Effect Of Bi, Y And Nd On Mechanical Properties Of AZ81 Magnesium Alloy

In order to improve the heat resistantance of Mg-Al system alloys, the effect of Bi and rare earth Y, Nd on the microstructure and mechanical properties of AZ81 magnesium alloy have been investigated systemically by optical microscope, scanning electron microscope, X-ray diffractometer and electronic tensile test system.The results show that, with proper content of Bi and rare earth Y, Nd addition, as-cast microstruture can be refined, and the aged microstructure have been improved. Meanwhile, the mechanical properties of the aged alloy have been increased dramaticly.For AZ81 alloy with 0.5wt%².5wt%Bi, with the increasing content of Bi, the microstructures of AZ81 magnesium alloy fine obviously, and Mg₁₇Al₁₂ phase and Mg₃Bi₂ phase which distribute at grain boundary and intracrystalline have been formed. After solid solution and aging treatment, with increasing content of Bi, the tensile strength and elongation of the cast-alloys first increase, then decrease. When content of Bi is up to 2.0wt%, the values of mechanical properties of the cast-alloy are up to their maximums. With increasing content of Bi, the tensile strength of the aging alloy increase, however,when the content of bismuth is 2.0wt%, the values of the elongation at room temperature and 150℃are up to their maximums.For AZ81-2.0Bi alloy with 0.6wt%².4wt%Y, with the increasing content of Y, the microstructures of AZ81-2.0Bi magnesium alloy fine obviously and the quantity of Mg₁₇Al₁₂ phases reduce, and Al₂Y and Mg₃Bi₂ phases form. After solid solution and aging treatment, with increasing content of Y, the tensile strength and elongation of the alloys (at room temperature, 150℃and 175℃) first increase, then decrease. When content of Y is up to 1.2wt%, the values of the tensile strength and elongation of the alloy at room temperature, 150℃and 175℃are up to their maximums simultaneously, 253MPa,223 MPa, 203MPa and 7.0%, 11.7%, 13.9% respectively.For AZ81-2.0Bi-1.2Y alloy with proper content of Nd, with the increasing content of Nd, the high melting-point granule rare earth compound Al₂Y, Al₂Nd and rod-shaped particles Mg₃Bi₂, distributed both in grains and at grain boundaries are observed. After solid solution and aging treatment, with increasing content of Nd, the tensile strength and elongation of the alloys at room temperature, 150℃and 175℃first increase, then decrease. When content of Nd is up to 1.2wt%, the values of the tensile strength and elongation of the alloy at room temperature, 150℃and 175℃are up to their maximums simultaneously.