Phase transformation and strengthening of the gas-atomized FeCoCrNiMo 0.5 Al 1.3 high-entropy alloy powder during annealing.

Phase evolution and strengthening of the FeNiCoCrMo0.5 Al1.3 powder alloy produced via inert gas atomization and annealed in the temperature interval of 300-800 °C have been studied by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, and microhardness testing. It was found that annealing at 300-600 °C leads to an increase of the element segregations between the several solid solutions with a rise of the lattice misfit (ε) to 1.5 % and microhardness growth to 1070 HV. It was assumed that elastic stress caused by the element partitioning is the main strengthening mechanism: microhardness rises linearly with misfit rise with dHV/dε  = 43400 MPa. Sigma arises after the maximum elastic deformation (in 1.5 %) was reached. Formation of the dispersed coherent sigma phase in the annealing interval 600-800 °C results in the microhardness rise. Oxidation that began at 800 °C in 27 h is accompanied with FCC formation due to a depletion of the B2 in Al caused by Al2 O3 formation. Estimation of the activation energy of the initial stage of the solid solution decomposition gives a very low value in 0.65eV, apparently caused by the high concentration of quenched vacancies. The activation energy of sigma formation approximately coincides with the activation energy of self-diffusion in BCC metals (about 2.60 eV).