Effect Of Al₂Sm And Al₂Ca On Microstructure And Thermal Stability Properties Of Mg-6Al Alloy

Although Mg-Al alloy has been widely used,its plasticity is poor due to its dense hexagonal structure.In order to expand its application scope,its performance needs to be improved.Grain refinement is an important method to improve the strength and plasticity of metal materials.Among them,Al₂X compounds have Cu₂Mg crystal structure and good lattice matching with Mg matrix,which is expected to become a new type of Mg alloy refiner.In this experiment,Al,Sm and Al,Ca elements were added to pure Mg to prepare the refiners containing Al₂Sm and Al₂Ca respectively.The effects of Mg-Al₂Sm and Mg-Al₂Ca refiners on the structure and properties of Mg-6Al alloy were studied.Research shows:1.The needle-like Al₁₁Sm₃ phase and granular Al₂Sm phase are formed when2%⁴%Al is added to Mg-7Sm alloy.With the increase of Al content,the needle-like Al₁₁Sm₃ phase decreases,while the granular Al₂Sm phase increases,and the particle size increases.Al₂Sm phase is stable at high temperature and Al₁₁Sm₃ phase is unstable.Al₁₁Sm₃ phase will change into Al₂Sm phase after heat treatment at 520?/5 h.2.Mg-Al₂Sm refiner can significantly refine Mg-6Al alloy.When the addition amount is 36.4wt.%,the grain size is refined from 126.13 to 75.68microns.The refinement mechanism is that the undissolved Al₂Sm particles in the refiner play the role of heterogeneous nucleation,and the solute effect of dissolved Al and Sm elements also refines the grain.3.Compound addition of Mg-Al₂Sm and Mg-Al₂Ca refiners can significantly refine Mg-6Al alloy.When the addition amount is 36.4 wt.%and 20.8 wt.%respectively,the alloy has the best refinement effect.The average grain size is refined from the original 126.13 micron to 46.57 micron.The grain refinement mechanism is due to the heterogeneous nucleation of Al₂Sm and Al₂Ca particles in the initial solidification stage of the alloy and the strong segregation ability of dissolved Al,Sm and Ca elements,which results in the undercooling of the composition,promotes the nucleation in the undercooled region and refines the grain.4.The mechanical properties of Mg-6Al alloy were significantly improved by adding Mg-Al₂Sm and Mg-Al₂Ca refiners.The maximum tensile strength and elongation reached 188.9 MPa and 10.23%respectively when the content of Mg-Al₂Sm and Mg-Al₂Ca refiners were 36.4 wt.%and 20.8 wt.%respectively.The strengthening mechanism is that the grain size is refined after adding refiner,and the fine grain plays the role of fine grain strengthening.The dispersed hard second phases Al₂Sm and Al₂Ca play a second phase strengthening role in the alloy.The solid soluble Al,Sm and Ca elements play a solid solution strengthening role after adding refiners.5.The grain thermal stability of Mg-6Al alloy can be improved by adding Mg-Al₂Sm and Mg-Al₂Ca refiners.The thermal stabilization mechanism is that the Al₂Sm and Al₂Ca phases are high temperature stable phases.When the alloy is at high temperature,the particles of Al₂Sm and Al₂Ca dispersed in the matrix play a pinning role relative to the grain boundary and hinder the growth of grains.The Ca element is segregated around the beta-Mg₁₇Al₁₂ phase,which improves the heat resistance of the beta-Mg₁₇Al₁₂ phase and the thermal stability of the grain.