract:

The equiatomic CoCrFeMnNi high entropy alloy (HEA) and its derivatives have attracted attention from
scientific community due to their mechanical properties, also at cryogenic conditions. Extensive
mechanical twinning is mainly responsible for their outstanding mechanical response under cryogenic
temperatures, e.g. increase in ductility and tensile strength. However, at room temperature dislocation
slip is dominant noting that for highly deformed samples twinning has been observed. Stacking fault
energy, temperature, crystallographic orientation and strain rate are a few of the key determinant factors
for the deformation dominance. Given the importance of twinning as a deformation mechanism for FCC
HEAs, in-depth understanding and quantitative insights of the stresses required for its activation are
crucial for advanced HEAs design. In this work, we aim to develop protocols to assess the critical resolved
shear stress (CRSS) required for twinning to occur by applying in situ micromechanical testing. Therefore,
three micromechanical geometries (micropillar, microcantilever and microshear) were chosen to activate
deformation twinning in different stress conditions. Specific orientations to favor deformation twinning
were carefully selected through electron backscatter diffraction (EBSD) analyses. Samples were then
micromachined by focused ion beam (FIB) and tested in situ with a strain rate of 10-2s-1 and overall 5%
deformation. Post-mortem analysis included scanning electron microscopy (SEM) imaging, EBSD and
transmission electron microscopy (TEM) to verify if twin microstructure could be observed. The results
provided quantitative insights essential for further understanding of the twinning mechanisms.