With the development of the space technology, the materials used in space with special properties are increasingly required. Because of their high specified strength and excellent mechanical performance, titanium and titanium alloys have been widely applied as structural components in the space. However, the complicated conditions of space may have the important influence on the behavior of materials. It is strategic to investigate the serving behavior, optimize the microstructure and improve the properties and adaptabilities of materials in the space.
The bimodal and basketweave microstructures were received by the heat treatment of the Ti-6Al-4V stick. To investigate the effect of the space conditions on the serving performance of Ti-6Al-4V alloy, the tensile experiments at different temperatures, the fatigue behavior of the alloy in vacuum and/or at low temperature and the ultra-high cycle fatigue were conducted. Again, the irradiation with high energy ions was carried out on the Ti-6Al-4V plates in order to investigate the influence of irradiation on the mechanical performance.
The results show that the strength increases and ductility decreases with decreasing the tensile temperatures. No obvious brittleness is found even tensioned at the liquid temperature. The alloy has the superior fatigue property in vacuum and/or at low temperature compared with that in laboratory air. Moreover, the coupling of high vacuum with low temperature can much improve the fatigue properties of the alloy. It is interestingly found that the vacuum affects the crack propagation and results in longer fatigue life, while the low temperature affects the crack initiation and leads to the higher fatigue strength. As for the fatigue in ultrahigh cycle regime, the fatigue S-N curves continue to decrease with increasing the number of cycles to failure and no conventional fatigue limits exist in the regime of 105 to 109 cycles for Ti-6Al-4V with both bimodal and basketweave microstructure. There is a transition of crack initiation from surface to interior of specimens with decreasing the stress level. As compared with the conventional loading frequency, the ultrasound frequency extends the fatigue life, but it has no obvious effect on the fatigue strength in the ultrahigh cycle regime. Unfortunately, the ultrahigh cycle fatigue behavior can not be much enhanced by shot peening with Al2O3 or steel pills in present conditions. So it is necessary to further investigate the application of the shot peen to improve the ultrahigh cycle fatigue properties. Under the present conditions of low temperature, high vacuum and the very long life regime, the alloy with the bimodal microstructure has superior performance to basketweave microstructure. Furthermore, the bimodal microstructure is less sensitive to the environmental conditions. Therefore, it is concluded that the Ti-6Al-4V alloy with the bimodal microstructure is more suitable than basketweave microstructure to be applied in the space environment. Additionally, the tensile and fatigue properties of Ti-6Al-4V plate deteriorate after irradiated with hydrogen or nitrogen ions. As a whole, irradiation with hydrogen ions plays more serious role than with nitrogen ions on the mechanical properties of the alloy. Meanwhile, the irradiated surface of the sample shows a kind of “dying color” effect, which has no much effect on the mechanical properties of the alloy.
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