Materials Science Forum Vols. 610-613

Abstract: The effect of solid solution treatment at temperature of 415oC and different holding time prior aging at 168oC for 16 hours on the mechanical properties and microstructure of AZ81-4Gd magnesium alloy were analyzed by X-ray and SEM. The results demonstrated that with increasing the solid solution time the primary β-Mg17Al12 phase dissolve in the matrix eventually. While the holding time increases to 20 hrs, the primary β-Mg17Al12 phase was completely vanished, but Al2Gd phase kept unchanged with increasing the holding time. In the mean while, the volume fraction of the β-Mg17Al12 phase precipitated during aging increased at first with increasing the holding time then decreased. Moreover, subgrains and twins began to form within the α grain matrix when the solid solution time increased to 12hrs, and the subgains parallele with each other or in the shape of oval. Increasing the solid solution time to 20 hrs the microstructure change dramatically and consist of very fine recrystallized α grains. Futher increasing the holding time the recrystallized grain size increased accordingly. The mechanical properties testing results showed that when the solid solution time increased to 24 hrs the ultimate tensile strength of AZ81-4Gd magnesium alloy was 270 MPa and the elongation was 11.6% respectively.

Abstract: The magnesium alloys have poor corrosion resistance because of their low equilibrium potential and loose oxide film, while the aluminum and its alloys have good corrosion resistance due to their compact oxide film. In this study, a kind of aluminum and magnesium bimetallic bars, 6063 aluminum alloy-clad AZ31B magnesium alloy bars, were acquired through hot extrusion. The experimental results showed that the extrusion temperature of 653K or lower, as well as an extrusion speed of 2m/min or lower be used during the extrusion. The interfacial structure and mechanical properties of the bimetallic bars were studied. The interface between aluminum alloy and magnesium alloy was metallurgic bonding with the shear bonding strength of 53Mpa for as extruded sample. As annealing temperature or holding time increased, the jointed layer between aluminum alloy and magnesium alloy was broadened, which decreased of shear bonding strength, indicating that the heat treatment is deleterious. The bimetallic bars have lower identity, good corrosion resistance and preferable mechanical properties, which are promising constructional materials.

Abstract: The mechanical properties and microstructure of the as-extruded ZM21 magnesium alloy and its modified alloy ZME210 with addition of 0.35wt% cerium were investigated with different extrusion ratios from 14 to 182 by using mechanical property test, microscopic structure quantitative analysis, SEM observation and energy spectrum analysis. The results showed that both ZM21 and ZME210 had an extrusion ratio limit for grain refining, and the grains were found to be finer with higher extrusion ratio when the ratio was not higher than the limit value. The extrusion ratio limit for the best effect for grain refining of ZME210 is lower than that of ZM21. It was found that the Ce can refine the grains effectively after hot extrusion with different extrusion ratios. The effects of Ce on the microstructure and mechanical properties were analyzed.

Abstract: The effect of Al contents on the properties and structures of AZ80, AZ91，AZ131 and AZ151 magnesium alloys has been studied. The experiments of homogenization treatment, hot extrusion, and annealing treatment are carried out in order to study the effects of these processes to the properties and structure of AZ80, AZ91, AZ131 and AZ151 magnesium alloys. The parameters of homogenization treatment (420°C×24h, 420°C×36h) determined by DSC and metallurgical photo are sufficient which ensure the eutectic phase melt into magnesium matrix greatly. The strength of extruded bars shortly after homogenization treatment increase (Rp0.2 from 232 to 310MPa), but elongation decrease (A from 12 to 2.5%) with Al contents. The differences and causes of structure and mechanical properties of extruded bar after the process of aging are analyzed in detail by tensile experiments, hardness measurement, and optical observation.

Abstract: Strain induced and melt activated (SIMA) is used for the preparation of semi-solid billet through equal channel angular extrusion (ECAE) as strain induced step in SIMA and completing melt activated step by semi-solid isothermal treatment, by which semi-solid billet with fine spherical grains can be prepared. As comparison, extruded ZW61 alloy is also remelted to the semi-solid state. Furthermore, both of the semi-solid isothermal treatment microstructures in as-cast and extruded ZW61 alloy are discussed. ECAE process is found more potential for semi-solid thixoforming for as-cast ZW61 alloy.

Abstract: The microstructures, mechanical properties, and formability of an Mg-1.5Zn-0.1Zr alloy with rare earth element of Erbium addition have been examined by using optical microscopy, scanning electron microscopy/energy dispersive X-ray spectroscopy, and thermal mechanical testing. The results showed that Erbium combined with Mg and impurity elements forming small particles dispersed on the matrix. Therm-mechanical simulation in the temperature range from 250°C to 400°C and strain rate from 0.1 to 1.0 indicated that the flow stress decreases with the increase of the temperature and the decrease of the strain rate. Sheet samples were prepared through ingot casting, hot rolling, cold rolling, and intermediate annealing. Remarkable grain size refinement was realized through recrystallization during the rolling process. Good combination of strength and ductility was achieved by applying intermediate annealing before the very last cold rolling. Further improvement of ductility and formability could be obtained by conducting proper heat treatment on the finishing cold-rolled sheets.

Abstract: The characteristic of dynamic recrystallization (DRX) in Mg-Y-Nd-Gd-Zr magnesium alloy had been investigated by compression test at temperatures between 523 and 723K and the strain rates ranging from 0.002 to 1s-1with maximum strain of 0.693. The flow behavior was described by a power exponent function. Processing map of this alloy was established on the basis of dynamic material model. Microstructure observations suggested that the peak value of dissipation factor was 0.36 at the temperature of 673K and the strain rate of 1s-1. The map exhibits flow instabilities as two domains, one is at the lower temperatures but higher strain rates, and the other is at higher temperatures and lower strains.The region at an intermediate temperature and a high strain rate is the region of the optimal mechanical working properties.

Abstract: The homogenization treatment, hot extrusion, and annealing treatment are carried out to study the effects of these processes on the mechanical properties and structure of AM140 magnesium alloy. The parameter of homogenization treatment (420°C× 24h) determined by DSC and metallurgical photo is sufficient which ensures the eutectic phase melt into magnesium matrix greatly. The Rm, Rp0.2 and A of billets after hot extrusion are 355MPa, 305MPa and 3.5% respectively. The variation of structure and mechanical properties during the process of aging is also studied by optical observation and tensile strength .

Abstract: The microstructures and mechanical properties of cold upsetting magnesium alloys were investigated upon anneal under different conditions. The results show that a large amount of twins were observed in the original grains of cold upsetting AZ31 magnesium alloys. The twins disappeared gradually and recrystal grains formed after anneal. The volume fraction of the recrystal grains increases as the strain of samples rises. Recrystal grain size grows large with the elevated annealing temperature. Recrystal grain size reduces at first and then grows as the annealing time is prolonged. In addition, compared with as-cast magnesium alloys, the yield strength of cold upsetting samples increase apparently due to grain refinement after anneals.

Abstract: After superplastic tensile tests and quantitative analysis of cavity volume fraction, the damage evolution equation based on the law of the micro-damage evolution and statistical mechanics was derived out. The characteristic parameters of damage evolvement and critical value of damage variables are achieved from the experiments.