Probing plastic mechanisms in gradient dual-phase high-entropy alloys under nanoindentation

The plastic deformation mechanisms of gradient nanostructured high entropy alloys (HEAs) subjected to nanoindentation tests have been examined. First, we have constructed gradient glass/crystal (GGC) dual-phase HEAs by embedding crystalline CoCrFeNi HEA grains into CoCrFeNiAlx (x = 0.5, 1, and 2) metallic glass (MG) matrix in a gradient manner, leading to a gradient change in both the grain size and MG matrix shell thickness. Then, nanoindentation tests on the specimens under proper boundary and loading conditions are simulated by molecular dynamics software. It is found that the gradient-distributed MG shell effectively reduces the degree of strain localization within the region around the indenter, which results in a considerable hardening effect, thanks to the coexistence of both dislocation motion in the grain phase and plastic flow activated simultaneously in the MG phase. The results also show that the damage tolerance of GGC specimens is higher than that of the polycrystalline specimen because gradient-distributed MG phases can effectively prevent FCC to HCP transformation. Besides, the Al addition in the gradient-distributed MG shell can reduce its deformation propagation capability, weakening the hardening effect. The proposed method of constructing GGC can be promisingly helpful, for example, in the surface engineering of high-entropy alloys.