Composition-dependent slip planarity in mechanically-stable face centered cubic complex concentrated alloys and its mechanical effects

Abstract

In metallic materials, enhancing strain hardening capacity positively affects ductility, fracture toughness, ultimate tensile strength, and other properties. Thus, activating mechanically-induced martensitic transformation or twinning mechanisms has been a captivating goal in the design of steels, titanium alloys, cobalt alloys, complex concentrated alloys (CCAs) and others, through modifications to composition or thermo-mechanical processing. Here instead, we focus on the most basic strain hardening effect, arising from dislocation kinematics, and interactions. For this purpose, we designed two model face centered cubic (FCC) CCAs, Ni2CoCrFe and Ni2CoCrFeTi0.2Al0.1. Both alloys develop single, mechanically-stable, FCC phase microstructures upon processing. Mechanical tests of these alloys reveal that the Al and Ti addition enhances the strain hardening capacity, leading to significant increases in strength and ductility. Microstructure analyses based on electron channeling contrast imaging (ECCI), electron-backscatter diffraction (EBSD), and transmission electron microscopy (TEM) confirm the absence of mechanically-induced twinning and martensitic transformation effects, revealing instead a transition from wavy slip to planar slip. In-situ synchrotron XRD tensile tests are used to discuss the origin of the dislocation glide mode transition and the effects on strain hardening. Based on these analyses, the increased degree of short-range ordering (SRO), rather than the changes in stacking fault energy (SFE), is proposed as the main cause for this transition, and the corresponding effects on strain hardenability.