Fatigue Crack Growth Mechanisms in HCP Single Crystals

关于HCP晶体疲劳裂纹扩展机制的研究很少。这可能部分是由于HCP晶体的滑移系统往往还没有被精确地识别出来。此外，由于HCP金属中的滑移系统数量有限，在HCP晶体中可能发生机械孪晶以适应变形。这也使得阐明HCP金属疲劳裂纹扩展机制变得困难。在HCP晶体中，a-钛及其合金因其优异的比强度和耐腐蚀性已被应用于航空航天结构和发动机部件。因此，解决这类材料的基本机制是很重要的。本文研究了室温下a-钛单晶在实验室空气中的疲劳裂纹扩展行为。制备了两种不同切口取向的CT标本E和F。E-和f -试件的缺口平面为（0001），缺口方向分别为[1010]和[2110]。在E-和f -试样中，裂纹扩展大致平行于基面，在裂纹面附近发现了对应于{1012}孪晶体系的痕迹。在疲劳表面上观察到平行于<1010>的脊和{1012}孪晶的痕迹和带。这些结果表明裂纹扩展可能是由于微孪晶的激活而发生的。

There have been few studies on fatigue crack growth mechanisms in HCP crystals. This may be partly due to the fact that the slip systems of HCP crystals have often not yet been identified precisely. In addition, mechanical twinning may occur in HCP crystals to accommodate deformation, since the number of slip systems in HCP metals is limited. This may also make it difficult to clarify fatigue crack growth mechanisms in HCP metals. Among HCP crystals, a-titanium and its alloys have been applied to aerospace structures and engine components because of their excellent specific strength and corrosion resistance. Therefore, it is important to address fundamental mechanisms in such materials. In this investigation, the fatigue crack growth behavior of a-titanium single crystals has been investigated in laboratory air at room temperature. Two types of CT specimens E and F with different notch orientations were prepared. In the E- and the F-specimens, the notch plane was (0001), and the notch directions were [1010] and [2110], respectively. The crack propagates roughly parallel to the basal plane in both the E- and the F-specimens, and the traces which correspond to {1012} twin systems are found near the crack plane. Ridges parallel to <1010>, and traces and bands due to {1012} twinning are observed on the fatigue surfaces. These results suggest that the crack growth might occur plausibly by the activation of micro-twins.