Mechanistic origin of abnormal annealing-induced hardening in an AlCoCrFeNi2.1 eutectic multi-principal-element alloy

Annealing-induced hardening, also known as anneal hardening, has been often observed in cold-worked solid solution alloys. Here, we report a peculiar case of annealing-induced hardening in a dual-phase (FCC + B2) as-cast AlCoCrFeNi _2.1 eutectic multi-principal-element alloy (EMPEA) without any prior cold-working history. Investigation of the relative hardness contribution of the two phases by nanoindentation experiments revealed that, although both the FCC and B2 phases harden with increasing annealing temperatures, only the FCC phase showed a similar hardening behavior while the B2 phase hardened with increasing annealing temperature. The abnormal hardening in the dual-phase AlCoCrFeNi _2.1 is thus proposed to result from the hardening of the FCC phase, which was caused by the ordering strengthening from the L1 ₂ phase embedded in the FCC matrix. This ordering strengthening is further supported by the fact that the volume fraction of the L1 ₂ ordered particles exhibit a similar peak as that of the hardness value of the FCC phase. By contrast, the growth of BCC particles within the B2 matrix phase was observed, which led to an increased volume fraction of BCC particles. Transmission electron microscopy observations reveal that the BCC particles were observed as strong pinning sites against dislocation motion, which could explain the temperature-dependent hardening behavior of the B2 phase measured by nanoindentation experiments. In this study, a quantitative evaluation of strengthening contributions from the L1 ₂ and BCC particles was also performed to understand the mechanical behavior of the alloy.