Controlled formation of coherent cuboidal nanoprecipitates in body-centered cubic high-entropy alloys based on Al₂(Ni,Co,Fe,Cr)₁₄ compositions

Microstructures and mechanical properties of Al-Ni-Co-Fe-Cr high-entropy alloys (HEAs) were investigated by systematically varying transition metals instead of Al, within the chemical formula of Al₂M₁₄ (M represents different mutations of transition metals). The formation of different crystal structures (FCC, BCC, or FCC+BCC mixture) and its effects on the resulting mechanical properties of this series of HEAs, both in tension and compression, were evaluated. It was found that, in the BCC-dominated HEAs, ordered B2 precipitates were always coherently dispersed in the BCC solid-solution matrix. The shape of these B2 precipitates was strongly affected by the lattice misfit between the disordered BCC and ordered B2. A uniform distribution of cuboidal B2 particles could be obtained by properly adjusting M, thus the lattice misfit, in a manner similar to that in Ni-based superalloys. Strengthening effects caused by different BCC/B2 morphologies were also estimated and compared with experimental measurements. The optimal strengthening as a function of the shape and size of the coherent precipitates was discussed in light of the lattice misfit in these HEAs.