Microstructure, phase formation and physical properties of AlCoCrFeNiMn high-entropy alloy

Abstract

Most known single-phase high-entropy alloys (HEAs) are metastable materials that decompose after long annealing into a mixture of two or more solid solutions. Besides simple FCC and BCC phases, the formation of σ-phase, which essentially affects mechanical properties, is often observed in HEAs. Moreover, long-time annealings affect essentially the microstructure of HEAs. Here we address these issues for AlCoCrFeNiMn high-entropy alloy and compare results with those for AlCoCrFeNi one. Performing long-time isothermal (up to 500 h) annealing at different temperatures, we estimate thermal stability region of σ-phase as 820−1230 K. Below 820 K, the equilibrium structure of the alloy is a mixture of BCC and FCC phases. Annealed AlCoCrFeNiMn alloy demonstrates a coarse-grained microstructure with characteristic size of phase areas of 20–30 μm, which are by the order of magnitude larger than for AlCoCrFeNi alloy. Such microstructure leads to unusual behaviour of transport properties, which is not typical for multicomponent systems. In particular, residual Lorenz number in AlCoCrFeNiMn alloy is very close to 1 which indicates pure electronic character of its thermal conductivity. We also observe that AlCoCrFeNiMn is an ultra-soft ferromagnetic material whose coercive force does not exceed 5 Oe.