Mechanistic insights and activation stress analysis of deformation twinning in the Cantor multi-principal element alloy

• Fundamental twinning mechanisms of FCC crystals were studied by micromechanical testing. • Single-slip based twinning mechanism are predominant in sub-micron pillars. • The size scaling for dislocation slip and twinning is identical. • A secondary twin type is observed in multi-slip activation cases. • The lower bound for the critical twinning shear stress for the Cantor alloy is around 130 MPa. Deformation twinning is an important deformation mechanism for low stacking fault energy face-centered cubic (FCC) alloys including multi-principal element alloys, however, its underlying mechanism remains incompletely understood. In this work, we applied in situ scanning electron microscope (SEM) micro-pillar compression combined with microstructural investigations to gain insights into the fundamental mechanism of deformation twinning and its stress and/or strain dependence. Our findings reveal that the morphology of the deformation twins and the controlling mechanism vary with micro-pillar size. In sub-micron pillars, single-slip based twinning models like the three-layer model were predominant as confirmed by in situ deformation and post-mortem microstructural analyses. For pillar diameters above 3 µm, two different twin variants were observed including one formed by the three-layer mechanism, although the secondary twinning mechanism remains unclear. When the pillar diameter increased to 10 µm, the applied stresses was insufficient to activate deformation twinning, and dislocation slip became the dominant deformation mode. A quantitative stress analysis of pillars ranging from 0.14 µm to 10 µm in diameter showed a lower bound for twinning stress of approximately 130 MPa. Finally, size dependence investigations revealed no significant difference between twinning stress and full dislocation slip critical resolved shear stress. This not only proves that dislocation slip is a prerequisite for twinning, but also indicates that, above a threshold stress, twinning could be more strain rather than stress-dependent.