Mechano-chemical synthesis, thermal stability and phase evolution in AlCoCrFeNiMn high entropy alloy

An equiatomic AlCoCrFeNiMn hexanary high entropy alloy (HEA) was successfully synthesized by mechanical alloying. The 40 h milled powder exhibits chemical homogeneity with an average particle size of less than 3 μm. This alloy, after 40 h of milling produces a solid solution BCC phase (a = 2.89 ± 0.03 Å), the lattice parameter of which is very close to the lattice parameter of Fe and Cr. Similar BCC solid solution phase is also obtained by following a two different milling schedule containing 3 elements i.e., AlCoCr & FeMnNi separately. Dynamic DSC thermogram of the hexanary alloy powder shows two exothermic peaks. The peak temperatures and heat evolution corresponding to each peak are sensitive to heating rates, indicating that the transformations are diffusional in nature. Phase evolution at these heat events was confirmed through high-temperature XRD and TEM study. It has been found that this alloy is stable upto 500 °C (773 K). Heat treatment at higher temperatures leads to the formation of a FCC phase closely related to Ni3Al type and Mn3Co7 type intermetallic phase. Microwave sintered samples show the similar kind of behaviour as in heat-treated powder. The BCC phase along with ordered Ni3Al type (L12) phase was observed in the bulk consolidated high entropy alloy. Thermo-physical properties of this alloy have been studied in order to elucidate its classification as high entropy alloy and to understand the essential differences with multicomponent glass forming alloys.