Study On Mechanical Properties Of Thermo-mechanical Treated Mg-re Alloy At Cryogenic Temperature

The tensile and impact properties of a magnesium rare-earth (Mg-RE) alloy with different thermomechanical treatment history were investigated within the temperature range of RT～-196℃by means of tensile test machine and drop impact test machine. The related tensile and impact deformation-fracture behaviors of the Mg-RE alloy were also studied by using optical microscopy and scanning electron microscopy techniques.The results show that that the suitable solution temperature range is 410℃～490℃, while the aging temperature is 180℃, for the investigated Mg-RE alloy. The phases such asα-Mg, Mg₃YZn₆, Mg₃Y₂Zn₃, Mg₁₂YZn, MgY and MgZn₂ have all been found in the as-forged, 490℃* 4h +180℃* 20h treated and 420℃* 4h +180℃* 8h treated Mg-RE alloys. Microstructure of the as-forged alloy is characterized by elongated grains along a certain direction, and small grains formed in the vicinity of large grains by dynamic recrystallization. Intergranular precipitation of second phases displayed a slightly uneven distribution. After a 490℃* 4h +180℃* 20h treatment, recovery was found to occur in the small grains. The second phases precipitated along the grain boundary formed net shape structure. But after a 420℃* 4h +180℃* 8h treatment, recovery was not obvious. The second phases precipitated along the grain boundary formed island shape structure, while the second phases within grains were well distributed.The tensile test results show that with the decrease of tensile temperature, the ultimate strength and yield strength of thermomechanical treated Mg-RE alloy increases, while the elongation decreases. The as-forged, 490℃* 4h +180℃* 20h treated and 420℃* 4h +180℃* 8h treated Mg-RE alloys exhibit excellent tensile properties at both room temperature and cryogenic temperature, and the as-forged Mg-RE alloy gives the best one. The ultimate strength, yield strength and elongation of the as as-forged Mg-RE alloy at room temperature and -196℃are 274.5MPa, 199.3MPa, 18.1% and 395.9MPa, 317.1MPa, 3.8%, respectively. SEM observations indicated that tensile fracture surface of the thermomechanical treated Mg-RE alloy exhibited obvious features of quasi- cleavage fracture at cryogenic temperature. A lot of tearing edges and steps could be observed on the flat fracture surface. As the second phases become brittle at cryogenic temperature, fracture mode of the Mg-RE alloy changed from intergranular fracture to transgranular fracture with the decrease of tensile temperature. The tensile deformation microstructure examinations showed that twinning seemed to be the dominated deformation mechanism at cryogenic temperature, by various ways, in the Mg-RE alloy. With the test temperature decreased, the effect of microstructure on tensile deformation increased.The impact test results show that with the decrease of test temperature, the impact energy of the as-forged Mg-RE alloy reduced gradually. The impact energy of the LD direction was found to reduce from 3.45J at room temperature to 3.12J at -196℃, and from 3.45J to 2.78J for the TD direction. No significant ductile-brittle transition was observed. Anisotropy in impact property was also found in the as-forged Mg-RE alloy. At the same testing temperature, the impact energy of the LD direction is higher than that of the TD direction. The impact fractography of the as-forged Mg- Re alloy along either the LD or the TD direction at room and cryogenic temperatures exhibited typical quasi-cleavage characteristics. With decreasing the impact temperature, the fracture mode of the as-forged Mg-RE alloy changed from the intergranular fracture to the transgranular fracture due to the increased brittleness of the second phases at cryogenic temperature.