Study Of Superplasticity On Mg-Al-Ca Alloys With High Ca/Al Ratio

As the lightest commonly used engineering alloy, magnesium alloy is widely used in aerospace and automobile. The addition of Ca to the Mg-Al alloy can improve the casting performance and reduce the grain size. The thermal stable intermetallic compounds in Mg-Al-Ca alloy have a very important influence on the properties of magnesium alloys.This paper takes four kinds of Mg-Al-Ca alloys (Mg-2Al-2Ca, Mg-3Al-3Ca, Mg-4Al-4Ca, Mg-5Al-5Ca) with high Ca/Al mass ratio as the research object, they were prepared by semi-continuous direct chill casting and extruded to improve their properties, the influence of extrusion process on microstructure and mechanical properties of the alloys, the superplasticity and mechanism of extruded alloys were investigated. It is concluded that:The as-cast alloys contain dendritic structure with secondary phase on the grain boundaries. Ca can refine the grain well, the more Al and Ca content is, the more secondary phases are, the finer the grains of cast alloy is, Mg-5Al-5Ca alloy has finest grain size among four alloys. The phase composition of the alloys is identified as a-Mg matrix+a large number of Al2Ca+few Mg2Ca.After extrusion, the grains of four alloys were refined, and secondary phases were refined into smaller particles. Extruded Mg-3Al-3Ca has highest strength, its yield strength and tensile strength were 303MPa and 333MPa, respectively, and the elongation-to-failure was 4.3%. Extrusion process makes the yield strength, tensile strength and elongation of Mg-3Al-3Ca increased by 168.1%,168.5%, and 26.5%, respectively. Mg-2Al-2Ca has best fracture toughness, its Kq value is 15.32 MPaml/2.Superplastic tensile test of the alloys was conducted at 250?,300?,350? and 400? and the strain rates of 3.6×10-3s-1 and 3.6×10-4s-1, the four alloys show remarkably high elongation-to-failure at 400?, and Mg-5Al-5Ca exhibits highest elongation 572% at 400? and 3.6×10-1, which is attributed to the grain refinement effect promoting grain boundary sliding (GBS).The strain rate sensitivity m and superplastic deformation activation energy Q of the alloys were calculated according to constitutive equation. The average m values of Mg-2Al-2Ca, Mg-3Al-3Ca, Mg-4Al-4Ca and Mg-5Al-5Ca are 0.181,0.209, 0.420,0.489, respectively, and average Q values are 147.6 kJ-mol'1,139.1 kJ-mol'1, 100.2 kJ-mol"1,87.1 kJ-mol"1. So superplastic deformation mechanism of Mg-2Al-2Ca and Mg-3Al-3Ca is dislocation climb creep accommodated by lattice diffusion (Dl-DCC), and superplastic deformation mechanism of Mg-4Al-4Ca and Mg-5Al-5Ca is grain boundary sliding accommodated by grain boundary diffusion (Dgb-GBS). For AC5 alloy, most of Al2Ca particles was 80nm in size, grain growth retardation effect is strong and the smaller particles at grain boundaries can accommodate deformation by GBS, thus Mg-5Al-5Ca alloy has best superplasticity among four alloys.