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/L21 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)0.7M5 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 L21-Ni2AlTi. Furthermore, the particle morphology of L21/B2 precipitates is closely related to the lattice misfit ε between L21/B2 phase and BCC matrix, in which a moderate ε (ε = 0.4-0.5%) can contribute to the formation of cuboidal L21/B2 precipitates in BCC matrix, while a large ε (ε > 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 σ phase, leading to a serious brittleness.