Enabling cold spray deposition of mechanically alloyed FeNiTiCuAl high-entropy alloy powder through annealing

This work investigates how low-temperature annealing improves the cold spray processability of mechanically alloyed equiatomic FeNiTiCuAl high-entropy alloy (HEA) powder while preserving a metastable phase constitution. FeNiTiCuAl powder was synthesized by mechanical alloying and heat-treated at 200, 300 and 500 °C to distinguish recovery-dominated relaxation from phase evolution. The as-milled powder exhibited an FCC-dominated metastable structure, pronounced X-ray diffraction (XRD) peak broadening and high Rietveld-derived microstrain. Cyclic differential scanning calorimetry (DSC) revealed partially irreversible thermal activity below approximately 330 °C, consistent with defect relaxation. Heat treatment at 200 °C reduced microstrain and microhardness while largely preserving the FCC-dominated phase constitution. In contrast, treatments at 300 and 500 °C produced additional reflections compatible with either disordered BCC or ordered B2 structures, increased chemical heterogeneity and renewed hardening. Cold spray deposition and single-particle splat tests compared the as-milled and 200 °C-treated powders using the same sieved 20–40 μm particle-size fraction under identical spraying conditions. The treated powder showed increased particle retention and improved coating formation, with the average thickness increasing from 78 ± 5 to 204 ± 19 μm. Coating XRD patterns showed no resolvable major phase transformation during spraying. The improved deposition response is therefore consistent with recovery-related reductions in powder microstrain and hardness. These results identify low-temperature annealing as an effective preconditioning strategy for improving the cold spray processability of this mechanically alloyed HEA without extensive phase transformation.