Successful transition from low-temperature superplasticity to high-strain-rate superplasticity with increasing temperature in an ultrafine-grained Mg–Y–Zn–Zr alloy

Abstract

Simultaneous achievement of low-temperature superplasticity (LTSP) below 0.5Tm (Tm: melting temperature) and high-strain-rate superplasticity (HSRS) above 0.5Tm from the identically processed material is advantageous for industrial applications. Severe plastic deformation by high-ratio differential speed rolling (HRDSR) with different roll speed ratios (2 and 3) was applied to cast Mg–Y–Zn–Zr alloys with the eutectic icosahedral phase (I-phase) structure. Two types of microstructures were obtained, depending on the roll speed ratio. The alloy processed at a speed ratio of 3 (HRDSR3) had an ultrafine-grained microstructure (∼1 μm) and finely broken eutectic I-phase particles. The alloy processed at a speed ratio of 2 (HRDSR2) also had an ultrafine grained microstructure, but with a higher fraction of non-fully fragmented original grains and a lower density of fine I-phase particles (< 1 μm in size). In both alloys, many I-phase particles were agglomerated. Below 523 K, both materials had good LTSP but HRDSR3 showed a better LTSP than HRDSR2. This was because HRDSR3 had a higher fraction of ultrafine grained area participating in grain boundary sliding and a higher density of fine I-phase particles contributing to the pinning effect on grain growth. Above 573 K, however, HRDSR2 showed HSRS but HRDSR3 failed to show HSRS. This resulted because thermal stability of ultrafine grained structure of HRDSR3 became poorer than that of HRDSR2 as the temperature increased above 0.5Tm. When solute solubility increased with temperature, the density of the small I-phase particles (<1 μm) exerting the effective pinning effect on grain boundary migration decreased at a more rapid rate in HRDSR3 than in HRDSR2 during the sample heating and holding duration. This was because a more significant reduction in particle size by higher shear resulted in a higher rate of dissolution of particles into matrix upon temperature increase. Retension of fine grains during the sample heating and holding duration was important in achieving HSRS because it allowed for active occurrence of grain boundardy sliding during deformation. Grain boundary sliding promoted progressive dispersion of agglomerated I-phase particles into the matrix, which was equally important in achieving HSRS because as the I-phase particles were more uniformly dispersed during deformation, the pinning effect of the particles increased, leading to a more effective suppression of dynamic grain growth of the fine grains. The obtained results suggest the importance of control on the size of I-phase particles and grains in achieving LTSP and HSRS from the identically processed Mg alloys in the low and high temperature ranges, respectively. The deformation behaviors of HRDSR2 and HRDSR3 as well as the condition for the successful transition from LSPT to HSRS with increasing temperature could be explained by using the model that assumes that the two deformation mechanisms of grain-size sensitive grain boundary sliding and grain-size insensitive dislocation climb creep compete each other during plastic deformation.