Mechanisms underlying the influence of Co and Ti on the microstructure, mechanical and wear properties of A2/B2 typed AlCrFeNi alloy

AlCrFeCoNi is one of widely studied high-entropy alloys, in which Co is considered to be beneficial. Here we demonstrate in detail that Co does not play a positive role. Instead, Co-free AlCrFeNi possesses higher ductility than AlCrFeCoNi while maintaining a similar mechanical strength. It is of interest to observe that removing Co results in a two-fold increase in the wear resistance. The improvements are related to the formation of a soft-hard alternating lamellar structure which is absent in the Co-containing alloy. We further tailor the alloy by replacing Co with Ti, leading to further enhanced wear resistance. AlCrFeCoxNi (x = 0.5 and 1) and Co-free AlCrFeNiTix (x = 0, 0.2, 0.5, 0.7 and 1) all contain a Fe-Cr-rich disordered BCC phase (A2) and a NiAl (Ti)-rich ordered BCC phase (B2). For x = 0.5, a small amount of L21 phase with embedded nanoscale A2 phase is observed. For x ≥ 0.7, an additional CrFeTi Laves phase forms mainly at grain boundaries. The increase in Ti content alters the lamellar eutectic microstructure of AlCrFeNi, forming a cellular eutectic microstructure. Ti considerably increases hardness and compressive strength at the expense of ductility. AlCrFeNiTi0.7 appears to have a proper balance between hardness and ductility with a superior wear resistance comparable to that of a well-known industrial wear-resistant high-Cr cast iron (Cr30C2.85). This HEA has demonstrated itself a promising candidate for the mechanical and anti-wear applications.