In-situ SEM observation of phase transformation and twinning mechanisms in an interstitial high-entropy alloy

The recently developed interstitial high-entropy alloys (iHEAs) exhibit an enhanced combination of strength and ductility. These properties are attributed to dislocation hardening, deformation-driven athermal phase transformation from the face-centered cubic (FCC) gamma matrix into the hexagonal close-packed (HCP) epsilon phase, stacking fault formation, mechanical twinning and precipitation hardening. For gaining a better understanding of these mechanisms as well as their interactions direct observation of the deformation process is required. For this purpose, an iHEA with nominal composition of Fe-30Mn-10Co-10Cr-0.5C (at. %) was produced and investigated via in-situ and interrupted in-situ tensile testing in a scanning electron microscope (SEM) combining electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD) techniques. The results reveal that the iHEA is deformed by formation and multiplication of stacking faults along {111} microbands. Sufficient overlap of stacking faults within microbands leads to intrinsic nucleation of HCP epsilon phase and incoherent annealing twin boundaries act as preferential extrinsic nucleation sites for HCP epsilon formation. With further straining HCP epsilon nuclei grow into the adjacent deformed FCC gamma matrix. gamma regions with smaller grain size have higher mechanical stability against phase transformation. Twinning in FCC gamma grains with a size of similar to 10 mu m can be activated at room temperature at a stress below similar to 736 MPa. With increasing deformation, new twin lamellae continuously nucleate. The twin lamellae grow in preferred directions driven by the motion of the mobile partial dislocations. Owing to the individual grain size dependence of the activation of the dislocation-mediated plasticity, of the athermal phase transformation and of mechanical twinning at the different deformation stages, desired strain hardening profiles can be tuned and adjusted over the entire deformation regime by adequate microstructure design, providing excellent combinations of strength and ductility. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.