Hot deformability, microstructural evolution and processing map assessment of high entropy alloys: A systematic review

The discovery of High entropy alloys (HEAs) and the deepened understanding of their physical metallurgy have raised the bar on microstructure design and choice materials selection for highly specialized, sensitive technological and structural integrity applications. This is on account of their unique phase constitution, facilitated by their multi-principal elemental compositions, which imparts excellent properties combinations in HEAs, which are rarely observed in conventional alloys. However, due to their shrinkage porosity, chemical composition heterogeneity, rough dendritic structure, and quasi-stable eutectic at grain boundaries make direct deployment in the as-cast condition, is problematic. As a result, using hot deformation techniques to create homogenous and fine microstructures in as-cast HEAs has gained attention as an effective strategy to resolve these shortcomings. A systematic review of studies on the hot deformation behaviour of different classes of high entropy alloys (HEAs), is summarized in this review paper. The plastic flow behaviour, phase, and microstructural evolution, mechanisms of hot deformation as well as the optimal hot deformation processing regions, derived from analysis of processing maps, were evaluated in this review. Factors such as alloy constitution, HEA phase(s) type, deformation processing parameters, and recrystallization mechanisms, were noted to influence the nature of microstructural evolution, plastic deformability, and material properties of the HEAs. The role and effectiveness of processing maps in establishing conditions for optimal tuning of the microstructures in HEAs to achieve the best product finish were also discussed.