Abnormal grain growth in a Zn-0.8Ag alloy after processing by high-pressure torsion

Abnormal grain growth (AGG) in a Zn-0.8Ag (wt%) alloy, produced through the application of high-pressure torsion (HPT), was systematically investigated using scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), high-resolution transmission electron microscopy (HR-TEM) and microhardness testing. The HPT-deformed alloy exhibits AGG at room temperature without any additional heat treatment. Analysis by EBSD revealed oriented grain nucleation in a $\left\{11\overline{2}0\right\}〈0001〉$ direction from the initial $\left(0001\right)$ fibre texture which agrees with the maximum energy release model. New grains were oriented according to the minimal Young's modulus direction (c-axis), parallel to the shearing direction. The strain-induced dissolution of nanocrystalline Zn₃Ag precipitates was identified as the main driving force for AGG in this alloy. The strains necessary for the initiation and termination of AGG were determined as ~4.0 and ~5.0, respectively. The increase in solid-solution strengthening caused an increase in hardness from ~47 HK in the fine-grained centre to ~84 HK in the coarse-grained region. A Hall-Petch investigation revealed grain refinement softening below a grain size of 23 µm. These results provide the first comprehensive description of AGG in metallic materials processed by a severe plastic deformation method at room temperature.