Study On Microstructures And Mechanical Properties Of Ultralow Temperature Ti-5Al-2.5SnELI Cast Alloy

With the putting into practice of the manned space flight and chang'e projects of our country, more and more ultralow temperature Ti-5Al-2.5SnELI cast alloy are needed. But we lack of data accumulation and theory research in the aspects of study and application of this alloy. Therefore,in order to provide the base of microstructures and properties for the further studying on the ultralow temperature properties of this alloy and provide theoretical basis for the further engineering application, the studies on the microstructures and mechanical properties of the ultralow temperature Ti-5Al-2.5SnELI cast alloy, have been carried out focusing on the effect of annealing treatment, vacuum dehydrogenation annealing treatment and solution heat treatment on the microstructures and mechanical properties, and the effect of impacting at -196℃(liquid nitrogen temperature) and tensioning at -253℃(liquid hydrogen temperature) on the microstructures and mechanical properties, and the deforming mechanisms of this alloy at 20℃and -253℃.The study proves that annealing at 700℃to 850℃, the tensile strength and the yield strength of the alloy gradually increase with the increase of temperature out of furnace under the condition of cooling in furnace. It is found that the mechanical property of the welding position has been improved after vacuum dehydrogenation annealing.The transition temperature from a phase toβphase for the Ti-5Al-2.5SnELI cast alloy is about 1020℃. The microstructure and property of the alloy have greatly affected by solution heat treatment process. Holding at 1020℃for one hour, then cooling to room-temperature in furnace, the structure of this alloy will be slice-like sawtooth , and the phase isα. The synthetic mechanical property is best. R_m=813MPa, R_p0.2=768MPa, A=11.3%, Z=22%, E=123GPa.When impacting at -196℃(liquid nitrogen temperature) and -253℃(liquid hydrogen temperature) for the alloy, a small amount of twin is found in the sectional microstructure of the impact fracture, but twin is not found in room-temperature impact sample, and the impact work of the alloy at liquid nitrogen temperature is obviously less than that at room-temperature, tensioning at -253℃(liquid hydrogen temperature) the saw tooth-like waving phenomenon is found in the stress-strain curve of tensioning and twin and slip line are found in the sectional microstructure of the tension fracture ,however, similar phenomenon have not been found in room-temperature tension the tensile testing under the condition of -253℃(liquid hydrogen temperature), strength and yield strength of the alloy have been greatly improved, and the plasticity is changed little. The deformation mechanism of the alloy is twinning and slipping combined action at -253℃(liquid hydrogen temperature),and slipping action at 20℃.