Everaerts J, Verlinden B, Wevers M
Department of Materials Engineering, Katholieke Universiteit Leuven, Leuven, Belgium.
J Microsc. 2017 Jul;267(1):57-69. doi: 10.1111/jmi.12547. Epub 2017 Mar 15.
In the very high cycle fatigue regime, internal crack initiation can occur in Ti-6Al-4V because of the formation of facets, which are α grains that have fractured in a transcrystalline and planar manner. Because this crack initiation phase occupies most of the fatigue life, it is essential to understand which mechanisms lead to facet formation. Fatigue tests have been performed on drawn and heat-treated Ti-6Al-4V wires, and the facets at internal crack initiation sites have been analysed in detail in terms of their appearance, their spatial orientation and their crystallographic orientation. The facets were not smooth, but showed surface markings at the nanoscale. In nearly all cases, these markings followed a linear pattern. One anomalous facet, in a sample with the largest grain size, contained a fan-shaped pattern. The facets were at relatively steep angles, mostly between 50° and 70°. Cross-sections of the fracture surfaces have been made by focused ion beam milling and were used to measure the crystallographic orientation of facets by electron backscatter diffraction. Most facet planes coincided with a prismatic lattice plane, and the linear markings were parallel to the prismatic slip direction, which is a strong indication that prismatic slip and slip band formation led to crack initiation. However, the anomalous facet had a near-basal orientation, which points to a possible cleavage mechanism. The cross-sections also exposed secondary cracks, which had formed on prismatic lattice planes, and in some cases early stage facet formation and short crack growth phenomena. The latter observations show that facets can extend through more than one grain, and that there is crack coalescence between facets. The fact that drawn wires have a specific crystallographic texture has led to a different facet formation behaviour compared to what has been suggested in the literature.
在超高周疲劳 regime 中，由于小平面的形成，Ti-6Al-4V 中会发生内部裂纹萌生，这些小平面是沿穿晶和平面方式断裂的α晶粒。由于这个裂纹萌生阶段占据了大部分疲劳寿命，了解哪些机制导致小平面形成至关重要。对拉拔和热处理的 Ti-6Al-4V 丝进行了疲劳试验，并对内部裂纹萌生部位的小平面在外观、空间取向和晶体取向方面进行了详细分析。小平面并不光滑，而是在纳米尺度上呈现出表面痕迹。几乎在所有情况下，这些痕迹都呈线性模式。在一个晶粒尺寸最大的样品中，有一个异常小平面包含扇形图案。小平面的角度相对较陡，大多在50°到70°之间。通过聚焦离子束铣削制备了断口表面的横截面，并用于通过电子背散射衍射测量小平面的晶体取向。大多数小平面与棱柱晶格平面重合，线性痕迹平行于棱柱滑移方向，这有力地表明棱柱滑移和滑移带形成导致了裂纹萌生。然而，异常小平面具有近基面取向，这表明可能存在解理机制。横截面还暴露了在棱柱晶格平面上形成的二次裂纹，以及在某些情况下的早期小平面形成和短裂纹扩展现象。后一种观察结果表明，小平面可以延伸穿过多个晶粒，并且小平面之间存在裂纹合并。拉拔丝具有特定晶体织构这一事实导致了与文献中所建议的不同的小平面形成行为。
