Existent Form And Action Of Rare Earth Ce And Y In AZ91D Magnesium Alloy

As the lightest structural metal materials, magnesium alloy has great application potential. However, the mechanical properties of magnesium alloy is lower correspondingly, strength and elongate need to further improve to meet the requirements of practical application. In this thesis, widely used AZ91D magnesium alloy was selected as matrix material and rare earth element Ce and Y as additive, through researching the existent forms of rare earth elements and the effect of rare earth elements on mechanical properties of as-cast and aging magnesium alloys to hope improving the mechanical properties of AZ91D magnesium alloy and to illuminate the action behavior of rare earth elements.Using various means to study the AZ91D alloy, such as optical microscopes, scanning electron microscope, X-ray diffraction and so on. The results show that the rare earth elements main exists in three forms in the as-cast magnesium alloys, which is solid solution inα-Mg matrix and the Mg17Al12 intermediate compound as well as formation of Al-RE phase. With the increasing of rare earth content, the grain of as -cast alloy refines, theβ-Mg₁₇Al₁₂ phase separates and reduces, acicular Al4Ce and nubby Al2Y phases increase and get together gradually. The rare earths can improve the mechanical properties of as-cast AZ91D magnesium alloy, when Ce additive amount is 0.8% mass, the mechanical property is optimal, tensile strengthσb can reach 185MPa, elongation have little change and hardness can reach 68.3HV. While the best additive amount for Y is 1.2%, under this condition the tensile strengthσb can reach 190.6MPa, the elongationδcan reach 6.3%, the hardness can reach 67.8HV. This shows that the tensile strengthσb and the elongationδrespectively are higher than 159.5MPa and 3.4% of alloys without rare earth. By contrast, Y is more effectively than Ce to improve mechanical properties. For the cast alloys, the morphology analysis of fracture shows that the main features character of the alloy without rare earth is cleavage. But the main features of alloys with feasible amount rare earth have visible change, appearing quasi-cleavage character even local dimple. So there are an optimal content of rare earth to improve mechanical property of as-cast AZ91D magnesium alloy, excessive additions make the mechanical properties down.The investigation to solid solution and aging processes and performance analysis of AZ91D magnesium alloy shows that, when solution at 420℃, theβ-Mg₁₇Al₁₂ phase in the alloy decomposes absolutely through by 12h. However, the decomposition ofβ-Mg₁₇Al₁₂ phase in the alloy with rare earth Ce or Y would be delayed. And moreover the Al-RE phases not decomposes at the temperature. When aging at 175℃, laminarβ-Mg₁₇Al₁₂ phase precipitates at grain boundary and forms lamellar structure withα-Mg phase. When aging for by 18h, the microstructure is fully ofα-Mg phase andβ-Mg₁₇Al₁₂ phase. Subsequently, some bulkβ-Mg₁₇Al₁₂ phase appears with delaying aging time. Rare earth element has action to inhibit decomposes ofβ-Mg₁₇Al₁₂ in the solution, and restrain precipitation ofβ-Mg₁₇Al₁₂ in the aging prophase. The Vickers hardness test shows that, the aging hardness peak value is corresponding with the full laminar microstructure, and rare earth can delays the aging hardness peak value. After the alloy is treated through solution at 420℃and aging at 175℃for by 18h, its tensile strength can be obviously improve, and elongation have little improve too. Both of tensile strength and elongation appeared rises then fall with the increasing of rare earth additions, and they can reach the maximal while the addition of Ce is 0.8% and Y is 1.2% respectively, that is the tensile strength reach 232MPa and 239MPa, higher than 223MPa and 4.4% of aging alloys without rare earth additions. In all, Y additions can get better mechanical. For the fracture of aging alloys, the main character of alloy without rare earth is the cleavage, but the main features of alloys with rare earth have visible change, more cleavage stair and more longer tortuous tearing ridge can be seen, furthermore local dimple can be discovered when addition 1.2%Y.