Ultrafine-grained Mg–Zn–Zr alloy with high strength and high-strain-rate superplasticity

We report the feasibility of fabricating ultrafine-grained ZK60 magnesium alloy sheets with a combination of superior mechanical properties at room temperature and excellent superplasticity at high strain rates by means of high-ratio differential speed (HRDSR). The ZK60 alloy processed under the optimum HRDSR condition showed a high yield strength of 378 MPa and a reasonably high ductility (a total elongation of 12.3%). The major strengthening mechanisms could be attributed to the grain size and particle strengthening. Its microstructure was quite thermally stable up to 573 K, such that high strain rate superplasticity could be achieved at 10−2 and 10−1 s−1. The development of high-density shear bands that occurred uniformly over the entire volume of a sample and dynamic precipitation of spherical or irregular shaped particles with the sizes of 20–32 nm along the lamellar boundaries in the shear bands and on the boundaries of grains newly formed within the shear bands via continuous dynamic recrystallization, resulted in an ultrafine-grained microstructure with high thermal stability. The room-temperature mechanical properties and superplastic ability of the HRDSR-processed ZK60 were comparable to those of a magnesium alloy of a same or similar composition processed via powder metallurgy. Because the currently developed HRDSR technique is based on a rolling process and only a few processing steps are involved, its transfer to industry that demands high-performance magnesium alloy sheets is promising.