Microstructure and Properties of Co-Cr-Fe-Mn-Ni-Based High-Entropy Alloys with Al Addition through Heat Treatments

The effects of heat treatment on microstructure and hardness of equiatomic CoCrFeMnNi high-entropy alloy (HEA) and two Al-containing variants, Al0.5CoCrFeMnNi1.5 and Al0.5CoCrFeNi1.5, were examined. Cast and hot isostatic pressed (HIP) materials were subjected to homogenization at 1120 degrees C for 24 hours followed by an aging treatment at 600, 700, or 800 for 72 hours. The microstructure and phase composition of the samples were characterized using scanning electron microscopy, energy-dispersive x-ray spectroscopy, and x-ray diffraction analysis. In addition, Vickers hardness of the materials was measured with micro-indentation to examine the aging and over-aging effects. The results demonstrated the phase evolution and precipitation hardening of the Al-containing HEAs under various heat treatment conditions. All alloys had a primarily face-centered cubic (FCC) structure as received. The Al-containing variants developed FCC-L12 Ni3Al and body-center cubic (BCC)-B2 NiAl precipitates during aging at 600 and 700 degrees C. Heat treatment of Al0.5CoCrFeMnNi1.5 at 600 degrees C maximized its hardness at 347 +/- 3 HV, corresponding to precipitation of NixAl enhanced by the presence of Mn. Al0.5CoCrFeNi1.5 reached its maximal hardness at 700 degrees C. The formation of NixAl were found to be significantly promoted by the presence of Mn.