Annealing-dependent microstructure, magnetic and mechanical properties of high-entropy FeCoNiAl_0.5 alloy

High-entropy FeCoNiAl_0.5 alloy was produced and subjected to heat treatment at different temperatures (400, 600, and 800 °C). The microstructure evolution, magnetic properties and mechanical properties of the alloy were found to be highly dependent on the annealing temperature. The as-cast alloy was composed of FeCoNi-riched matrix (face-centered cubic structure) and the Al-riched region (body-centered cubic structure). After annealed at 400 °C, Al–Ni-based ferromagnetic rod-like precipitates with body-centered cubic structure were abundantly produced within the matrix, improving saturation magnetization (M_s), coercivity (H_c) and hardness. Increasing the annealing temperature to 600 °C, not only gave rise to a higher number density of the rod-like precipitates, but also to the formation of some nanosized spherical precipitates. This microstructure feature is more effective in strengthening the material (achieving a peak hardness of ~304 HV) but at the same time results in slight declination of M_s and H_c. When the annealing temperature was up to 800 °C, the most strengthening nanoscale precipitates dissolved, and the hardness of the alloy was reduced accordingly. The main strengthening mechanisms are discussed, and a good combination of mechanical and magnetic properties under optimized heat treatment may be available.