Superplastic behavior of an ultrafine-grained Mg-13Zn-1.55Y alloy with a high volume fraction of icosahedral phases prepared by high-ratio differential speed rolling

An ultrafine-grained (UFG) Mg-13Zn-1.55Y alloy (ZW132) with a high volume fraction (7.4%) of icosahedral phase (I-phase, Mg3Zn6Y) particles was prepared by applying high-ratio differential speed rolling (HRDSR) on the cast microstructure following homogenization. The alloy exhibited excellent superplasticity at low temperatures (tensile elongations of 455% and 1021% 473 K- 10−3 s−1 and 523 K- 10−3 s−1, respectively). Compared with UFG Mg-9.25Zn-1.66Y alloy (ZW92) with a lower volume fraction of I-phase particles (4.1%), which was prepared using the same processing routes, the UFG ZW132 alloy exhibited a higher thermal stability of grain size. Rapid grain coarsening, however, occurred at temperatures beyond 523 K, leading to a loss of superplasticity. The high-temperature deformation behavior of the HRDSR-processed ZW132 alloy could be well described assuming that the mechanisms of grain boundary sliding and dislocation climb creep competed with each other and considering that the grain-size was largely increased by accelerated grain growth at the temperatures beyond 523 K.