Abstract
Fatigue tests were carried out at the stress ratio R = -1 using a 20 kHz ultrasonic testing facility to investigate the effects of low temperature nitriding on the fatigue properties of Ti-6Al-4V alloy in the very high cycle fatigue (VHCF) regime in detail. The oscillation and fatigue behavior of the nitrided Ti-alloy were characterized by measuring parameters like the ultrasonic generator power, the displacement of the specimens and dissipated energy under ultrasonic cyclic load. Moreover, the surface microstructure of the nitrided Ti-alloy was characterized using a micro-Vickers hardness tester, an optical microscope, scanning electron microscopy (SEM), X-ray diffraction (XRD) and electron backscatter diffraction technique (EBSD) to clarify the fatigue fracture mechanism. The Ti-alloy nitrided at the temperature of 873 K showed duplex S-N properties consisting of the respective fracture modes of the surface fracture and the subsurface fracture. The low temperature nitriding reduced the surface fatigue life of Ti-alloy in comparison to the un-nitrided one due to the formation of a brittle titanium nitride (Ti2N), whereas the subsurface fatigue life in the VHCF regime was increased by the low temperature nitriding. In addition, the fatigue fracture mechanisms of the low temperature nitrided Ti-alloy were discussed from viewpoints of fractography and fracture mechanics.
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