Study of heterostructure composition by regulating lamellar LPSO phase and the related strengthening mechanism in the Mg-Gd-Y-Zn-Zr alloy

The heterostructure preparation in Mg-rare earth (RE) alloy has attracted much attention due to the excellent enhancement of strength and ductility. However, the effect of heterostructure composition on mechanical properties in Mg-RE alloy is still not clear. In this work, three types of heterostructures with different composition induced by lamellar 14H long period stacking ordered (LPSO) phase were achieved in the Mg-Gd-Y-Zn-Zr alloys after cyclic extrusion and compression (CEC). The heterostructure was mainly composed of dynamic recrystallization (DRX) grains, deformed coarse grains, multiscale LPSO phase (blocky, granular, lamellar LPSO phase). The strength and ductility of Mg-Gd-Y-Zn-Zr alloy with heterostructure were simultaneously improved. The DRX behavior during CEC process was largely affected by the lamellar LPSO phase. The lamellar LPSO with large spacing (~92 nm) and low thickness (~13.46 nm) is easy to occur kinking behavior and the zigzag kinking area can serve as nucleation sites to promote DRX behavior. While the lamellar LPSO phase with high thickness (~23.41 nm) and similar spacing (~82 nm) was ruptured into granular LPSO phase and thus increase the volume fraction of granular LPSO phase, which made a great contribution to DRX behavior by particle stimulated nucleation. The main deformation mechanism of solution treatment + furnace cooling (SF) sample during CEC process is dominated by the multiple slips composed of basal slips, prismatic slips and pyramidal slips. For the solution treatment + air cooling (SA) sample and solution treatment + ageing treatment (ST) sample, the activation of basal slips is the critical deformation mechanism. The main contribution to yield strength is from the grain boundary, dislocation and hetero-deformation induced (HDI) strengthening. Moreover, the HDI strengthening in the SF and SA sample after CEC deformation is much larger than that of ST sample due to the distinct heterostructure composition.