Study On Microstructure And Mechanical Behavior Of Mg-6Gd-3Y-0.5Zr Magnesium Alloy Subject To Thermal Cycling Treatment

As the current research hotspot of magnesium alloys,Mg-Gd-Y series of high-strength and heat-resistant rare earth magnesium alloys are expected to be used in aeronautics and spacecraft as structural materials,and will have a broad application prospect in aerospace and national defense equipment industry with high demand for lightweight.On the other hand,aeronautical and spacecraft mainly serve in the air,space,near space and extremely cold environment,and often bear the combined action of high temperature,low temperature and cyclic load.Therefore,the room temperature,high temperature and low temperature mechanical properties of Mg-Gd-Y series alloys under deep cryogenic/deep cryogenic-elevated temperature cycling conditions are important technical indexes.Therefore,based on the application requirements and project objectives in the aerospace field,this paper studied the mechanical behavior of Mg-6Gd-3Y-0.5Zr(GW63K)magnesium alloy after deep cryogenic/deep cryogenic-elevated temperature cycling treatment,including room temperature,high temperature and low temperature tensile behavior,room temperature impact behavior and high cycle fatigue behavior.The results of tensile study of as-cast GW63K alloy show that the yield strength,tensile strength and elongation of as-cast GW63K alloy without deep cryogenic/deep cryogenic-elevated temperature cycling treatment are 156 MPa,212 MPa and 6.2%,respectively.With the increase of tensile temperature,the yield and tensile strength increased at first and then decreased,reaching the highest at 150?,while the elongation increased all the time.Compared with the untreated as-cast alloy,the yield and tensile strength of the as-cast alloy after deep cryogenic/deep cryogenic-elevated temperature cycling treatment increased continuously with the increase of the number of cycles at room temperature,and reached 181 MPa and 242 MPa respectively at 5 cycles,while the elongation decreased all the time.Low temperature cycling(from-196?to room temperature)has little effect on its mechanical properties.The high temperature stage in the process of deep cryogenic/deep cryogenic-elevated temperature cycling treatment can be equivalent to the aging process,which greatly releases the stress concentration caused by low temperature and leads to the improvement of strength.The tensile study of T6 state GW63K alloy shows that the yield strength,tensile strength and elongation of T6 state GW63K alloy without deep cryogenic/deep cryogenic-elevated temperature cycling treatment are 237 MPa,309 MPa and 5.2%,respectively.After deep cryogenic/deep cryogenic-elevated temperature cycling treatment at±196?,the yield and tensile strength first increased and then decreased,and the highest in the three cycles,respectively,251 MPa and 340 MPa,increased by 5.9%and 10.0%respectively compared with the untreated alloy,and then decreased with the increase of the number of deep cryogenic/deep cryogenic-elevated temperature cycles.The change law of mechanical properties after deep cryogenic/deep cryogenic-elevated temperature cycling treatment at±150?is consistent with that at±196?.The deep cryogenic/deep cryogenic-elevated temperature cycling treatment at±100?and±50?and deep cryogenic cycling have no obvious effect on its mechanical properties.At high temperature,the change of mechanical properties of untreated T6 state alloy is the same as that of as-cast alloy,and the yield and tensile strength reach the peak at 150?,which are237 MPa and 325 MPa respectively.Compared with the untreated alloy,the yield and tensile strength of the T6 state alloy treated at±196?for 3 times at150?increased by 6.3%and 8.9%,respectively.At low temperature,with the decrease of tensile temperature,the yield and tensile strength increase gradually,while the elongation decreases greatly.The yield,tensile strength and elongation of untreated T6 state alloy at-150?are 356 MPa,383 MPa and4.2%,respectively.After 3 times of deep cryogenic/deep cryogenic-elevated temperature cycling treatment at±196?,the yield and tensile strength at-150?were increased to 404 MPa and 413 MPa,respectively,and the elongation decreased to 2.6%.Compared with the alloys without deep cryogenic/deep cryogenic-elevated temperature cycling treatment,the changes of low temperature and high temperature mechanical properties of the alloys after deep cryogenic/deep cryogenic-elevated temperature cycling treatment are consistent with those at room temperature.The deep cryogenic/deep cryogenic-elevated temperature cycling treatment makes the??phase fine and uniformly distributed,and with the increase of the number of cycles,the??phase decomposes in situ to form?₁phase,and its yield and tensile strength decrease with the further increase of the number of deep cryogenic/deep cryogenic-elevated temperature cycling treatment.The results of high cycle fatigue study showed that the fatigue strength of T6 state GW63K alloy without deep cryogenic/deep cryogenic-elevated temperature cycling treatment was 130 MPa,but after 3 cycles at±196?,the fatigue strength of T6 state alloy increased by 7.7%to 140 MPa.With the increase of the number of deep cryogenic/deep cryogenic-elevated temperature cycles,the fatigue life of as-cast GW63K alloy increases continuously.The fatigue life of T6 state GW63K alloy reaches the highest at three cycles,and then decreases gradually.After deep cryogenic/deep cryogenic-elevated temperature cycling treatment,the??phase becomes finer and more dispersed than the untreated T6 state alloy,the pinning effect on dislocations is enhanced,and the fatigue strength is improved.The results of impact behavior study show that the room temperature impact toughness of as-cast and T6 state GW63K alloy without deep cryogenic/deep cryogenic-elevated temperature cycling treatment is 45.5 J/cm²and 35.6 J/cm²,respectively.The impact toughness of as-cast alloy after high and low temperature cycling treatment decreased with the increase of cycle times,and decreased to 10.9 J/cm² after 5 times deep cryogenic/deep cryogenic-elevated temperature cycling treatment.However,the impact toughness of T6 state alloys after deep cryogenic/deep cryogenic-elevated temperature cycling treatment decreased at first and then increased with the increase of deep cryogenic/deep cryogenic-elevated temperature cycles,and reached the minimum at three deep cryogenic/deep cryogenic-elevated temperature cycles,which was 21.8 J/cm².The impact toughness changes little before and after cryogenic cycle treatment.There is a strong correlation between impact toughness and elongation.After deep cryogenic/deep cryogenic-elevated temperature cycling treatment,the number of twins decreases,which hinders the enhancement of dislocation slip,so that the plasticity of the alloy decreases,so the impact toughness is also lower.