Mechanical Properties Of AZ31B And Az91D Magnesium Alloys At Cryogenic Temperatures

The tensile and compressive properties of as-extruded AZ31B and as-cast AZ91Dmagnesium alloys at room temperature（RT）,0℃,-50℃,-100℃,-150℃,-196℃,have beeninvestigated respectively by use of electronic universal material testing machine. Andthe fatigue properties of as-extruded AZ31B magnesium alloy in ambient air at RT andin vacuum (10^-4Pa) at cryogenic temperature （83K） has been studied by using the MyBPfatigue testing facility. Meanwhile, cryogenic fracture and deformation features of theAZ31B and AZ91D magnesium alloys were characterized by employment of opticalmicroscope （OM） and scanning electron microscope （SEM）.It shows that the microstructure of as-extruded AZ31B alloy consists of primaryα-Mg matrix phase and distributed Mg17Al12precipitates, with an average grain size ofabout25um. With decreasing tension temperature, the yield strength and ultimatetensile strength of AZ31B alloy increases while the elongation decreases all the time,especial between-100℃and-150℃. The AZ31B alloy exhibits ultimate tensilestrength and elongation of270MPa,17.65%and419MPa,8.2%at RT and-196℃,respectively. Tearing ridge and dimples could be seen on fracture surfaces ofthe as-extruded sample tested at room temperature, indicating favorable plasticity ofAZ31B alloy at RT. However, the fracture is characterized by quasi-cleavage plain forthe sample tested at cryogenic temperature. The OM observations indicate that theamount of twins found in the fractured AZ31B alloy samples increases with decreasingthe tension temperature.The fatigue life of AZ31B magnesium alloy in vacuum at cryogenic temperature isfound to enhance greatly, compare with in air at room temperature, especially at lowstress amplitude. For both test conditions applied in this study, the fatigue crack alwaysinitiated in the surface or near surface of the AZ31B magnesium alloy. The proportionof crack growth area in sample fatigue fractured in vacuum at cryogenic temperature isbigger than that in air at room temperature, and the fatigue striations is dense and clear.The fracture surface of the sample fatigued in air at room temperature exhibits features amixture of plastic and brittle fracture, and tends to be brittle fracture at cryogenictemperature in vacuum. In addition, the OM observations indicate that the amount of twins found in the AZ31B alloy samples fatigue fractured in vacuum at cryogenictemperature is larger than that in air at room temperature, and it is similar for thesamples fractured at low stress amplitude compared with at high stress amplitude.The microstructure of the as-cast AZ91D magnesium alloy mainly consists of-Mg matrix phase, β-Mg₁₇Al₁₂and divorced eutectic （α+β）. The effect of cryogenictemperature on tensile properties of the as-cast AZ91D magnesium alloys is obvious.Generally the tensile strength and ultimate yield strength of AZ91D alloy, either alonglongitudinal or transverse direction, decrease firstly and increase with decreasingtemperature, while elongation reduce all the time. The samples tensile fractured at RTor0℃show cellulite surface, and the samples fractured at lower temperature showsmooth surface. The compressive yield strength and fracture strength of the as-castAZ91D alloy increase with decreasing temperature, and the compressive fracture strainincreased firstly and then decreased, but both of them are suddenly varied at-100℃.