Working hardening behaviors of severely cold deformed and fine-grained AZ31 Mg alloys at room temperature

AZ31 Mg alloy extrusion wires were drawn to a maximum cumulative area reduction of 61% at room temperature, and the 61%-drawn sample was subjected to various annealing treatments for grain refinement. Tensile tests were performed on all the as-drawn and as-annealed samples at a constant strain rate at room temperature. The entire stress strain curves of each investigated sample were analyzed for the dependence on drawn area reduction and mean grain size. The results show that the cold drawn samples exhibit a constant elastic modulus; however, the stresses are significantly dependent on the deformation level. The corresponding theta-sigma curves (where theta is the strain hardening rate, d sigma/d epsilon) show extended stage II and suppressed stage IV of strain hardening. The recrystallized samples exhibit enhanced yield stress with the grain size refinement and typical hardening stages of polycrystalline metals: II, III, IV and V. Additionally, decreasing in stage IV with refining grain size is observed probably due to the contribution of grain boundaries slipping. The different hardening behaviors demonstrate the various hardening mechanisms between the cold drawn and recrystallized materials.