Effects of annealing treatment on microstructure and hardness of bulk AlCrFeNiMo0.2 eutectic high-entropy alloy

• Two distinct eutectic cells are observed in bulk eutectic HEA casting ingot.
• Bulk AlCrFeNiMo0.2 eutectic HEA displays high thermal stability.
• σ phase firstly precipitates from and then dissolves into the FeCrMo-rich BCC phase.

Bulk AlCrFeNiMo0.2 (in molar ratio) alloy ingot was prepared by vacuum induction melting and casting methods. The effects of annealing temperature variations (550–1050 °C) on the crystal structure, microstructure, and hardness were investigated. The hardness of the as-cast alloy was HV467, and the alloy exhibited a typical eutectic cell microstructure consisting of FeCrMo-rich solid solution (BCC structure) and NiAl-rich intermetallic compound (B2 structure). After annealing at 750–950 °C, a Mo-rich σ phase precipitated from the FeCrMo-rich solid solution, and apparent annealing hardening appeared. The hardness increased from HV479 to HV542. The hardening of this alloy was attributed to the transformation of the BCC phase to the σ phase. The σ phase changed back to the BCC phase at 1050 °C and the hardness decreased to HV478 which was nearly equal to that of the as-cast alloy. Overall, the AlCrFeNiMo0.2 alloy exhibited excellent annealing softening resistance.

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