Papers by Author: Wei Neng Tang

Abstract: Rolling processing has been carried out on samples cut from the extruded Mg-9Gd-3Y-0.5Zr seamless tubes. Effects of rolling reduction, 5%, 20% and 70% per pass, on its microstructure and mechanical properties were investigated. The results showed that the strength and ductility varied with increasing rolling passes at different rolling reduction. The strength of the alloy rolled by 5% reduction per pass gradually improved with increasing rolling passes, and its ductility remained basically constant. However, when 20% reduction per pass was applied, the strength and ductility of the alloy after rolling increased at first and then decreased a little after the accumulative strain higher than 52%. Moreover, as reduction reached 70% per pass, macro-cracks were induced when rolling at 420°C, while the samples were rolled at a high temperature of 500°C and a larger reduction of 70% per pass exhibited the mechanical properties comparable to those fabricated by 5% and 20% reduction. This indicated that a relatively higher productivity via rolling as well adequate mechanical properties can reach for the large scale of industrial products.

Abstract: Evolution of microstructure and mechanical properties of the Mg-9Gd-3Y-0.5Zr base alloys without 0.5% Zn addition (GWK930) and with Zn addition (GWZK9300) have been investigated during processing by extrusion, rolling and ageing. It was found that the yield/ultimate strength of the two as-rolled alloys increased with the rolling strains. Twinning was the main deformation mechanism in GWK930 alloy while few twinning could be detected in the GWZK9300 alloy due to the dense secondary phases such as LPSO structure. The (0002) texture intensity in two as-rolled alloys increased with the rolling reduction, however, a typical rolling basal plane texture with single peak was found in GWZK9300 while a bimodal texture was found in the GWK930 alloy. Compared with GWK930 alloy, GWZK9300 alloy exhibited higher strength and ductility after rolling, annealing and ageing treatments. The finer grain size was due to more recrystallization fraction, while more fine secondary phases dispersed in matrix. And the basal texture strengthening after rolling deformation and subsequent heat treatment are suggested to be responsible for the superior comprehensive mechanical properties of GWZK9300 alloy.

Abstract: The flow behavior and microstructural evolution of the ZW61 (Mg-6Zn-0.6Y-0.5Zr ) alloy during uniaxial compressive deformation at temperatures of 250-400°C and strain rates of 0.5-0.001s^-1.were investigated. The results indicated that the flow stress could be described with a power law equation related to the temperature and strain rate. Furthermore, the deformation microstructures at different strain rates and temperatures were different. Microstructural evolution deformed under 350 °C and 0.001s^-1 found that the twinning and different modes of slip systems were selectively activated during deformation, and that misorientation in some grains increased with lattice rotation during hot deformation, which resulted in the gradual formation of new grain boundaries. In addition, dynamic recrystallization (DRX) preferably took place nearby the boundaries of original grains/twinning/slipping bands and coarse particles, which resulted in an inhomogeneous deformation microstructure, i.e. necklace microstructure.

Abstract: Superplastic deformation (SPD) behaviors of two fine-grained materials produced by ECAE and hot rolling methods have been contrastively studied in this paper. It is found that the optimum superplastic condition in as-ECAEed material was at 350°C and 1.7×10-3s-1 with elongation to failure about 800%; while in as-rolled material, the largest elongation to failure about 1000% was obtained at 480°C and 5.02×10-4s-1. Microstructure observation showed that grain evolution and cavitation behavior were different in these two materials during superplastic deformation. The controlled mechanisms for superplasticity, i.e. grain boundary sliding (GBS), dislocation creep and diffusional creep, at different deformation conditions were discussed in terms of strain rate sensitivity coefficient, stress exponent and activity energy.

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.

Abstract: The variation of the flow stress and the microstructural evolution during hot deformation at different test temperatures and strain-rates are studied. During hot deformation, dynamical recrystallization did not occur completely; after annealing at 523K, microstructural changes due to static recrystallization and small grain sizes of several micrometers and fine nanometer-precipitates can be attained. The stress exponent (n) and the activation energy (Q) for high temperature deformation have been evaluated and the deformation mechanisms at different temperature have also been discussed.