演讲人： 张桢教授 Prof. Zhen ZHANG
题目: Micromechanistic and Microstructural Investigations Into The Deformation of Ti Alloys in Cold Dwell Fatigue
Cold dwell fatigue has been known as an important engineering issue in component failure, featured by cyclic dwell-stress induced creep accumulation at relatively low-temperatures (i.e. T < 0.3Tm) and low stress levels (i.e. less than yield stress). The cold dwell behaviour was first acknowledged from the in-service failure in RR RB211 engine Ti-alloy disks on Lockheed Tristar aircraft in the 1970s. The inclusion of dwell period in loading introduces time-dependent stress redistribution, which makes cold dwell fatigue very different from classical fatigue. An interesting phenomenon in cold dwell fatigue is that the dwell debit has been found to be most severe at around 120oC and tends to diminish at around 200oC.
This work presents the rate dependent behavior of titanium alloys under cold dwell fatigue. A micromechanical study has been conducted on low temperature dwell fatigue resistance in multiphase polycrystalline Ti alloys. The origin of the observed peak in strain rate sensitivity over temperature has been explained by the transition from high-stress/low-temperature to low-stress/high temperature thermally activated dislocation release. Interestingly, the rate dependent behavior of Ti alloys were found to be dependent on the texture and phase morphology in multiphase Ti alloys, which is different from some conventional binary alloys.
Within the framework of cold creep and temperature sensitivity, relations between microscopic slip and macroscopic creep are established in dwell fatigue. The local rate dependent responses were found to be logarithmic creep in high stress state and exponential form in low stress state, respectively. Micromechanical evidence was provided in rationalizing cold creep at room temperature and creep saturation towards around 200oC. The roles of local rate-dependence, material elastic and plastic anisotropy in load shedding are discussed on possible dwell facets nucleation. These understandings are important in microstructural design of titanium alloys for resisting cold dwell fatigue.