Strain-induced selective grain growth in AZ31 Mg alloy sheet deformed by equal channel angular pressing


In this work the microstructure development during annealing of AZ31 sheets processed by equal channel angular pressing (ECAP) is presented. Samples with dimensions of 200 × 200 × 1.8 mm3 were processed in a single ECAP pass at 225 °C using a channel angle of 130°. Subsequent isothermal and isochronal heat treatments were used to reveal the microstructure stability of the ECAPed sheets at temperatures between 180 and 300 °C. A systematic analysis of the microstructure development was conducted using optical microscopy and electron backscatter diffraction (EBSD). It was found that a texture modification produced by the profuse activation of View the MathML source101-2 extension twinning during ECAP is lost during annealing as a result of thermal detwinning. Moreover, a bimodal microstructure composed by small and coarse grains was observed at 250 °C and 300 °C as a result of the fast growth of a few grains. The reason for the selective grain growth is related with the activation of strain induced grain boundary migration which was stimulated by the large stored strain contained in the as-ECAP processed sheets. The dissolution of Mg17Al12 particles during annealing is also seen as a possible factor for the onset of rapid grain growth.
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