Materials Science Forum Vols. 488-489

Abstract: The effect of manganese on grain refinement of a commercial AZ31 alloy has been investigated using an Al-60%Mn master alloy splatter as an alloying additive at 730 °C in aluminium titanite crucibles. It is shown that grain refinement by manganese is readily achievable in AZ31. Electron microprobe analyses reveal that prior to the addition of extra manganese the majority of the intermetallic particles found in AZ31 are of the Al8Mn5 type. However, after the addition of extra manganese in the range from 0.1% to 0.8%, the predominant group of intermetallic particles changes to the metastable AlMn type. This leads to a hypothesis that the metastable AlMn intermetallic particles are more effective than Al8Mn5 as nucleation sites for magnesium grains. The hypothesis is supported by the observation that a long period of holding at 730 °C leads to an increase in grain size, due probably to the transformation of the metastable AlMn to the stable Al8Mn5. The hypothesis has also been used to understand the mechanism of grain refinement by superheating.

Abstract: Additions of grain refiners based on Ca, Ca/Si, Al3Ti and Al4C3 to pure magnesium and AZ31 alloy result in remarkable microstructure refinement. With proper amount of these refiners addition, the grain size in as-cast ingots is one order of magnitude lower than that without refiner addition. Small amount of refiner addition to alloy AZ31 increases both ultimate and yield strength significantly, and the ductility of the alloy is similar to that without refiner addition. Additions of refiner improve the deformation formability of the AZ31 alloy.The extruded and hot rolled specimens (rods or sheets) with refiner addition are crack free and exhibit higher surface quality and mechanical properties than that without refiner addition.

Abstract: Minor alloying elements such as RE, Sb, and Si were added to Mg-Al based commercial alloys and their effects on the mechanical properties, corrosion resistance, and castability were investigated.The result show that small amount of cerium-rich misch metal (RE) addition with antimony enhances the heat resistance of AZ91 alloy effectively without deteriorating other good properties. The addition of Si with RE was also found to improve the heat resistance and casting capabilities (fluidity and hot cracking resistance) of AM50 alloy.

Abstract: Effect of heat treatment on the microstructure and mechanical properties of high purity MA2-1(Mg-5wt.%Al-1wt.%Zn-0.4wt.%Mn) alloy sheet were investigated. X-ray diffraction analysis indicated that the microstructure of high purity MA2-1 alloy sheet annealed consisted of α-Mg solid solution, β (Mg17Al12) phase and Al-Mn phases such as Al6Mn and Al10Mn3. β phase dissolved into α-Mg solid solution during the solution treatment and formed supersaturated α-Mg solid solution. After aging at the temperatures of 423 K, 473 K and 523 K for 12 hours, β phase precipitated from the supersaturated α-Mg solid solution. Optical microscope observation found that the grain size of the MA2-1 alloy sheet became larger after heat treatment. As a result, the mechanical properties of the MA2-1 alloy sheet such as the tensile strength and yield strength declined after the heat treatment.

Abstract: The effects of Al, Zr additions on the microstructure and properties of a β matrix Mg-Li-Al-Zr alloy of hot forged, cold rolled and heat- treated states have been systematically investigated. The tensile properties of the alloy was tested, and microstructure was studied by X-ray diffraction and SEM methods. The results showed that the addition of zirconium in the aluminum containing Mg-Li alloy was effective for grain refinement. The alloy containing zirconium of rolled state exhibited higher strength and satisfied ductility compared with the alloy without zirconium. The existence of zirconium in the alloy was mainly in the form of Al-Zr compounds of Al2Zr andAl3Zr. The reason for less age-hardening effect may be caused by the capture of aluminum by zirconium and as a consequent by the lake of aluminum in the matrix.

Abstract: Microstructure and mechanical property changes brought by the addition of Nd element in Mg-5wt%Zn-2wt%Al alloy are investigated, results show Al element takes a priority to react with Nd element over Mg and Zn, forming binary phases Al2Nd or Al11Nd3, tensile test showed that the total elongation is improved by the addition of Nd, and the UTS is also elevated when the Nd content surpass 1wt%. With a larger Nd addition, the phase dominating grain boundary is Al11Nd3 phase.

Abstract: When a casting solidifies and cools down in a metallic mould, stresses and strains develop as a result of uneven temperature distributions, metal shrinkage and mould constraints. At ejection from the mould, the casting usually springs back slightly when the elastic part of the strain is released. However, due to the low flow stress at high temperatures it is likely that the casting also has experienced some plastic deformations, meaning that the dislocation density has increased. This paper discusses how the existence of remaining plastic deformations affects the initial flow behavior and the yield strength during tensile testing of as cast Mg-Al alloys with different degrees of mould constraints.

Abstract: Recent experiments and first-principles calculations in the literature revealed the existence of a C36 laves phase in the Al2Ca-Mg2Ca pseudo-binary system in addition to the C14-Mg2Ca and C15-Al2Ca laves phases. In the present work, special quasirandom structures (SQS) for all three laves phases were constructed. The structures possess local pair and multisite correlation functions that mimic those of the corresponding random structures. First-principles calculations were carried out based on the SQS developed to predict the enthalpy of formation in the Al2Ca-Mg2Ca pseudo-binary system. It was observed that the enthalpy of formation of C36 is very close to that of C14 at the Mg2Ca end and decreases with the addition of small amount of Al, while the enthalpy of formation of C14 increases with the addition of Al. It is thus energetically plausible that C36 is stable in the Al2C- Mg2Ca pseudo-binary system.

Abstract: The room temperature tensile strength and ductility of Mg-3.3%Li alloy were improved simultaneously by two kinds of different equal channel angular pressing (ECAP) treatments. Microstructural analyses showed that grain refinement and texture modification are the principal reasons for the improvement of mechanical properties.