Papers by Author: Dao Kui Xu

Abstract: In this paper, the fatigue crack propagation (FCP) behaviors of the forged Mg-Zn-Y-Zr alloy with different tempers were investigated. At the stage 1 of the FCP, the
K-da/dN curves were distinguished from each other. However, at the stage 2 of the FCP, the
K-da/dN curves were almost the same. SEM fracture analysis indicated that at the stage 2 of FCP, the fracture characteristics of the forged and T5 samples were similar, with many deep dimples on the fracture surfaces, whereas the fracture surfaces of the T4 and T6 samples were covered with lots of lamellar cleavage planes. SEM observations also indicated that the microstructures were distinguished from each other. For the forged and T5 samples, many big bulk w-Mg3Y2Zn3 phases distributed in the Mg matrix. However, for the T4 and T6 samples, due to the solid solution treatment, w-Mg3Y2Zn3 phases were almost completely decomposed and many coarse rod-like  1 precipitates appeared in the interior of the grains. These firmly confirmed that the variation of the microstructures had great influence on the fatigue crack propagation (FCP) characteristics of the Mg-Zn-Y-Zr alloy.

Abstract: A conventional extrusion has been carried out to induce significant grain refinement in Mg-Zn-Y-Zr alloy. The results showed that good superplasticity have obtained in this extruded alloy. The simple and rapid processing route may allow it to be put into successfully practical use in industry. The effects of temperature and strain rate on superplastic deformation of the extruded alloy were studied. The optimum condition with the elongation of more than 450% was found to be at 450°C and 3.3×10-4s-1. Jump tests were carried out at 300-450°C and 8.3×10-5 ~ 1.7×10-2s-1. The activation energy for superplastic deformation at 300-450°C is 106kJ/mol and the stress exponent is about 2.8. The superplasticity observed in this studied condition may be attributed to mechanisms of dislocation creep mainly within large grains and grain boundary sliding (GBS) of small grains.

Abstract: The effects of Y addition to the Mg-Zn-Y-Zr alloy on the change of the microstructure and the mechanical properties (with the Y content range of 1 to 3 wt%) have been investigated. It shows that when Zn content is constant (5.65wt%), the alloys with Y content between 1.17 and 1.72wt% nearly reach its highest strength. With the composition near the optimums, the extruded Mg-6%Zn-1%Y-Zr alloy shows high strength and excellent ductility. The deformation behavior of this new alloy at high temperature has also been studied. Moreover, the super-long fatigue behavior of the Mg-6%Zn-1%Y-Zr alloy has also been tested, the results show the alloy with a high fatigue strength of about 85-90MPa in the super-long fatigue life regime of 1×109 cycles.