Microstructure And Mechanical Properties Of Mg-Al Heatresistant Magnesium Alloys

After entering the 21st century,the protection of resources and environment and sustainable development has become a primary problem,energy saving and environmental protection has become a prominent feature of modern industry.Today,a lot of traditional metals tend to exhaust,it will be of important in strategic significance to accelerate the development of magnesium material for balance of resources and environment and sustainable development.Magnesium alloys are widely used in automotive,aerospace and other fields because of their excellent properties.With the rapid development of automobile and aviation industry,the requirements for the comprehensive properties of magnesium alloys are becoming higher and higher,and the research and development of new high-strength heat-resistant magnesium alloys become more and more important.In the design of heat resistant magnesium alloy,the main design principles include how to reduce the diffusion rate of alloy elements,how to strengthen the the grain and grain boundary by adding elements and how to restrict the movement of dislocations in the matrix and prevent the grain boundary sliding.Therefore,based on Mg-5Al alloy,the effect of Ca and Sn elements addition on the hot tearing,microstructure,mechanical properties and high temperature creep properties of Mg-5Al-xCa-ySn alloy are investigated by sample casting SEM,XRD and TEM analysis,as well as tensile and creep test.The aim is to develop an economical heat-resistant magnesium alloy without rare earth elements.Which also provids a theoretical basis for the development and application of new alloy.In the hot tearing experiments,the effect of Ca on the hot tearing of Mg-5Al-xCa?x=1,2,3,4,5?alloys was considered.It was found that the critical sizes of hot cracking sample produced by the cast rod were 10,8,6,4 and 6 mm,respectively,with Ca increasing.The improved hot tearing sensitivity coefficient?HSC?shows that the alloy exhibits a insensitivity when the Ca content exceeds 3wt.%.At same time,the eutectic phase Mg₁₇Al₁₂ is decreased,while eutectic phase Mg₂Ca phases increased with the increase of Ca content obtained by the microstructure observation for Mg-5Al-xCa alloys.The eutectic structure changes from discontinuous to continuous.Therefore the area of the eutectic microstructure is increased.In addition,Mg-5Al-xCa alloy tensile strength increased first and then decreased,the alloy with3wt.%Ca content has the highest strength.The fracture surfaces analysis showes that the fracture mechanism changes from cleavage fracture to quasi cleavage fracture and to ductile fracture with Ca content.The microstructure of Mg-5Al-xCa-0.7Sn alloy is also observed in this paper,and it is found that the eutectic microstructure has a continuous skeleton distribution at x>3,which is similar to that of typical MRI-230d alloy.In Mg-5Al-xCa-2Sn alloy,with the increase of Ca content,eutectic structure of Mg₂Ca phases increase,and a continuous network structure is formed.The CaMgSn phases change from feathery to rodlike.At room temperature,the tensile properties of as cast Mg-5Al-xCa-2Sn alloy increase with the increase of Ca content.The alloy with 5%Ca content shows the excellent mechanical properties.In Mg-5Al-5Ca-ySn alloys,the phases constituents for the alloy without Sn are mainly Mg₂Ca phases companied by a small amount of Mg₂Ca phases.With the addition of Sn,the CaMgSn phase is formed.While the as-cast microstructure is refined.The eutectic structure is increased and distributed continuously as a skeleton structure.The precipitates in the matrix is increased by heat treatment.Tensile properties of as cast Mg-5Al-5Ca-ySn alloy increase with the increase of Sn content at room temperature.While at high temperature,tensile properties increase first and then become stable.The tensile properties of the alloy without Sn were slightly reduced after heat treatment at room temperature,but increased at high temperature.Therefore,the alloy with 2%Sn content showes excellent mechanical properties.The creep tests for the as-cast Mg-5Al-5Ca-2Sn alloy with different applied stresses at the temperatures of 175 and 200?,respectively,were carried out.The results show that the creep properties are sensitive to the temperature and stress,and the creep rate increases with the increase of applied stress.At the initial stage of creep,the creep rate decreases rapidly,the lower the creep life,the faster the creep rate decreases.The strain and creep rates follow different rules during creep.The dynamic precipitation is facilitated at a certain temperature and applied stress during creep,which result in the precipitation of Al₂Ca phases.The dominant creep mechanism is phase boundary cracking and sliding.Which results in grain movements accommodated with either diffusion or with dislocation motion.Therefore,dislocation-accommodated grain boundary sliding is expected to be the dominant creep mechanism.