Achieving ultra-high hardness of Mg-Sm-Ca alloy with the unique nanostructure

Most magnesium rare-earth (Mg-RE) alloys have limited hardness. Here a new strategy is proposed, that is, the solution treated Mg-4.97Sm-0.84Ca (wt.%) alloy was subjected to hot rolling processing and high-pressure torsion (HPT) processing, followed by aging for various periods. The ultra-high hardness value of ~175 H V for Mg-Sm-Ca alloys processed by hot rolling and HPT after aging at 150 °C for 3.5 h is achieved which is ~109 H V higher than as-solution alloy (~66 H V). Such an unprecedentedly high hardness value is mainly attributed to the unique nanostructure in the alloy which contains high-density nano-precipitates which have low lattice misfit with the matrix. We found that the morphological of the Mg₄₁Sm₅ phase was changed from rod-like to kink band-like by HPT processing and then convert into dense, near-spherical shape particles during the subsequent aging process to form isotropic structure which provides exceptional strengthening for the alloy.