Twinning mechanism asymmetry in body-centered cubic tantalum under [001] uniaxial compression/tension

Plasticity of body-centered cubic tantalum with preexisting dislocations is investigated under $[001]$ uniaxial compression/tension loading using a molecular dynamic simulation method. At low temperature or high strain rate, twinning is the main deformation mechanism under both $[001]$ compression and tension. However, the twinning mechanism under compression is different from the conventional twinning mechanism under tension, which is formed by $1/6[1\overline{1}1]$ twinning dislocations moving on adjacent $(\overline{1}12)$ planes along the twinning direction. The twin nucleates from six-layer dissociation of a $1/2[111]$ screw dislocation and grows by the glide of disconnections that step the twin boundary by two $(\overline{1}12)$ planes. Further study shows that this type of twinning mode generates a finite antitwinning shear strain, the magnitude of which is one-half of that generated by the conventional deformation twinning. Finally, the two twinning mechanisms are discussed in terms of the energy landscape.