Low cycle fatigue behaviour of titanium alloy 685

Abstract

The low cycle fatigue behaviour of a near α titanium alloy, IMI 685, has been studied in its single phase microstructural state with martensitic platelets of α′, at room temperature and at 500°C. There is excellent cyclic stability at low strain amplitude (Δε{lunate}p/2≤1.12%) both at room temperature and at 500°C. However, there are marked differences in the cyclic stress response of the material at the two test temperature at higher strain amplitudes (Δε{lunate}p/2 > 0.12%). On the one hand there is mild softening at room temperature and on the other there is significant hardening at 500°C. The cyclic hardening at 500°C is attributed to strong dynamic strain aging. Fatigue life based on the plastic strain component is greater at 500°C by nearly a factor of two than that at room temperature. However, the disparity in fatigue life at the two test temperatures is observed to decrease at the lower strain amplitudes, when the evaluation is based on the total strain. The mode of fracture is shown to be transgranular both at room temperature and 500°C. A few smooth facets are seen in the peripheral region of the fracture surface from the test at room temperature. These facets appear to be feature-less at low magnification but reveal distinct striations at high magnifications. There are no such smooth facets on the fracture surface of the specimens tested at 500°C; however, striations are observed from the surface towards the interior to a much larger extent than that at the room temperature. Also there is a strong tendency for secondary cracking, with crack propagation at 500°C. © 1988.