High-strain-rate superplasticity and tensile behavior of fine-grained Mg97Zn1Y2 alloys fabricated by chip/ribbon-consolidation

A new combined processing procedure is applied to a Mg97Zn1Y2 alloy with a long-period stacking ordered (LPSO) phase. The procedure involves three processes: cooling-rate-controlled solidification, chipping of the solidified master alloy, and extrusion for chip/ribbon-consolidation. Three types of chip/ribbon-consolidated alloys are fabricated from gravity-cast ingots, twin-roll-cast sheets, and melt-spun ribbons using this procedure and are denoted as GCC, TCC, and RRC, respectively. The cooling rate in the cooling-rate-controlled solidification process strongly affects the grain size of the α-Mg matrix and the morphology of the LPSO phase; increasing the cooling rate promotes reduction of the dendrite arm spacing in addition to grain refinement. Extrusion during chip/ribbon-consolidation promotes dynamic recrystallization of α-Mg grains, resulting in the formation of fine equiaxed grains with random crystallographic orientation. The GCC alloy and the TCC alloy consist of fine dynamically recrystallized α-Mg grains and a small amount of worked LPSO grains. The RRC alloy has fine dynamically recrystallized α-Mg grains with thin basal plate-shaped LPSO phase precipitates in their interior. The GCC alloy and the TCC alloy show large elongation with reasonable strength and slight work-hardening after yielding. By contrast, the RRC alloy shows a high strength of more than 450 MPa, but the flow stress decreases with increasing strain during tensile testing. The TCC alloy and the RRC alloy exhibit high-strain-rate superplasticity at a strain rate of 3 × 10-2 s-1 and extremely large elongation values of ~600% and ~1000%, respectively.