| Tensile and vacuum fatigue properties and deformation-fracture behavior at cryogenic temperature of a 720℃/1h annealed TC4 alloy have been studied by means of universal material testing system (MTS), fatigue testing machine, optical microscope, scanning election microscope(SEM) and transmission electron microscope (TEM). Influence of Co60 irradiation on tensile properties of TC4 alloy has also been investigated. The cryogenic tensile test was carried out at -100℃,-150℃and -195℃, while the fatigue test was performed in laboratory air, in vacuum(10-3Pa) and in cryogenic temperature-vacuum(-195℃, 10-4Pa), respectively.The results show that the 720℃/1h annealed TC4 alloy possesses anα+βbimodal microstructure and exhibits excellent tensile properties, with an UTS and elongation of 1018.4MPa and 14.8% at room temperature, and 1524.1MPa and 14.5% at -195℃,respectively. The UTS and yield strength of TC4 alloy were found to monotonously increase, while the elongation slowly decreases with decreasing the testing temperature, and the elongation shows slightly increasing at -195℃. The Co60 irradiation has a little effect on tensile properties of TC4 alloy, resulting in a reduction of 20～30MPa in UTS and 1% decrease in elongation at room temperature. SEM observations indicated that the fractured surfaces of TC4 alloy tested at room and cryogenic temperatures displayed dimple fracture features, and the uniformity of dimples size distribution decreased with decreasing the testing temperature. TEM examinations showed that the dominant deformation mechanism of TC4 alloy tensioned at room and cryogenic temperatures would be dislocation slip.The fatigue property of the annealed TC4 alloy was apparently different as tested in the three environments of laboratory air, vacuum(10-3Pa) and cryogenic temperature-vacuum(-190℃, 10-4Pa), and the highest fatigue life could be obtained in the cryogenic temperature-vacuum, while the lowest one in the laboratory air. It has been found that the fatigue life of TC4 alloy in vacuum is about two times of that in laboratory air and the fatigue life in cryogenic temperature-vacuum increases obviously, indicating the effect of temperature on fatigue life is greater than that of vacuum. SEM observations showed that the crack initiation site, crack prorogation path and rapid failure zone were distinguishable on the fractured surfaces of the specimens fatigued in laboratory air and cryogenic temperature-vacuum. The crack initiated at surface of the fatigue specimens, and a lot of dimples could be found on the rapid failure zone. Some melted zone could be observed on the fractured surface of specimens tested in vacuum, because of the low thermal conductivity of titanium alloy which leading to the generation of heat produced by cyclic stressing. No such features could be found in the specimens tested in air or in vacuum at cryogenic temperature. TEM examinations showed that dislocation slip is the main deformation mode of the TC4 alloy fatigued in laboratory air or in the cryogenic temperature -vacuum. |
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