| Mg-Li alloys, as the lightest metallic structural materials, have great potential application in spacecraft and automotive industries due to their low density, high specific strength and specific stiffness, good damping characteristics and excellent machinability etc. However, because of the rapid decrease of strength and poor creep resistance, the application of these alloys is restricted at the temperature higher than 120℃. The deterioration of high temperature mechanical properties in the Mg-Li alloys is attributed to the softening of discontinuous phase. So far, many efforts have been dedicated to improve the high temperature mechanical properties of Mg-Al based alloy, accordingly a series of commercial alloys have been explored, such as Mg-Al-Sn, Mg-Al-RE and Mg-Al-Ca-Sr systems. The aims of these efforts are decrease of Al content and produce the high melting point intermetallics. So rare earth, copper and tin can also be used in Mg-Li alloys, improve alloy high temperature resistant performance. Due to the disadvantage of Mg-Al alloy, the Mg-Zn-Al (ZA) systems alloy with Zn content is proposed as a low-cost, which were considered a promising high temperature creep resistant magnesium alloys. The ZA systems alloy exhibits low ductility due to the formation of a large amount of network phase at the grain boundaries, moreover the alloy is prone to hot tearing, and the die castability is sensitive to the alloy composition. Until now, except for using as decorative material, there is still limited commercial application of this alloy system.The optimized LAZ532-2RE,LAZ532-2Cu and LAZ532-1Sn alloys were extruded at 553K. The diameter of specimens after extrusion was changed from 55mm to 13mm. The microstructure and mechanical properties of the alloys were investigated.The extruded LAZ532-2RE alloy at 150℃, high temperature tensile deformation mechanism is the coordination of twins and dislocation deformation mechanism. When the temperature rises to about 200℃, the grain softening, and the deformation mechanism is dislocations and grain boundary sliding. With temperature increasing, the softening (3 phase precipitates from the a-Mg solid solution, which occurred more in harmony with a large deformation. The high temperature tensile deformation mechanism at this time is the grain boundary sliding mechanism, and accompanied by the precipitation of second phase precipitation. The extruded LAZ532-2RE alloy was performed on creep tests at 398K,423K and 448K, The stress exponent of n varies from 4.25 to 6.23, and the activation energy varies from 104 to 134kJ/mol. There is a transition between dislocations climb dominated creep mechanism and dislocation creep controlled by non-basal planes slip. The stress exponent is between 4.25-4.98, the creep mechanism is dislocation climb creep. When n is greater than 6, are subject to non-basal dislocation motion-controlled creep. At high temperature creep test, the rare earth phase hindered the dislocation movement, improved high temperature creep properties of the alloy.At the temperature 150℃, the extruded LAZ532-2Cu alloy still has higher strength, when the temperature reaches 200℃, the strength decreased a lot, but the elongation up to 44% at the strain rate 1×10-2. Under the temperature of 125℃,150℃and 175℃, the stress of 60Mpa,80Mpa and 100Mpa, the creep test of extruded LAZ532-2Cu alloy was carried out. The stress exponent n values are 3.72,4.8 and 6.1, the creep activation energy Qc values are 94.8kJ/mol,123.9kJ/mol and 128kJ/mol respectively. At the temperature range of 125℃～150℃, the creep mechanism is dislocation climb creep mechanism. When the temperature reaches 175℃, the non-basal plane dislocation motion is dominated creep mechanism. During the creep test, the copper compounds hindered dislocation movement, the alloy occurred strengthening effect of aging. The extruded LAZ532-2Cu alloy creep properties are very close to the extruded LAZ532-2RE alloy.At temperature 150℃, the extruded LAZ532-1Sn alloy has higher strength. When the temperature reaches 200℃, the strength decreased markly, the elongation up to 75% at the strain rate 1×10-2. At temperature range of 125℃～175℃, stress range of 60MPa～100Mpa, the stress exponent n values are 2.5,3.7 and 5.8, the creep activation energy Qc value are 104kJ/mol,118kJ/mol and 135kJ/mol respectively. At the stress exponent n≈2, the creep activation energy around 100kJ/mol, the corresponding creep mechanism is the grain boundary sliding creep. When the temperature reaches 150℃, the stress exponent n≈3～4, the creep mechanism is dislocation climb creep. When the temperature continues to rise, the stress exponent n≈6, the creep activation energy Qc=135kJ/mol, the creep mechanism is the non-basal plane dislocation creep motion control.The extruded LAZ532-2RE alloy, there were two plastic instability phenomena in the tensile curve. The first instability of small fluctuations is the effect of atoms on dislocation, which is "dynamic strain aging" mechanism. The second largest zigzag instability phenomenon is caused by fluctuations in the shear-induced deformation twins. This phenomenon of plastic instability is caused by the deformation twins of the "steady-state twin plastic instability" mechanism. The extruded LAZ532-2RE alloy at room temperature tensile appears abnormal phenomenon can be interpreted as positive strain rate sensitivity is caused by strain hardening, the negative strain rate sensitivity is due to the twin plastic instability.Li elements decrease the lattice constant c value of magnesium. According to X-ray step scanning results, the LAZ532-2RE alloy of a-Mg grain axial ratio c/a values decreased from 1.624 to 1.6074. Therefore, the alloy has a better plastic at room temperature, produce more deformation twins, which greatly enhanced the elongation of the alloy. The{1012} and {1011} deformation twins were observed from the extruded LAZ532-2RE alloy.
|
PDF Full Text Request