Study On The Microstructure And Mechanical Properties Of Magnesium Alloys Under Simulated Lunar-environment Temperature

Magnesium alloy spacecrafts (i.e., the lunar rover) the parts of which have great weight saving effects often work under low temperature or even high-low temperature alternating cycle environments. So it is of great significance to make it clear that the mechanical properties and fracture mechanism of magnesium alloy under these conditions. In this paper, the ambient temperature on the moon was simulated with laboratory conditions, the commercial wrought AZ31 alloy and new high-strength rare-earth GW103K alloy were processed by cryogenic (-196℃, 10 days) or high-low temperature cycles treatments (-196℃×11h+200℃×11h) firstly. Then the mechanical properties, microstructure and fracture mechanism of these two alloys at low temperatures (-196℃), room temperature (25℃) and high temperature (200℃) were studied separately, and the two alloys'plastic deformation mechanism at different temperatures were explained tentatively.The study on AZ31 alloy show that compared to the alloy(σ0.2=217MPa,σb=288MPa, =18.3%) without heat treatment, cryogenic or high-low temperature cycle treatments have no obvious effects on the alloy's tensile properties at room temperature, but the hardnesss tend to decrease. The fracture mechanisms of all alloys at room temperature are mainly quasi-cleavage fracture. Compared to the tensile mechanical properties at room temperature, AZ31 alloys'yield strength and tensile strength increase greatly at -196℃, while the elongations also decrease obviously. The alloys'tensile strengths increase slightly at low temperature after cryogenic or high-low temperature treatments compared with the alloy (σ0.2=333MPa,σb=442MPa, =7.1%) without heat treatment. In addition, the ruptures of all states alloys are brittle fracture with poor ductility at low temperature. The yield strength and tensile strength of all AZ31 alloys at high temperature are significantly lower than the corresponding values at room temperature and low temperature, while the elongations increase substantially. There are no significant properties differences among all the states, and the tensile fracture mechanism is maily ductile fracture.The research on as-cast GW103K alloys show that as-cast GW103K without heat treatment has good strengths and poor plasticities at both room temperature and high temperature. The alloy has a higher yield strength and tensile strength (σ0.2=311MPa,σb=358MPa) at low temperature while the elongation almost keeps constant. Cryogenic soaking has no obvious impact on the alloy's yield strength and tensile strength, but the values decrease after high-low temperature cycle treatments. And after 20 cycles, the high temperature yield strength and tensile strength decrease to 121MPa and 146MPa separately. Cryogenic soaking can improve the the critical stress of dislocation slip in the alloy, and it can promote the internal energy by causing the substrate lattice distortion, which is reinforcing factor; at the same time, the low temperature further exacerbate the hard brittle properties of Mg24(Gd, Y)5, which is weakening factor. At room temperature, the weakening factor takes the leading position and the cryogenic soaking reduces the strength; at high temperature, the dominant factor is the reinforcing factor, the alloy's strength increase. The fractures of all the as-cast GW103K alloys at different temperatures mainly belong to intergranular fracture, which is caused by the hard brittle characteristics of the eutectic phase.The study on as-extruded GW103K alloy show that the alloy has good mechanical properties at room temperature:σ0.2=295MPa,σb=398MPa and =5.2%. After cryogenic soaking the alloy's mechanical properties reduce slightly, while the high-low temperature cycle treatment can improve alloy's mechanical properties and the alloy reaches the optimal mechanical properties after 20 cycles. At high temperature, the yield strength and elongation of as-extruded GW103K alloy are 317MPa and 8.6% respectively, which are obviously higher than the corresponding values at room temperature. However, the tensile strength (362MPa) was significantly lower than that at room temperature. Cryogenic soaking has no distinct effect on the alloy's high temperature properties, while the high-low temperature cycle treatment can bring down the high-temperature strengths, and the values decrease more and more with the cycle number increases.At low temperature, all the as-extruded GW103K alloys have higher mechanical properties than that at room temperature and high temperature. The yield strength and tensile strength of alloy without heat treatment are 349MPa and 506MPa respectively at low temperature; and it is greatly different from AZ31 and as-cast GW103K that the as-extruded GW103K's low temperature elongation increases by 37% than room temperature elongation relatively, from 5.2% to 7.1%. Although the cryogenic soaking can improve the alloy's yield strength, it also reduces the tensile strength and elongation. By contrast, high-temperature cycle treatment can further enhance the low-temperature strength of as-extruded GW103K. The rupture of as-extruded GW103K alloy at room temperature mainly belongs to quasi-cleavage fracture; the shallow dimples and little tearing edges comes into being when the temperature goes to 200℃; at -196℃, both ductile fracture and cleavage fracture characteristics are observed in the morphologies.