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

时间：2020年12月4日14：00~15：15

地点：H栋508室

内容摘要：

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.

个人简介:

张桢教授毕业于华中科技大学和武汉理工大学，并在新加坡国立大学取得博士学位。2010-2013年在新加坡国立大学担任研究员，从事结构振动与微观结构力学的研究。2014年-2018年在英国帝国理工大学材料系从事博士后工作，研究材料保载疲劳与断裂等微观力学行为。2018年5月在英国布里斯托大学机械系从事博士后工作，研究下一代核电结构的长期响应。2018年11月起担任华中科技大学教授，博导，主持国家自然科学基金和华中科技大学自主创新基金的研究工作，学术兼职包括中国材料研究学会疲劳分会理事。