Implementing high entropy alloy synthesis via friction stir processing: Simultaneous solid-state alloying and ultra-refinement of copper

Using the multi-principal element based high entropy alloy (HEA) concept, single-step ultra refinement and solid-state alloying of Cu with NiTi, Y, Fe, Cr, and Co were achieved simultaneously for the first time via friction stir processing (FSP). A bimodal microstructure with an average grain size smaller than 1.5 μm was attained. The microstructure consisted of inter-dispersed regions of high and low density of geometrically-necessary dislocations. The fraction of high-angle grain boundaries was observed to increase from ∼33% in the unstirred region to ∼80% in the processed alloy. Intercalated bands having sharp variations in grain size were formed in conjunction with the bimodal microstructure. The entropic stabilization due to addition of diverse elements assisted multi-faceted heterogeneity and grain refinement along. This study paves the way for the development of HEAs via FSP.