Effect of the volume fraction of the icosahedral phase on the microstructures, hot compressive behaviors and processing maps of Mg-Zn-Y alloys

The effect of the volume fraction (0.6-8.4 vol %) of the icosahedral phase (I-phase, Mg3Zn6Y1) on the microstructure, hot deformation mechanism and hot workability of cast Mg-Y-Zn alloys was studied. As the volume fraction of I-phase increased, the fraction of dynamically recrystallized (DRXed) grains increased, indicating that I-phase promoted dynamic recrystallization (DRX) by providing the preferred nucleation sites for DRXed grains through particle stimulated nucleation mechanism. There was, however, a critical amount of I-phase (about 5 vol %) beyond which the fraction of DRXed grains decreased. This was attributed to the formation of relatively coarse divorced eutectic I-phase and lamellar I-phase structures at the volume fraction of I-phase ≥5%. This result indicates that besides the amount of I-phase, the size of I-phase structure is another important factor affecting DRX. Processing maps were constructured to evaluate the hot workability of the alloys with different volume fraction of I-phase. The hot workability increased as the volume fraction of I-phase increased up to about 5 vol % but decreased afterward, confirming that DRX and hot workability are correlated with each other. The hot workability increases with increasing I-phase amount because the occurrence of extensive DRX delays the onset of power-law breakdown and flow instability to higher strain rates and lower temperatures.