Role of Cu addition in enhancing strength-ductility synergy in transforming high entropy alloy

The mechanical properties of transformation induced plasticity high entropy alloys (TRIP HEAs) are affected by tailoring the metastability via alloying and processing. The current work focuses on as-cast and friction stir processed alloy where the initial microstructure is altered by Cu addition (a γ-fcc phase stabilizer) to the ε-hcp dominated HEA. With the Cu addition, the tensile stress-strain curves exhibited improved ductility and a delay in TRIP effect, suggesting an increased stacking fault energy, along with improved strength and work hardening. Improved properties with Cu addition are credited to almost 100% stabilized γ-fcc phase, increased type and number of interfaces: Cu-rich precipitates, shorter faults, phase separation, and increased grain boundary fraction. The new alloy was then friction stir processed (FSPed) to further improve the properties. An advanced TRIP effect is observed with FSP as compared to as-cast alloy, attributed to increased ε-hcp fraction and finer grain size. Based on microscopic observations, the improved strength is due to finer grain size, increased dislocation density, low density of faults, whereas reduced ductility is reasoned to be due to dissolution of Cu-rich precipitates and increased width of modulations from phase separation.