Synergistic effect by Al addition in improving mechanical performance of CoCrNi medium-entropy alloy

The effect of 7.5 at.% Al addition in Al7.5CoCrNi medium-entropy alloy (MEA) was investigated. The recrystallized Al7.5CoCrNi exhibited the microstructure of fine-grain face-centered cubic (FCC) phase altogether with the formation of NiAl-rich B2-phase precipitate around the grain boundary and grain interior. The formation of NiAl-rich precipitate led to the significant grain size refinement compared to the Al-free CoCrNi MEA. The small addition of Al was also discovered to be able to significantly improve the mechanical properties of CoCrNi matrix, with a substantial yield strength increment from 400 MPa to 680 MPa was obtained. This increment occurs owing to the combination of several strengthening mechanisms by the addition of Al via substitutional solid solution, grain refinement, and the B2-phase precipitation strengthening that has a good agreement with the theoretical results. The presence of NiAl-rich B2 phase also changed the plastic deformation mechanism that the dislocation slip without ε martensitic transformation was found to replace the deformation twinning mechanism which generally observed in a CoCrNi matrix during the plastic deformation.