Effects of normal strain on pyramidal I and II ⟨c plus a⟩ screw dislocation mobility and structure in single-crystal magnesium

In this study, atomistic simulations were used to analyze the effects of nonglide stress and temperature on the mobility and structure of pyramidal-I (Pyr-I) and pyramidal-II (Pyr-II) < c + a > screw dislocations in single-crystal Mg. At a very low temperature (10 K), the pyramidal screw dislocations stably exist on Pyr-II planes and tend to glide on Pyr-I planes. The critical resolved shear stresses (CRSSes) of the pyramidal screw dislocations depend on the migration direction. Once a Pyr-II dislocation is transformed into a stuck core, a very high shear stress (243 and 391 MPa) is required to escape from the immobilized structure. Furthermore, their CRSSes increase with increasing compressive strain and decrease with increasing tensile strain normal to the slip planes. At the intermediate temperature range of 200 K <= T <= 400 K, the CRSSes of Pyr-I screw dislocations are weakly affected, whereas those of Pyr-II screw dislocations drastically decrease. Thus, both Pyr-I and Pyr-II screw dislocations have similar CRSS values at 400 K. At a higher temperature (500 K), Pyr-I screw dislocations frequently emit basal- < a > dislocation loops, and the remained dislocations are momentarily immobilized. The basal- < a dislocation loops emitted from the < c + a > dislocations are quickly retracted, and the core structure is recovered as the shear deformation continues. This phenomenon can reduce the mobility of Pyr-I < c + a > screw dislocations at a higher temperature. The emission of basal- < a > dislocation loops is enhanced under compression.