Friction stir gradient alloying: A novel solid-state high throughput screening technique for high entropy alloys

Selecting an optimal high entropy alloy (HEA) from the copious compositional space of multi-principal HEAs remains a substantial challenge. To accelerate the exploration process to obtain the most desirable composition and microstructure, a novel solid-state gradient alloying method for high throughput compositional screening technique was developed. This prototype technique consists of a tapered section of a pure alloying element retrofitted in the base alloy groove created via milling. Subsequent friction stir processing of the assembled region resulted in a continuous increase in alloy content of the additional element. In the present study, this method was applied on a recently-developed transformation induced plasticity-(TRIP) based HEAs, where the effect of gradient variations of Cu on the phases (epsilon-hcp and gamma-fcc) and the mechanical property response of the base Fe40Mn20Co20Cr15Si5 (at. %) (CS-HEA) were studied. The base material is epsilon-dominant CS-HEA, which by addition of Cu shifts towards a mixture of epsilon + gamma phases. Further addition along the alloying path transforms the whole microstructure into a supersaturated gamma phase that rejects extra Cu. Apart from phase fraction, grain size refinement was observed with increased Cu content. The impact of evolution to mechanical properties was correlated via microhardness and nanoindentation tests. The Young's modulus values of completely epsilon-dominated base material increased from similar to 154 GPa to similar to 224 GPa for a completely gamma microstructure.