A magnetically soft yet mechanically strong and ductile Ta free CoFeNi high entropy alloy with Al and Ti additions

Next generation high speed, large torque, rotating electrical machines demand materials with an excellent balance of magnetic and mechanical properties. High entropy alloys (HEAs) can exhibit this unique combination of properties, e.g., CoFeNi based HEAs containing Ta. The high Ta content makes these alloys prohibitively expensive for industrial applications. Hence, we develop Ta free HEA using a thermodynamic approach. The addition of Al and Ti to the base CoFeNi alloy is studied. We investigate two compositions: Ti_0.24CoFeNi_0.78 and Al_0.3Ti_0.2CoFeNi. In their solution treated and quenched condition these HEAs exhibit a low coercivity H_c ~ 1.8-2.4 Oe, high saturation magnetization M_s ~ 96.5-137.3 emu/g, coupled with a high yield strength, YS ~ 780-850 MPa, ultimate tensile strength, UTS ~ 1200 MPa, large tensile elongation, ε ~ 29-35%, and with desired high electrical resistivity ( ~ 269.5-308.3 µΩ·cm), making them attractive for high frequency electrical machines. Importantly, the latter HEA is further strengthened to a very high YS ~ 1140-1200 MPa with excellent ductility (ε ~ 18-25%) and electrical resistivity (296.1-459.3 µΩ·cm), but at the cost of higher H_c values ~ 8.8-48.6 Oe.