Effects of Non-Schmid Stresses on 〈a〉-Type Screw Dislocation Cores in α-Ti

〈a〉-type dislocations are a major carrier of plasticity in HCP metals. Their mobility is known to be limited by their screw component. In α-Ti, 〈a〉-type screw dislocations have been observed to have a jerky glide: gliding at high velocity over long distances on the prism plane, but occasionally getting locked in position, and sometimes gliding slowly on the pyramidal plane over short distances. The motion is controlled by the dislocation core morphology which impacts the energy barriers of the prism-, pyramidal-, and cross-slip. Density functional theory computations have shown that the difference between these energy barriers is small, within tens of meV per burgers vector. From a metallurgical standpoint, this is of particular interest. If one can tune the magnitudes of these barriers, one can enhance pyramidal or prism slip, and one can thus tune the material’s mechanical properties.<br/>Non-Schmid stresses, i.e., stresses that do not drive the motion of dislocations, are known to impact the core morphology of 〈a〉-type screw dislocations in α-Ti. We have developed a model to quantitively assess the interaction of the dislocation core with non-Schmid stresses. We show that non-Schmid effects are caused by an interaction of the dislocation core displacement field with non-Schmid stresses and that this displacement field is accurately characterized by an elastic dipole tensor. Density functional theory was used to compute the elastic dipole tensor of screw dislocations during a pyramidal and prism slip and a cross-slip from a pyramidal to a prism plane. We illustrate the effects of non-Schmid stresses on the mobility of dislocations through the case of an oriented single crystal undergoing tensile stress. We predict a change in the dislocation pattern (planar to wavy) depending on the crystal orientation.<br/>The model allows one to assess the effect of complex stress fields on dislocation mobility. One could use it to understand how to leverage non-Schmid effects to engineer dislocation cores through alloying. Indeed, point defects generate a stress field that will interact with the core field and change the energy barriers of slip events. This problem can now be tackled with a quantitative approach.<br/><br/>The authors gratefully acknowledge funding from the U.S. Office of Naval Research under grants N00014-17-1-2283 and N00014-19-1-2376. The final portions of this work were completed with funding from the National Science Foundation under contract 2324022.