Microstructure evolution and strengthening mechanism of directional solidification Ni_40.5Co₃₀Cr₁₁Fe₁₁Al₆Ta_1.5 high entropy alloy

Directional solidification Ni_40.5Co₃₀Cr₁₁Fe₁₁Al₆Ta_1.5 high entropy alloys (HEAs) with different withdraw rates (5 μm/s-100 μm/s) were prepared to reveal the microstructure evolution, the deformation mechanism and the strengthening mechanism. The results show that the solid-liquid (S-L) interface of the alloy changes from convex to planar with the increase of withdraw rates. The primary and second dendritic arm spacing decreases from 248 μm and 53 μm (20 μm/s sample) to 177 μm and 37 μm (100 μm/s sample), respectively. The alloy consists of FCC, L1₂ and topologically close packed (TCP) phases, which is in good agreement with the analysis of phase volume fraction. The L1₂ phases are formed in IR due to the lower mixing enthalpy. The ultimate tensile strength of the alloy increases from 412.36 MPa and 25.75% to 547.89 MPa and 48.1%, respectively. The strength of the alloy is mainly attributed to precipitation strengthening.