Effect of faceting on twin grain boundary motion in zinc

Abstract

Faceting and migration of incoherent twin grain boundary in Zn [112̅0] flat single crystals has been investigated. The stationary shape of the slowly migrating incoherent twin grain boundary of the twin plate tip was studied and migration velocity was measured in situ in the range from 473 K to 692 K using polarized light. Below 623 K the incoherent twin grain boundary represents the facet which forms at a 43° angle to the coherent twin grain boundary. Above 623 K the incoherent twin grain boundary represents the facet, whose position changes from the initial to 75° to the coherent twin grain boundary as temperature increases. Below 623 K the incoherent twin, grain boundary moves at very low experimentally determined activation enthalpy 19.3 kJ/mol of facet migration. Above 623 K the experimentally determined activation enthalpy for the facet migration is 154.4 kJ/mol, which is higher than the activation enthalpy of grain boundary diffusion in Zn. These results clearly indicate that there is a strong effect of the grain boundary shape on the migration velocity of the twin tip. We suppose that there is a grain boundary structural phase transition in this system: facet with high coincidence site lattice transforms to facet with low coincidence site lattice with disordered structure at 623 K.