Realizing enhanced superplasticity in an AZ91 Mg alloy featuring layered heterogeneous microstructure and texture

• Friction stir processing (FSP) generated a heterogeneous microstructure with layered variations in texture and grain size. • Superplastic elongation of 977% was observed for samples deformed at high temperature. • Both strain hardening and strain rate sensitivity were found to play an important role in achieving high elongation. • Distinct deformation mechanisms governed the individual layers within the heterogeneous microstructure. • Synergistic effect of different deformation mechanisms operating in the layers resulted in enhanced superplasticity. Commercial as-cast AZ91 Mg alloy exhibiting a dendritic microstructure with an average grain size exceeding 300 µm was subjected to multipass friction stir processing (FSP). FSP generated a heterogeneous microstructure characterized by layered variations in texture and grain size, which were demarcated as the Top (close to the shoulder), Middle, and Bottom (pin-tip) layers. The processed material predominantly exhibited non-basal texture components, with an average grain size ranging from approximately 2 µm to 20 µm across the layers. To elucidate the influence of this microstructural heterogeneity on superplastic deformation, tensile specimens with gauge length aligned along the processing direction were tested at strain rates ranging from 10⁻⁴ to 10⁻³ s⁻¹ and temperatures between 473 and 623 K. A 10-fold increase in elongation was achieved at 573 K and a strain rate of 4 × 10 −4 s −1 . The microstructural evolution under these optimum conditions was examined using interrupted tensile tests at various true strain levels. The results revealed that distinct deformation mechanisms dominate within different layers, governed by the local texture and grain size. The final microstructure at fracture exhibited a homogenized grain size distribution and significantly weakened texture intensity. The combined effects of predominantly non-basal texture and grain-size distribution were analyzed using EBSD, enabling a correlation between microstructural evolution and the observed enhanced superplasticity.