Processing of High-Entropy AlCoCr_0.75Cu_0.5FeNi Alloy by Spray Forming

Compositionally complex AlCoCrCuFeNi alloy system is one of the extensively studied high-entropy alloys (HEA). Most of these studies have been carried out on as-cast materials that witness slow cooling rate during solidification. In the present investigation, HEA based on AlCoCrCuFeNi system was processed employing spray forming so as to realize its inherent rapid solidification effect and unique microstructural evolution mechanism. The spray formed as well as the as-cast alloys were compared in terms of their microstructural features, phase constitution and hardness. These microstructural features were also compared with that of the overspray powders (< 250 µm) obtained as a by-product of spray forming. Both the as-cast and spray formed alloys showed a mixture of mainly two phases, i.e., face-centered and body-centered cubic (BCC) phases, in contrast to a single BCC phase in overspray powders. However, the spray formed alloy revealed unidentified phases, as against the as-cast material, which disappeared after heat treatment at 1473 K for 3 h. The grain size of the as-cast alloy was around 600-1000 μm compared to a highly refined grain size of 25-30 μm in the as-spray formed alloy. It was found that the hardness of the regions dominated by the constituent phases as well as the bulk of the spray formed material was higher than that of the as-cast alloy. The results validate the efficacy of the process in achieving drastic refinement in grain size, in a single step of large-scale spray forming. The characteristic microstructural features and the phase constitutions have been discussed in light of the prevailing mechanisms of structural evolution and possible solidification phenomena during spray atomization and deposition. This study gives a new direction in processing of these alloys and understanding their solidification behavior under the conditions of layer-by-layer deposition of semi-solid/liquid droplets.