Precipitation of the lamellar α phase in an Al-containing VCoNi medium-entropy alloy

VCoNi-based multicomponent alloys have attracted considerable attention owing to their exceptional mechanical properties. Understanding the microstructures and formation mechanisms at the atomic scale contributes to the development of new materials with desired attributes. In this study, the precipitation behavior of a VCoNi-based medium-entropy alloy (MEA) containing Al was analyzed using aberration-corrected scanning transmission electron microscopy (STEM) technique. Our investigation revealed that the introduction of Al led to the creation of a dual-phase alloy exhibiting an Al-poor face-centered cubic phase alongside an Al-rich body-centered cubic (BCC) phase. Subsequent high-temperature annealing resulted in the precipitation of a substantial amount of lamellar hexagonal close-packed (HCP) α phase within the BCC matrix. These lamellar HCP-α phases exhibited a width of approximately 25 nm and a length of about 5 μm. Further analysis revealed that these α phases exhibited a high Ni content and a low Al content, with a notable enrichment of Ni observed at the phase boundaries. Importantly, these precipitates displayed a semi-coherent relationship with the BCC matrix, characterized by a habit plane of {2 \(\stackrel{\text{-}}{1}\stackrel{\text{-}}{1}\) $\stackrel{\text{-}}{1}\stackrel{\text{-}}{1}$ } and an orientation relationship of (0001)_α // (01 \(\stackrel{\text{-}}{1}\) $\stackrel{\text{-}}{1}$ )_BCC and [11 \(\stackrel{\text{-}}{2}\) $\stackrel{\text{-}}{2}$ 0]_α // [111]_BCC. Our findings confirm the precipitation of the α phase in Al-containing VCoNi MEAs, even in the absence of group-IV elements. These results are expected to provide a theoretical foundation for the design of novel precipitation-strengthened, high performance alloys in the future.