Study On Elevated Temperature Properties Of Magnesium-Strontium Alloys

The development of heat-resistant magnesium alloy is always focused by the research of magnesium alloy. The application of magnesium alloy could be widened according to improving its heat-resistant properties. The application of magnesium alloy in the aircraft and automobile could significantly alleviat its own weight, save fuel and reduce trail gas to relieve pressures from energy shortages and environmental deterioration. It has important economic value and social meanings. However the commercial magnesium alloys fail to meet the requirement of driving parts and other systems in automobiles,because of their weak strength and bad creep resistance at elevated temperature.In this paper, microstructures, fracture, elevated temperature properties as well as elevated temperature strengthen mechanism of Mg-Al based alloys with alkaline earth Sr have been investigated by melting furnace, optical microscope(OM), scanning electron microscope(SEM), X-ray difractometer(XRD), keeping treatment, Gleeble-1500D elevated temperature testing machine etc.During the melting, Mg-Sr intermediate alloy is an effective process alloy which could avoid excessive amount of Al in the alloy. Mg-4Sr alloy is composed of a-Mg, Mg₁₇Sr₂ phase. Dendritic and reticular Mg₁₇Sr₂ distributes along the grain boundaries. With the increasing of contents, Sr could promote diffusion of Al and Zn towards toα-Mg matrix and eutecticum. Furthermore because of the electronegativity difference of elements, Sr tends to separately form stable Al₄Sr with Al prior to Mg in the alloys and some Al would formβ-Mg₁₇Al₁₂ phase with Mg.β-Mg₁₇Al₁₂ phases distribute dispersively. The new phase Al₄Sr(1040℃) has higher melting point than Mg₁₇Al₁₂(437℃) which could more effectively pin the grain boundaries and hinder dislocation climbing and slipping along the grain boundaries. So that the elevated temperature properties could increase.At 150℃, both as-cast and keeping-treatment AZ91-Sr and AJ62 have better tensile strength, yield strength, and worse elongation than AZ91. Elevated temperature properties of Mg-Sr alloys have a stable comprehensive elevated temperature properties after keeping for eight hours at 200℃. Elevated temperature fractural morphology and microstructure distribution show that the elevated temperature fractural modes of Mg-Sr alloys are"intergranular fracture and cleavage fracture". With addition of Sr, Mg₁₇Al₁₂ and more stable Al₄Sr phase distribute along the grain boundaries which could hinder the dislocation moving effectively. The cleavage fracture is restrained and, the trends of intergranular fracture extend.After keeping-treatment, according to the microstructure of AZ91-Sr and AJ62 alloys, rapidly solidified and slowly solidified microstructure, grain size and distribution have no obvious differences comparing with as-cast alloys. All of those indicate that two alloys are not sensitive to the temperature below 200℃. Alloys show good heat-resistance.