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In this study, calcium, Ca-, and strontium, Sr-added AZ91 magnesium alloys, which were composed of magnesium, Mg–9 wt.% aluminum, Al–0.8 wt.% zinc, Zn–x wt.% Ca–y wt.% Sr, were formulated and designated as AZXJ91xy alloys. The optimum composition of die-cast AZXJ alloys has been studied by micro-structural analysis and through the application of the constitutive equation of deformation behavior at elevated temperatures that includes the stacking fault energy. The normalized plots of the tensile test results of AZXJ alloys at 448 K by the constitutive equation of deformation behavior indicated that the deformation mechanism of the alloys was climb-controlled dislocation creep. The value of the constant A′ in the constitutive equation differs in the different AZXJ alloys, even though the effect of the solid solution of added elements, which is considered in the stacking fault energy term, was eliminated from A′. Using A′ as an indicator parameter of the creep resistance with the tensile strength at ambient temperature, the optimum composition of the creep-resistant Mg–Al–Zn–Ca–Sr alloy has been determined, and its creep resistance was comparable to those of known heat-resistant magnesium alloys.

The effect of Ca and Sr content on the microstructure and mechanical properties of a cast AZ91 magnesium alloy is investigated. Ca and Sr additions in AZ91 magnesium alloy are expected high creep resistance. The microstructure of the alloy exhibits the dendritic α-matrix and the second-phases forming networks on the grain boundary. Tensile tests at elevated temperatures between 448 and 523K reveal that the creep resistance was improved with increasing the additional amount of Ca, especially more than 1.0wt%. From the perspective of grain refinement effect, it is expected that the additions of Ca and Sr to AZ91 magnesium alloy not only improve creep resistance but also improve mechanical properties at room temperature.

Twin roll casting process combines casting and hot rolling into a single process. In this study, mechanical properties at room temperature and microstructure of the twin roll cast AZ91 magnesium alloy are investigated. The alloy exhibited a good combination of high ultimate strength of 343MPa, yield stress of 224MPa and elongation to failure of 13%. The mechanical property was very excellent compared with AZ91 die-cast alloy. EPMA analysis reveals that the Al concentration in Mg matrix is higher in twin roll cast alloy than that in die-cast alloy. This high Al concentration must be the origin of the good mechanical properties of twin roll cast alloy at room temperature.

Grain refinement during forging was found to be due to dynamic recrystallization of four kinds of Mg–x wt.%Al–1 wt.%Zn (x = 3, 4, 5, 6) alloys with grain sizes of ∼30 μm, which were cast using a thixo-molding machine. Superplastic forming combined with forging was successfully performed at a low temperature (473 K) and high-strain rate (3.3 × 10−2 s−1).

The climb-controlled dislocation creep behavior was investigated in Mg–Al–Zn alloys of AZ31, AZ61 and AZ91 with different aluminum contents. The flow stress increased with the increase of aluminum content under the same deformation conditions. At high temperatures, the stress exponent was 5 and the activation energy was close to that for lattice diffusion of magnesium, whereas at low temperatures the stress exponent was 7 and activation energy was close to that for pipe diffusion for all alloys. By incorporating the effective diffusion coefficient and stacking fault energy into a constitutive equation, the dislocation creep behavior of magnesium alloys can be described by a single relation.

The cast AZ91 magnesium alloy having excellent mechanical properties, such as high strength and high creep resistance was developed by additional elements of Ca and Sr. From the microstructural observations, in order to obtain the fine grained AZ91 magnesium alloys, the optimal concentration of elements, such as Ca and Sr were at least 1.0 and 0.5 wt%, respectively. The present developed cast AZ91 magnesium alloy had fine grain sizes of 19 μm. The tensile strength and elongation at room temperature were 250 MPa and 3.5%, respectively. The strength at an elevated temperature of 448 K on developed AZ91 magnesium alloy was also drastically larger than that on precious cast magnesium alloys.