Effect of Yttrium Addition on Microstructural Characteristics and Superplastic Behavior of Friction Stir Processed ZK60 Alloy

As-extruded ZK60 and ZK60-Y magnesium alloy plates were successfully processed via friction stir processing (FSP) at a tool rotation rate of 1600 r/min and a traverse speed of 200 mm/min. FSP resulted in the formation of equiaxed recrystallized microstructures with the average grain sizes of ∼8.5 and ∼4.7 μm in the ZK60 and ZK60-Y alloys, respectively. Moreover, FSP broke and dispersed the MgZn2 and W-phase (Mg3Zn3Y2) particles and dissolved MgZn2 phase in the FSP ZK60 alloy. With the addition of rare earth element yttrium (Y) into the ZK60 alloy, the ratio of the high angle grain boundaries (HAGBs) in the FSP alloys increased from 64% to 90%, and a certain amount of twins appeared in the FSP ZK60-Y alloy. The maximum elongation of 1200% and optimum strain rate of 3 × 10−3 s−1 achieved at 450 °C in the FSP ZK60-Y alloy were substantially higher than those of the FSP ZK60 alloy. This is attributed to the fine grains with high ratio of HAGBs and the distribution of a large number of dispersed second phase particles with high thermal stability in the FSP ZK60-Y alloy. Grain boundary sliding was identified as the primary deformation mechanism in the FSP ZK60 and ZK60-Y alloys from the superplastic data analyses and surficial morphology observations.