Deformation behavior of ultra-fine-grained Mg-0.3 at% Al alloy in compression

Fine and coarse grained (5–20 μm) and ultra-fine-grained (down to 1 μm) samples of a binary Mg-Al alloy were fabricated by extrusion at various temperatures. When tested in compression, a slight stress drop was observed after yielding, followed by a plateau in flow stress. This behavior is typical of textured fine-grained samples. Examination of deformed structures within the ultra-fine-grained samples at various levels of deformation showed that the stress drop after yielding is due to an occurrence of long, thin twins across low angle boundaries and in unrecrystallized regions. In the plateau region, extensive twinning occurs inside individual grains. Then, in a rapid strain hardening region that follows the plateau region, stress concentrations were found to build up on boundaries. Such pile-ups could be responsible for a lowering of fracture strain. •Compression deformation of an ultra fine grained Mg-Al alloy has been studied.•Yielding occurs by formation of long twins in unrecrystallized regions.•Limited number of twins in each grain gives rise to a plateau in the flow curve.•Refining grain size to ∼1 μm leads to activation of non-basal slips.•Intense activity of non-basal slip results in strain hardening beyond 6% strain.