Dislocation Dissociations in hcp Metals

Abstract

Equations are derived for nonplanar dissociations of dislocations in hcp metals. Various components of AB, A_μ, φθ, [AB+φθ], and s′ [AB+φθ] dislocations on the basal, prism, and pyramidal planes meet the criteria for dissociation into twinning partials on$\text{{2̄11l}}$ and$\text{{1̄01l}}$ planes, and the Burgers vectors of the twinning partials are consistent with minimum shear strain and small atomic shuffling criteria for twinning. The repulsive force γ_m on each twinning partial is computed using anisotropic elasticity and compared with estimates of the twin lamella distortion energies γ. For [AB+φθ] and φθ dislocations γ_m is quite large and γ_m/γ > 1 in many cases. In several cases γ_m/γ > 1 for dissociation of AB on the basal and prism planes, leading to an increase in dislocation resistance, locking, or twinning at lower temperatures. The results are consistent with the difference in flow behavior of metals like Zr and Ti on one hand and Cd and Zn on the other. Successive dissociations for twin growth and double twinning processes are studied, with the latter shown to be consistent with reported behavior in magnesium.