| Mg-Gd-Y-Zr alloys are considered to be a new kind of advanced heat resistant magnesium alloy with high strength and have a wide application prospect in the field of aerospace industry. However, there is a problem that crack failures tend to appear in complicated structural components with thin walls when using traditional water cooling method after solid solution and aging heat treatment. Therefore, it is necessary to explore a new heat treatment process based on other cooling method. In this study, the heat treatment process of sand-cast Mg-10Gd-3Y-0.5Zr alloy which was taken air cooling method after solid solution was optimized. The microstructures were studied. The mechanical properties, fracture behavior at different temperatures of optimized T6treated Mg-10Gd-3Y-0.5Zr alloy were investigated. The mechanism of anomalous strength behavior of T6treated Mg-10Gd-3Y-0.5Zr alloy at elevated temperatures was synthetically discussed. The plane strain fracture toughness was also studied.The results show that the microstructures of sand-cast and T4treated alloy are α-Mg+Mg24(Gd,Y)5+Zr core and α-Mg+γ+Zr core.β’phase gradually precipitates during the process of aging and the microstructure of peak-aged Mg-10Gd-3Y-0.5Zr alloy is α-Mg+β’+Zr core.With the increase of the aging time, the strength and elongation of525℃×12h treated Mg-10Gd-3Y-0.5Zr alloy increase rapidly and get the peak values at peak-aging state. After that, the strength and the elongation slowly decrease with the continual increase of the aging time. The peak-aging states for sand-cast Mg-10Gd-3Y-0.5Zr alloy which is taken air cooling after solid solution are525℃×12h+225℃×14h and525℃×12h+250℃×12h. The hardness, TYS, UTS and elongation of T6treated Mg-10Gd-3Y-0.5Zr alloys under these two heat treatment processes are120.9HV,251.6Mpa、339.9Mpa、1.5%and110.9HV,247.3Mpa、359.6MPa、2.7%at room temperature, respectively. The optimum T6heat treatment for sand-cast Mg-10Gd-3Y-0.5Zr alloy is525℃×12h+250℃×12h according to strength and toughness synthetically.In the temperature range from room temperature to300℃, the strength of525℃×12h+225℃×14h and525℃×12h+250℃×12h treated alloys firstly increases with the increase of the temperature and reach peak values at the temperatures of200℃and125℃, respectively. The peak TYS and UTS of these two conditions are250MPa、350MPa and225MPa、368MPa, respectively. Then the strength of the alloys decreases with the continual increase of the temperature. The elongations of the T6treated alloys increase monotonously with the increase of the temperature and reach their peaks at300℃whose values are11.8%and14.9%respectively according to225℃and250℃aging.The fracture modes of sand-cast, T4and T6Mg-10Gd-3Y-0.5Zr at room temperature are all mainly quasi-cleavage fracture. With the increase of tensile test temperature, the fracture modes turn to intergranular fracture. The proportion of grain boundary sliding increases and toughness improves with the increase of tensile temperature.The mechanism of anomalous strength behavior, namely both tensile yield strength and ultimate tensile strength of the alloy firstly increase with the tensile temperature, and subsequently decrease as the temperature increases further, of T6treated Mg-10Gd-3Y-0.5Zr alloy at elevated temperatures is due to the interaction of dispersive distribution of thermal stable β phase and more deformation mechanisms at elevated temperatures.The plain-train fracture toughness of525℃×12h+225℃×14h and525℃×12h+250℃×12h treated Mg-10Gd-3Y-0.5Zr alloys is14.0MPa.ml1/2and14.7MPa.m1/2at room temperature, respectively. The fracture of plain-strain fracture toughness specimens of T6treated alloy is mainly composed of cleavage planes and grain boundaries and the cracks extend both transgranularly and intergranularly, which indicates the fracture mode is a mixed mode of transgranular and intergranular fracture. |
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