| AZ80A magnesium alloy is widely used in aerospace,transportation,and other fields due to its low density and high specific strength.In this thesis,the effects of heat treatments on the microstructure,tensile mechanical properties,and creep resistance of AZ80A magnesium alloy were studied using an optical microscope（OM）,scanning electron microscope（SEM）,transmission electron microscope（TEM）,tensile mechanical properties test,and high-temperature tensile creep performance test.The continuous precipitation（CP）and discontinuous precipitation（DP）behavior of theβphase were revealed.The influence of theβphase precipitation behavior on the high-temperature creep resistance was explored.By studying the CP and DP behaviors ofβphase in AZ80A magnesium alloy during T5 and T6 treatments,the evolution laws of continuous precipitates（CPs）and discontinuous precipitates（DPs）during aging were revealed,the growth mechanism of DP regions was investigated,and the influence mechanism of T5 and T6 treatment on the tensile mechanical properties was studied.When DP regions grew into CP regions at the beginning of aging,the following reaction occurred:supersaturated matrix+fine continuous precipitates→balanced matrix+discontinuous precipitates,in which the fine CPs dissolved again on the reaction front and transformed into DPs.The evolution of DPs was faster than that of CPs.When DPs coarsened,CPs were still in the growth and precipitation stage.During aging at 175°C,the scope of CP and DP regions of T6 state account for 42%and 58%of the alloy as a whole（hereinafter referred to as volume fraction）,respectively.The volume fraction of CP regions of T5 state is smaller than that of T6 state,only10%.The strengthening effect of DP regions was better than that of CP regions at room temperature but worse at high temperatures.It contributed to that the room temperature tensile mechanical properties at T5 state were higher than those at T6 state,but the high-temperature tensile strength at T6 state was higher than that at T5 state.The tensile strengths at T6 state and T5 state at 150°C were 194 MPa and 189 MPa,respectively.The effects of solution treatment on the microstructure of AZ80A magnesium alloy and its tensile mechanical properties at room temperature and high temperatures were studied.With the increase in solution temperature in the range of 420-480°C,the volume fraction of CP regions increased gradually,and that of DP regions decreased gradually after aging at 175°C.After solution at 480°C and peak aging,the CP region fraction of 68%and the DP region fraction of 32%were obtained.At room temperature,the tensile mechanical properties of the peak-aged alloy gradually decreased with the increase in solution temperature,but increased at high temperatures.Cracks were easy to initiate and propagate in the CP regions at room temperature.However,they were more likely to initiate and propagate in the DP regions during tensile tested at high temperatures.The alloy had a tensile strength of 206MPa at 150°C after solution treatment at 480°C and peak aging at 175°C,which was 12 MPa higher than that after solution treatment at 420°C and peak aging at 175°C.The effect of the aging process on the microstructure and mechanical properties of AZ80A magnesium alloy was studied.After solution treatment at 480°C,the volume fraction of CP regions increased gradually from 51%to 80%with the increase in aging temperature from150°C to 225°C.When the aging temperature increased,the scope of CP region gradually increased and the precipitates coarsened,the peak-aged alloy at 200°C had a higher high-temperature tensile strength.The tensile strength reached 215 MPa when the tensile temperature was 150°C,and the yield strength and elongation were 146 MPa and 13.5%,respectively.The effect of different heat treatment processes on the high-temperature creep resistance of AZ80A magnesium alloy was studied,and the constitutive relationship between the high-temperature creep resistance and the content of CP and DP region components was revealed.The creep resistance of the alloy at T6 state was better than that at T5 state,and increasing the solution temperature could significantly improve the creep resistance of the alloy.The alloy solution treated at480°C obtained excellent creep resistance after peak aging at 200°C.During creep at 150°C/90 MPa,the steady creep rate was only 3.67×10-8s-1.The steady creep rate of the alloy gradually decreased with the increase in the ratio of the CP region fraction to the DP region fraction,and the creep resistance gradually increased.The constitutive equation was established accordingly. |
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