Additional dislocation slip determined excess yield stress in titanium

The excess yield stress induced slip system variations at the early stage of plastic deformation in Ti was in-depth studied. As a common sense, both the normal grain refinement according to the Hall-Petch relationship and the suppressed dislocation source activation in refined-grain can extensively increase the yield stress. In this study, combined calculations of Schmid factor and critical resolved shear stress suggest that small-grain samples with much higher yield stress could incorporate both <a>-prismatic slip and <c+a>-pyramidal slip, while only <a>-prismatic slip can be activated in the large-grain counterparts. This slip system transition can explain the abnormal fine-grain strengthening effect, which the experimental yield stress of small-grain sample deviates from the value that predicted by using Hall-Petch relationship, as well as the stress drop right after yielding in Ti. These findings could also provide a new strategy for designing high-strength metallic materials.