The Effect Of Stress Aging On Mechanical Properties Of AZ80Magnesium Alloy

In this paper, AZ80magnesium alloy was aged under different conditions oftemperature and holding time with and without applying elastic stress in a hightemperature creep and stress aging furnace. We used equipments like micro Vickershardness tester, tensile testing machine and some other machines to test alloy mechanicalproperties. Metallographic microscope, scanning electron microscope （SEM）, X-raydiffraction, and transmission electron microscope were used to analyze stress aging testspecimen. The effect of stress aging on mechanical properties and microstructure of AZ80magnesium alloy was systematically studied.Results show that the organization of as-cast AZ80magnesium alloy is mainlycomposed of α-Mg solid solution and intermetallic compound β-Mg₁₇Al₁₂phase, hardbrittle β-Mg₁₇Al₁₂phase distribute along the grain boundary, the elongation of the as-castalloy sample is very low. After solid solution treatment, β-Mg₁₇Al₁₂phase was dissolved.And the elongation greatly improved compared to as-cast. While the alloy was aged, theβ-Mg₁₇Al₁₂phase precipitated in two forms-continuous and discontinuous. And with theincrease of the number of β-Mg₁₇Al₁₂phase, alloy elongation decreased, but hardnessand yield strength increased.When aged under an applied45MPa stress for six hours, along with the rising ofaging temperature, alloy hardness and its yield strength and tensile strength increase firstand then decrease, and its elongation decreases in all the process. Alloy obtained the bestcomprehensive mechanical properties at220℃, the hardness, yield strength and tensilestrength arrives at the value of82HV、90.9MPa and189.3MPa, respectively. Theperformance is superior to the traditional aged alloy.When the alloy was aged under an applied30MPa stress at220℃, its hardness andyield strength increase first and then decrease over time, but the elongation decreases allthe time. At6th hour, the value of hardness reached94HV, and its mechanical propertiesexceed the traditional aged at220℃for36hours. Its hardness value increased by22%,yield strength increased by7%, tensile strength increased by6.2%and elongation increased by23%. The better performance of the alloy was because stress aging made thesolute atoms move together to grain boundary, and it promoted the discontinuousprecipitation in nucleation and growth, at the same time elaborated the intracrystallinecontinuous precipitation phase.The precipitated phase growth mechanism model with an elastic stress was proposedin this paper. The vacancy and solute atom combined as a whole and diffused in matrix,and when the complex was spread to grain boundary, it broke up into vacancy and soluteatom again. Under the elastic tensile stress, grain boundary can trap solute atom, and thispromoted the diffusion of solute atom.