In this study, AZ91 magnesium alloy rods were used to investigate the effects of torsion deformation on microstructure and subsequent aging behavior. Extruded AZ91 rod has a uniform microstructure and typical fiber texture. Torsion deformation can generate a gradient microstructure on the cross-section of the rod. After torsion, from the center to the edge in the cross-section of the rod, both stored dislocations and area fraction of {10-12} twins gradually increase, and the basal pole of the texture tends to rotate in the ED direction. Direct aging usually generates coarse discontinuous precipitates and fine continuous precipitates simultaneously. Both twin structures and dislocations via torsion deformation can be effective microstructures for the nucleation of continuous precipitates during subsequent aging. Thus, aging after torsion can promote continuous precipitation and generate gradient precipitation characteristics. Both aging treatment and torsion deformation can reduce yield asymmetry, and torsion deformation enhances the aging hardening effect by promoting continuous precipitation. Therefore, combined use of torsion deformation and aging treatment can effectively enhance the yield strength and almost eliminate the yield asymmetry of the present extruded AZ91 rod. Finally, the relevant mechanisms are discussed.

An Fe-23Cr-8.5Ni alloy was used as a model material to study the grain size effect on the mechanical behavior of metastable duplex metal. Alloy samples with different grain sizes ranging from 0.1 to 2 μm were prepared by cold-rolling and annealing. A structural refinement to about 0.1 μm results in a high yield strength but very limited ductility. A significant improvement of ductility occurred at the grain size of about 0.4 μm. A further increase in grain size results in a decreased strength and a slightly improved ductility. The alloy with a grain size of about 0.4 μm exhibits an excellent combination of strength and ductility, where the yield strength and tensile elongation are increased up to 738 MPa and 29% as compared to 320 MPa and 33% of a coarse-grained (about 2 μm) sample, respectively. The origin of the excellent mechanical properties was attributed to the unique deformation characteristics associated with the transformation induced plasticity and the development of back stress.