Heat treatment impacts the micro-structure and mechanical properties of AlCoCrFeNi high entropy alloy

AlCoCrFeNi alloy was prepared by arc melting and examined as-cast and after various heat treatments using XRD, SEM, micro-hardness and compression tests. It was found that the alloy solidified dendritically with an Al- and Ni-rich dendrite core and inter-dendritic regions rich in Co, Cr, and Fe. The dendrite core of the as-cast material presented a relatively soft matrix with nano-sized precipitates, while the inter-dendritic regions consisted of a relatively hard matrix with nano-sized particles larger than those in the dendrite core. Heat treatment did not change the dendritic morphology, however, different phase transformations occurred, especially in the inter-dendritic regions. Heat treatment between 650 and 975 °C caused transformation of the BCC matrix in the inter-dendritic regions to the σ phase, leading to a further increase in hardness. Heat treatment at 1100 °C caused transformation of the σ phase in the inter-dendritic matrix to a BCC structure and softening of the alloy. Heat treatment at 1200 °C caused partial phase dissolving and homogenization, which in turn enabled the alloy to re-enter the miscibility gap and decomposition to a BCC matrix, with B2 precipitation in the dendrite core and inter-dendritic regions. The impact of these changes on morphology and phase composition upon heat treatment in terms of mechanical behavior was discussed.