Materials Science Forum Vols. 546-549

Abstract: The present work studied the precipitate microstructures in as-cast Mg-Zn-Y-Zr alloys. The experimental result showed that there is significant number of small precipitates within the grains besides the icosahedral quasicrystals along the grain boundaries. Among these precipitates, a new phase has been identified. The new phase displays square morphologies with the size in the range of 200 nm to 2 μm. According to the energy dispersive spectrometer (EDS), this phase could be a metallic Y-riched compound. The diffraction patterns can be indexed with an f.c.c. structure with the lattice parameter a = 0.52±0.1 nm. The structure does not agree with any precipitate structures that have been reported from the previous studies of Mg-Zn-Y alloys.

Abstract: Electromagnetic Vibration Casting (EVC) is a new effective technical processing for optimizing solidification microstructures of light alloys. In this paper, this technology was induced into magnesium alloy DC casting for providing high-quality large diameter billets. The effects of vibration frequency and current intensities on the microstructures of large diameter billets were investigated. The results indicate that electromagnetic vibration would refine grains and the reduce macrosegregation effectively.

Abstract: A series of alloys were designed and prepared using vacuum melting furnace. The as-cast microstructure and phase compositions of these alloys were characterized by optical microscope, X-ray diffraction (XRD) and scanning electronic microscope (SEM). The hardness and mechanical properties of alloys at room temperature were measured as well. The results show that the addition of RE leads to the formation of intermetallic Al3La distributed within grain boundaries. The intermetallic compound with oriented structure impedes grain growth, and refines the grain. The strength and the hardness of the alloys increase with the increase of the RE content in the range of 0.2-0.8 wt %.

Abstract: The effects of non-flux purification on the mechanical properties, microstructure and fracture pattern of AZ31+xCa magnesium alloys have been studied. The results show that the purification process of ceramic foam filters incorporating gas bubbling process can improve obviously the ultimate tensile strength σb and elongation δ of AZ31 alloy. With 20ppi MgO filter incorporating gas bubbling treatment under Ar flow rate of 2 l/min and temperature of 730°C, the σb and δ of AZ31 can be improved from 144.6MPa and 5.21% to 180.8MPa and 9.06% by 25% and 73.9%, respectively. Addition of 0.13%Ca to magnesium can not only increase the strength but also improve the elongation. With the further increase of more than 0.13% Ca content, the strength and elongation decrease. The experimental results show that under low Ca content, filtration purification process can greatly improve the σb and δ of AZ31+xCa alloy. However, with the increase of Ca content, the filtration effects decrease. The effective sequence of different filters is as follows: 20ppi MgO>20ppi Al2O3>10ppi MgO>10ppi Al2O3.

Abstract: Casting magnesium alloys are heterogeneous materials containing numerous voids. Assuming the voids are spherical, in the present investigation, the evolution equations of the growth and nucleation of the voids have been presented. Combining the evolution equation of the void growth with that of the void nucleation, the evolution rule of the voids was obtained. Based on the void evolution rule a nonclassical elastoplastic constitutive model involving void evolution was developed. The corresponding numerical algorithm and finite element procedure were developed and applied to the analyses of the elastoplastic response and the porosity of casting magnesium alloy. The calculated results show the satisfactory agreement with experiments.

Abstract: Magnesium alloys and very light metals that can be used for structural application. A variety of Mg-Li-Zn alloys including Mg-11%Li-1%Zn, Mg-9%Li-1%Zn, Mg-9%Li-1%Zn-0.2%Mn, Mg-9%Li-1%Zn-1%Al-0.2%Mn and Mg-9%Li-3%Al-1%Zn-0.2%Mn were processed by equal channel angular extrusion (ECAE) to see how effective is this process in refining the grain structure. After 4 passes of the ECAE process, TEM studied the microstructural diversifications, especially showing the existence of compounds, MgLiZn, MgZn2 and ZnO. The room temperature and high temperature mechanical properties were also investigated. After the ECAE process, the room temperature strength was found enhanced at a modest cost of reduction in elongation.

Abstract: In this study, AZ31 in form of sheet, plate and extruded rod and AZ61 wire with different initial grain sizes were used to investigate the effect of initial grain size on recrystallization behavior and the formation of fine recrystallized grain (the so-called necklace) structure. Nucleation and growth of recrystallized grains along grain boundaries has been examined. In order to observe the effect of initial grain size and deformation on static recrsytallization and necklace formation, the specimens were annealed at 100-250°C for 10min--2hr. Specimens are also stretched to a total strain of 10, 20 and 40% at 300°C at an initial cross head speed of 1x10-3 s-1 for dynamic recrystallization studies. The results suggest that there exists a grain size limit, below which a necklace structure is not observed.

Abstract: The elevated temperature mechanical behavior of Mg-Y-Zn alloys was investigated. It was found that the extruded Mg-Y-Zn alloy exhibited excellent mechanical properties both at ambient temperature and elevated temperature. With the increase of tensile temperature, the ultimate tensile strengths of Mg-Y-Zn alloys decreased and their elongations increased. The ultimate tensile strengths increased and elongations decreased with the increase of yttrium content. However, a gradual increase in the ultimate tensile strength and elongation both at ambient temperature and elevated temperature was obtained by increasing both yttrium and zinc contents. The fracture modes of Mg-Y-Zn alloys at different tensile temperature were also investigated.

Abstract: Rolling and punching techniques of AZ31 alloy were investigated in this paper. Various rolling experiments were carried out to make fine-grained Mg sheets. Punching tests were conducted at the temperatures range from 70 to 300 oC. The analysis revealed that there existed an excellent warm forming temperature for as-rolled AZ31 alloy. A warm deep punching tool setup using heating elements was designed and manufactured to produce the cell phone. Microstructures were observed using optical and scanning electron microscope equipped with EBSD. The textures in as-rolled and as-annealed specimens attribute to different mechanical properties along the various direction.