ABSTRACT

This study investigates heterogeneity in the deformation of an annealed sheet of commercially pure titanium (cp-Ti), its variation with loading direction and its influence on dwell fatigue response. Cp-Ti sheet with a typical cold rolling texture was deformed, along its rolling direction (RD) and transverse direction (TD), to the same strain level in tension. Higher level of grain boundary sliding in TD led to increased surface roughness (30% higher) in TD when compared to RD. Whereas, the higher extent of grain boundary-affected zones in TD resulted in a higher level of strain gradient, as evident from higher Kernal Average Misorientation values observed in TD. The reason for the higher extent of grain boundary sliding and formation of grain boundary-affected zones in TD samples was higher compatibility issues in TD. This work proposes a criterion defining the compatibility of a boundary. This criterion, which is based on the c-axis deviation of constituent grains with respect to the loading axis and the geometric compatibility of slip systems, is demonstrated to define the compatibility of a boundary completely. Using this criterion, a higher fraction of incompatible boundaries is identified in TD. Moreover, the markedly superior dwell fatigue response of RD samples when compared to TD is partly attributed to the lower heterogeneity in the nature of deformation in the former. Furthermore, textures with less damage prone grain boundaries and hence better dwell fatigue resistance is also proposed. Taken together, the present work underlines the principal role of crystallographic texture on the mechanical behaviour in general and dwell fatigue in particular.

摘要

本研究调查了商用纯钛退火板（cp-Ti）的变形异质性，随载荷方向的变化及其对保压疲劳响应的影响。具有典型冷轧织构的Cp-Ti板沿其轧制方向（RD）和横向（TD）变形至相同的拉伸应变水平。与RD相比，TD中较高的晶界滑动水平导致TD中的表面粗糙度增加（高30％）。TD中较高的晶界影响区域导致较高的应变梯度，这可从TD中观察到的较高的平均核取向差值看出。TD样品中较高的晶界滑动和形成晶界影响区的原因是TD中较高的相容性问题。这项工作提出了定义边界兼容性的标准。该标准基于构成晶粒相对于加载轴的c轴偏差和滑移系统的几何相容性而得到证明，可以完全定义边界的相容性。使用此标准，在TD中识别出更高比例的不兼容边界。此外，与TD相比，RD样品的明显的持久疲劳响应部分归因于前者变形性质的较低异质性。此外，还提出了具有较少的易于损坏的晶界并因此具有更好的耐持久疲劳性的织构。综上所述，目前的工作强调了晶体织构在一般的机械性能上，特别是在保压疲劳方面的主要作用。