Materials Science Forum Vols. 488-489

Abstract: Alloy phases in the as-aged (T6) ZK60 magnesium alloy were investigated. More attentions were paid to types and morphologies of main alloy phases in the as-aged ZK60 magnesium alloy. The experimental results indicated that there was an abundant flower-shaped segregation in the solution-treated ZK60 magnesium alloy. A small quantity of undissolved compounds, MgZn2, was also found. They distributed irregularly in the form of similar parallelogram, with a dimension of 200nm~500nm and not sensitive to heat-treatment. Mg2Zn3 was found to be a main secondary compound in the as-aged ZK60 alloy, which precipitated during aging. Another precipitation phase, MgZn, in the as-aged ZK60 magnesium alloy was also found in the form of long and thin rods with a dimension of 500nm.

Abstract: Depending on its initial texture and external strain condition differences in deformation mechanisms, kinetics of dynamic recrystallization or even superplastic behaviors may emerge in magnesium alloys leading to distinct microstructure and texture evolutions. When imposed strain condition is altered, e.g. from plane strain compression to rolling or uniaxial compression, the deformation anisotropy will decrease in different rates and basal slip and {1012} twinning will dominate deformation process. This work examines this strain sensibility by inspecting the σ-ε curves, microstructures and textures in a quasi plane-strain compressed ZK60 alloy and compares the results with those of AZ31 alloy compressed in channel die.

Abstract: The microstructure of three dilute Mg-Mn and two dilute Mg-Zr alloys which had been heat treated at different temperatures and times were studied using optical microscopy and various electron optical techniques, including FEGSEM and TEM. It was found that the Mg-Zr alloys homogenized at 500°C and annealed at 350°C contained a fine dispersion of Zr-containing particles at grain boundaries and within grains. In contrast, annealing Mg-Zr alloys at 300°C for up to 3 h led to little modification of the as-cast structures. When the Mn content was less than 0.9 weight percent, homogenization of the Mg-Mn alloys at 550 to 600°C resulted in the dissolution of small rod-like and needle-shape particles, which then grown up as fine particles when aged at 300°C. Mg- 0.6Mn and Mg-0.9Mn alloys annealed at 300 to 400°C without solution treatment contained a large volume fraction of nano-sized precipitates.

Abstract: Orientation-controlled Mg single crystals were tensile tested at temperatures between 473 K and 673 K at a strain rate of 4.2 x 10-4 s-1 in vacuum. Though all the single crystals showed high ductility compared with that of polycrystals, the ductility of the single crystals strongly depended on the crystal orientation. The [27 -1 -26 1] single crystal showed 0.57 fracture strain, while the [3 8 -11 -1] single crystal showed superplastic behavior of ductility over 1.8 strain. The observed strong orientation dependence of ductility seemed to be caused by orientation dependence of ease occurrence of dynamic recrystallization (DRX) in the single crystals during high-temperature deformation. The orientation dependence of ductility of Mg single crystals will be discussed in detail concerning crystallographical orientations of the single crystals, occurrence of DRX and fracture.

Abstract: The effects of silicon and AlP on microstructure and properties of Mg-8Zn-4Al-0.3Mn alloy were studied. When 0.96%Si was added in the base alloy, Chinese script Mg2Si formed, the precipitation of τphase was restrained while the precipitation of φ phase was facilitated for decreasing of the concentration of solute in the residual liquid phase, at the same time, the fluidity was remarkably improved. To modify Chinese script type Mg2Si, a new green modificator--Al-AlP master alloy was added in Mg-8Zn-4Al-0.3Mn-1Si alloy. It showed that AlP addition promoted the formation of fine polygonal type Mg2Si particles by providing the nucleation sites, refined the grains, facilitated the precipitation of τphase while inhibited the precipitation of φ phase by changing the concentration of solute in the residual liquid phase and shortening the diffusion distance of the atoms. The improved microstructure of the modified alloy resulted in the large improvement in tensile properties at both ambient temperature and 150 as compared to the base alloy.

Abstract: The microstructure evolution of magnesium alloy AZ91D solidified with different electric current pulses and cooling rates was investigated and a new method, Low-voltage Electric Current Pulses (LVECP), to produce semi-slurry magnesium alloy was developed in this paper. The experimental results showed that the electric current pulses during solidification changed morphology of dendrites and the equiaxed, non-dendritic grains formed. The size of the primary a-Mg particles in semi-solid AZ91D alloy and the sphericity of the particles decreased with increase of discharging the voltage and treating time of LVECP. The increase of the cooling rate during the solidification of AZ91D alloy with LVECP promoted the formation of finer a-Mg particles, but the value of the sphericity of the particles rised. The formation of equiaxed, nondendritic structure by LVECP might be attributed to the electric current pulses increase the nucleation rate, restrained growth of the dendrites, and made dendrite arms remelted during the solidification of AZ91D alloy.

Abstract: The morphology and crystallography of precipitates formed in three Mg-Al-Zn alloys containing various Al/Zn ratios, conventionally designated as AZ91, AZ64 and AZ46, were characterized, and the mechanical properties determined. Three morphologically and crystallographically different types of lath- or rod-shaped γ-(Mg17All2) precipitates were identified in AZ91, and needle-like η-(MgZn2) in addition to γ-(Mg17All2) occurred in the alloys containing higher Zn. With increasing Zn content, i.e. with decreasing Al/Zn ratio, the fraction of γ-(Mg17All2) decreased, while that of η-(MgZn2) increased. The peak aged hardness and tensile properties of the alloys were slightly decreased, rather than increased, with increasing the Zn content from AZ91 through AZ64 to AZ46. The mechanism by which the γ-(Mg17All2) and η-(MgZn2) precipitates affected the age strengthening of the alloys was discussed in terms of the morphology, crystallography, interfacial structure, and relative quantity of the precipitates, and of their ability to block dislocation basal slip in the Mg-rich matrix.

Abstract: Bulk armorphous Mg65Cu25Gd10 alloy of 12mm rods were prepared by water quenching the molten alloy in the stainless steel tube. Neither cavities nor voids are seen over the whole range and no contrast revealing a crystalline phase is seen over the transverse cross section. The glass transition temperature (Tg), crystallization temperature (Tx), the melting temperature (Tm) and the liquidus temperature (Tl) are measured to be 419K, 492K, 702K, 735K, respectively, for the Mg65Cu25Gd10 alloy with a diameter of 12mm. These temperatures are similar with those of the alloy with a diameter of 8mm produced by metallic mould casting. Both alloys have the same Vickers hardness about 260. Water quenching can further improve the critical diameter of glassy rods, so it is qualified for the formation of the bulk alloys.

Abstract: The deformation behavior of AZ31 magnesium alloy has been investigated by isothermal compression at temperatures between 573-723K and at constant strain rates ranging from 10-3 -1s-1. It is shown that the form of flow stress curves is very sensitive to temperature and strain rate. In the experimental domain studied, the flow stresses are modeled using a power law with an average activation energy of 145.16 kJ/mol, and dynamic recrystallization (DRX) occurs. The critical strain for DRX is determined by analysis of flow stress curves. The ratio of the critical strain to the peak strain falls in the range of 0.4-0.5. At low temperatures and high strai rates, the deformation become macroscopically inhomogeneous, and the fracture of the specimens is accompanied by shear banding. Grain refinement resulting from DRX is less effective at high temperatures due to rapid grain growth. It is also shown that there is no difference between peak stress and stable state stress at high temperatures and lower strain rates, presenting the feature of continuous dynamic recrystallization (CDRX).

Abstract: Various amounts of antimony and mischmetal (Ce-rich) were added into AZ91 cast magnesium alloy and the microstructural characteristics of the cast magnesium alloys were studied by optical microscopy, electronic microscopy and X- ray diffraction techniques. Both antimony and mischmetal had grain refinement effect to AZ91 alloy. When they were added together, a further refinement was observed. SEM-EDX and XRD analysis indicated that a plate-shaped phase, Al11RE3, formed at the boundaries of dendritic α-Mg grain. A rod-like Mg3Sb2 compound distributed both within α -Mg dendrites and along grain boundaries. A new spherical particle, ReSb, formed in the magnesium alloy when 0.8wt%RE and 0.4wt%Sb were join added. The individual additions of antimony or mischmetal to AZ91 alloy did not result in a noticeable improvement to its room temperature tensile properties, but when 0.8wt%RE and 0.4wt%Sb were added together, the as-cast tensile properties of the alloy were increased dramatically.