Structural evolution during mechanical milling and subsequent annealing of Cu–Ni–Al–Co–Cr–Fe–Ti alloys

This study reports the structural evolution of high-entropy alloys from elemental materials to amorphous phases during mechanical alloying, and further, to equilibrium phases during subsequent thermal annealing. Four alloys from quaternary Cu0.5NiAlCo to septenary Cu0.5NiAlCoCrFeTi were analyzed. Microstructure examinations reveal that during mechanical alloying, Cu and Ni first formed a solid solution, and then other elements gradually dissolved into the solid solution which was finally transformed into amorphous structures after prolonged milling. During thermal annealing, recovery of the amorphous powders begins at 100 °C, crystallization occurs at 250–280 °C, and precipitation and grain growth of equilibrium phases occur at higher temperatures. The glass transition temperature usually observed in bulk amorphous alloys was not observed in the present amorphous phases. These structural evolution reveal three physical significances for high-entropy alloys: (1) the annealed state of amorphous powders produces simple equilibrium solid solution phases instead of complex phases, confirming the high-entropy effect; (2) amorphization caused by mechanical milling still meets the minimum criterion for amorphization based on topological instability proposed by Egami; and (3) the nonexistence of a glass transition temperature suggests that Inoue's rules for bulk amorphous alloys are still crucial for the existence of glass transition for a high-entropy amorphous alloy.