Microstructures and mechanical properties of body-centered-cubic (Al, Ti)0.7(Ni,Co,Fe,Cr)5 high entropy alloys with coherent B2/L21 nanoprecipitation

Coherent precipitation of cuboidal B2/L2(1) nanoparticles in body-centered-cubic (BCC) based high entropy alloys (HEAs) is crucial for the improvement of mechanical properties. The present work investigated the influence of Ti addition on the phase constitutions and particle morphology in (Al,Ti)(0.7)M-5 HEAs (M = Ni1Co1Fe1.5Cr1.5 and Ni1Co1Fe1Cr2) with different Al/Ti ratios. Series of alloy rods were prepared by suction-cast processing, and their microstructures and mechanical properties were then characterized comprehensively. It was found that the substitution of Ti for Al can change the phase structures of ordered precipitates, from B2-NiAl to L2(1)-Ni2AlTi. Furthermore, the particle morphology of L2(1)/B2 precipitates is closely related to the lattice misfit epsilon between L2(1)/B2 phase and BCC matrix, in which a moderate epsilon (epsilon = 0.4-0.5%) can contribute to the formation of cuboidal L2(1)/B2 precipitates in BCC matrix, while a large epsilon (epsilon > 1.0%) will cause a lath-like microstructure. High compressive yield strengths (above 1500 MPa) of these Al/Ti-contained HEAs with cuboidal coherent microstructure are primarily attributed to the particle size in light of the precipitation strengthening mechanism. In addition, the excess addition of Ti will transform the matrix structure of alloys from the BCC to the sigma phase, leading to a serious brittleness.