Significant hardening due to the formation of a sigma phase matrix in a high entropy alloy

The hardening in Al0.3CrFe1.5MnNi0.5 high-entropy alloy not only nearly triples the hardness of the alloy, but also shows a quick hardening response and the absence of overaging. However, the crystal structure, morphology, and composition of the hardening phase have not yet been confirmed. Here, such information regarding the hardening phase is investigated. It was found that the hardening phase is a Cr–Mn–Fe ternary sigma phase. Unlike in conventional engineering alloys, the sigma phase is not precipitated from the matrix, instead, the whole BCC matrix transforms to sigma phase almost without changing its composition. Therefore, the hardening phenomenon is not a precipitation hardening reaction as suggested before.

► Age hardening phenomenon in Al0.3CrFe1.5MnNi0.5 high entropy alloy is unraveled.
► The hardening reaction is not precipitation hardening as suggested before.
► Hardening is due to the transformation of the matrix from BCC to sigma phase.
► The sigma phase becomes the matrix in the hardened alloy, which is rarely seen.
► Existence of the sigma phase as a matrix may provide a new design route.