Modification of the grain structure, γ phase morphology and texture in AZ81 Mg alloy through accumulative back extrusion

Abstract

The capability of accumulative back extrusion (ABE), as a recently developed severe plastic deformation technique, has been considered to modify the microstructural characteristics of a Mg–Al–Zn alloy composing higher Al content. The results indicate that applying the ABE process up to five passes led to simultaneous modification of the γ phase morphology, grain structure and deformation texture of the experimental alloy. The morphology of the eutectic γ phase has been sequentially altered from initial coarse network to elongated and finally spherical morphologies with an average globularity and diameter of 0.85 and 7.5μm, respectively. This has been justified considering the mechanical fragmentation and thermal disintegration of the eutectic particles through necking phenomena. In addition the initial coarse grain structure of the cast alloy (~330μm) has also been significantly refined (d~1μm). This substantial grain refinement is attributed to the dynamic recrystallization enhancement in connection with pinning effect of crushed γ particles during successive passes. The typical basal texture has been modified to a weak random texture having grains with their normal distribution in a desirably wide range of deviation angles from normal direction. The occurrence of particle stimulated nucleation, particle pinning, and shear banding phenomena are suggested as the main reasons causing the weak scattered deformation texture.