The phase evolution and stability of Al_0.5CoCrFeNi high-entropy alloy at 600 °C

The Al_0.5CoCrFeNi alloy has been shown to possess a high potential for applicability by virtue of its mechanical properties. Its duplex nature proved to be stable after heating cycles to near melting and after aging treatments. In the as-cast state, this alloy contains ∼ 90 vol% of dendrite core (DC) regions having FCC structure, while the remainder consists of inter-dendritic (ID) regions with B2/BCC mixture, characterized by its typical ‘Chinese letter’ morphology. At 600 °C, ordered FCC (L1₂) nano-precipitations have been shown to form uniformly within the DC regions. These precipitates can potentially increase the DC hardness due to precipitation hardening during aging treatments. This article investigates the phase evolution and changes in the L1₂ phase morphology during a variety of aging treatments at 600 °C. The effect of aging on the alloy's hardness was also evaluated. It was found that during aging, the L1₂ phase changed its morphology from individual particles to a ‘tweed-like’ morphology, which caused a sharp increase in hardness (600 °C/17d: 388.5 ± 11.4HV) compared to a non-aged sample (600 °C/q: 218.7 ± 5.4HV). With further aging, the L1₂ symmetry evolved into orthorhombic symmetry before finally transiting into a lamellar B2 morphology at the over-aged state, associated with a decrease in hardness. The increase in the DC hardness during aging seems to cause a delay in the formation of a tetragonal (σ) Cr-Fe rich phase, which tends to precipitate along the DC/ID boundaries in this alloy. In the ID region, the phase evolution starts with an HCP precipitation that remains stable as long as the B2/BCC mixture is stable, but once the BCC phase is transformed to FCC, the HCP particles become extinct. The experimental results were compared with the predicted phase content using the TCHEA4 thermodynamic database, and the differences are discussed.