Materials Science Forum Vols. 546-549

Abstract: Tensile creep behaviour of the die-cast magnesium alloys AZ91 and AE42 has been studied at temperatures between 85oC and 200oC and at stresses in the range from 30 to 100 MPa. Microstructural investigations, mainly by TEM, have been performed on the selected crept samples to characterise the microstructural change during the plastic deformation, which reveals several phenomena related to the creep process including formation of dislocation sources, denuded zones around grain boundaries and microvoids, and changes in the nature of intermetallic phases. The active creep mechanisms have been discussed on the basis of the creep data in combination with the microstructural change during creep.

Abstract: Some differences between the growth behaviors of small fatigue crack of cast AM50 magnesium alloy at different elevated temperature and at open and closed states were investigated based on in-situ observations with scanning electron microscope (SEM). These results indicate that the growth rates of small fatigue cracks depend on not only the stress levels but also the elevated temperature and crack states. The fatigue crack growth rates were estimated based on novel and conventional methods.

Abstract: The stress-strain behavior of cast magnesium alloy (AM60) was investigated by strain-controlled cyclic testing carried out on MTS. In order to describe the cyclic stress and strain properties of AM60 by means of the energy storing characteristics of microstructure during irreversible deformation, a plastic constitutive model with no yielding surface was developed for single crystal by adopting a spring-dashpot mechanical system. Plastic dashpots reflecting the material transient response were introduced to describe the plasticity of slip systems. By utilizing the KBW self-consistent theory, a polycrystalline plastic constitutive model for Magnesium alloy was formed. The numerical analysis in the corresponding algorithm is greatly simplified as no process of searching for the activation of the slip systems and slip directions is required. The cyclic stress-strain behavior, based on this model, is discussed. The simulation results show good agreement with the experimental data for AM60.

Abstract: The dynamic stress-strain behavior of the AZ91 alloys in different treatment conditions (as-cast, T4 and T6) was investigated by means of split Hopkinson pressure bar. It was found that the flow stress increased at first, and then declined with the strain rate increasing at the range of 102~103s-1 for the alloys in these three conditions. And the alloys exhibited both positive and negative strain rate effects. The former was caused by strain rate strengthening and the latter was caused by strain rate weakening. However the flow stress for the alloy in aged condition at the same strain rate was higher than both of the alloys in as-cast and solution conditions. The study also showed that the maximum strains of the alloys in different conditions increased with the strain rate increasing, and the strain rate to fracture for the alloy in solution condition was higher than those of other two alloys. The work-hardening of α–Mg matrix and the reinforcement of β-Mg17Al12 phases led to the strengthening of the alloy, while thermal softening of matrix, the fracture of β phases and initiation and propagation of the cracks were responsible for the weakening of the alloy.

Abstract: The evaluating rules for semi-solid microstructure were brought up, and the analysis software was developed on MATLAB 7.0 platform in this study. Furthermore, this system can be used not only in the Thixomolding AZ91D magnesium alloy microstructure analysis, but also in the other semi-solid processing microstructure.

Abstract: The microstructure of Mg-3wt%Nd-0.2wt%Zn-0.4wt%Zr (NZ30K) alloy after solution treatment was investigated and several kinds of interesting phases containing zirconium were found in this study. NZ30K was gravity cast using permanent die casting. After high tempering solution treatment at 540°C, cluster particles were observed inside the grains under optical microscopy. The detailed investigations were carried out on transmission electron microscopy (TEM). Four kinds of precipitates were visible inside grains with different shapes: block-like, globular, short rod-like, long rod-like. The block-like particle was identified as ZrH2 phase and the globular one was Zn2Zr3 phase. The other two were still Zr-containing phases, which could not be identified to any of existed compounds containing Zr. The formation of Zn-Zr compounds would probably be due to relative high ratio of Zr to Zn in the center of grains. ZrH2 would be the results of reaction of zirconium with H element during solution treatment, which probably came into the alloy during melting.

Abstract: The microstructure, mechanical properties, creep and corrosion resistance of Mg-Gd-Y-Zr(-Ca) alloys were studied. Small additions of 0.4-0.6 wt% Ca to Mg-(9-10)Gd-3Y-0.4Zr(wt.%) alloys led to a slight improvement in creep resistance and a remarkable increase in corrosion resistance, but an obvious decrease in elongation to fracture. UTS and TYS of the Mg-Gd-Y-Zr(-Ca) alloys are obviously higher than those of WE54, especially in the temperature range from room temperature to 200 oC. TEM images and corresponding energy dispersive x-ray spectra showed that the Ca element primarily segregated to the grain boundaries and existed in the cuboid-shaped particles with a trace concentration, and the small addition of Ca had no obvious effect on the orientation, morphology, and distribution of β′ phase, which is responsible for the peak hardness in Mg-Gd-Y-Zr alloys.

Abstract: Microstructure and mechanical properties of Mg-Zn-Er-Zr alloys were characterized in detail. The grain size of as-cast Mg-Zn-Er-Zr alloy was greatly decreased by the Mg-Zn-Er phases formed at grain boundaries. The addition of Er can increase the yield strength (YS) but decrease the ultimate tensile strength (UTS) and elongation of as-cast Mg-Zn-Zr alloy. The thermally stable Mg-Zn-Er phases were just partially dissolved into the matrix during solution treatment. And the addition of Er can prolong the precipitation process of Mg-Zn-Zr alloy. Solution-plus-ageing treatment can increase the strength of both the Mg-Zn-Zr and Mg-Zn-Er-Zr alloys, but the strengthening effect of Mg-Zn-Er-Zr alloy was greatly weakened, for the incompletely solution of Mg-Zn-Er phases. Er can greatly enhance the high temperature elongation of Mg-Zn-Zr alloy, but the increase of high temperature tensile strength was just a little.

Abstract: Physical and mathematical models of microstructure evolution during the solidification process of die casting AZ91D alloy were investigated in this paper. Coupled with solute concentration, a modified three-dimensional cellular automaton (CA) model was proposed. Considering the solute enrichment and the formation of eutectic microstructure, these models can reproduce the whole microstructure evolution process of Mg alloy, from the formation of primary phase to the eutectic transformation. The microstructure of the AZ91D alloy cylinder head cover die casting was simulated with the proposed models. The simulated results are in agreement with the experimental.

Abstract: This paper analyzes the casting process characteristics of Mg alloys compared with Al alloys in the production of high quality (i.e. structural) structural vehicle components. The disadvantages of traditional squeeze casting for high quality Mg components has led the authors to develop a novel squeeze casting process which overcomes the drawbacks of the current methods. The details of the new process are presented here and show the increased productivity and quality of castings that can result from changing the way molten metal is handled and processed. The feasibility of this new squeeze casting process was applied to Mg wheels for LX150 motorbikes.