Study On The Microstructure And Elevated-Temperature Property Of Mg-9Li-3Al Alloys With Addition Of Sr

Mg-Li alloy is known as the lightweight alloy.Due to its low density,high strength-to-weigh, rigidity-to-weigh ration and good damping capacity properties, Mg-Li alloys are applicable to aerospace,electronics and communication industries ,which would be likely to expanding at a high rate.Research works of this thesis aim at Mg-Li-Al alloy. Mg-Sr intermediate alloy and Mg-9Li-3Al alloy were prepared using"Incorporation method". The influence of Sr on the microstructure of as-cast,heat-treated, and hot-extruded test alloys were investigated by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and high-temprature mechanical property test. Microstructure and high-tempreture mechanical properties of Mg-9Li-3Al alloy with Sr addition were studied, the fracture mechanism of the Mg-9Li-3Al-xSr alloys was investigated as well.The results showed that Sr has refining effect on test alloys and the new stable phase Al₄Sr is formed in the alloys, which distributes along grain boundary.When the as-cast alloy with 2.5wt% Sr content, the grains are refined efficiently. The dynamic recrystallization occurs during the hot-extruding ,so the grains are refined more smaller than as-cast alloys which makes the tensil strength of test alloy increase greatly. Besides,Al₄Sr phase distributes dispersively in the form of block structure or particle structure after extrusion.Mg-9Li-3Al-xSr alloys as-cast and extuded were subjected to the high-tempreture mechanical properties test at 100℃and 150℃.The result of mechanical properties test indicates that mechanical properties of hot-extruded test alloys are much better than the as-cast alloys. Mg-9Li-3Al alloy of high-tempreture tensile strength declined clear compared with the test alloy with Sr addition which indicated that amount of Sr performed good mechanical properties. Elevated temperature fractural morphology showed that the trends of intergranular fracture extended of Mg-9Li-3Al alloy with addition of Sr.Not only the elongation is all above 10% for hot-extruded test alloys at 100℃,the tensile strength of the hot-extruded alloy can reaches to its peak value 235.2MPa with 2.5wt%Sr content. The tensil strength, yield strength respectively increases by 19.8% compared with the Mg-9Li-3Al alloy. So the Mg-9Li-3Al-2.5Sr extruded alloy shows the best high-tempreture mechanical properties with the highest strength and a relative good elongation. Moreover,the improved high-tempreture properties with Sr addition are analysed. Sr tends to separately form stable Al₄Sr(1040℃) phase with Al in the alloys which leads to the reduction in amount of Mg₁₇Al₁₂,AlLi phases. The highly thermal stablility of Al₄Sr phase could hinder dislocation climbing and slipping along the grain boundaries. The addition of Sr might cause the phase Al₄Sr to be concentrated to the boundaries, which reduce the high-temperature properties of Mg-9Li-3Al alloy. On the contrary, minor content of Sr can improve the mechanical properties.