Due to the initiation of fine granular area inside the material and the formation of nanoscale grains, it is difficult to conduct in-situ observation and high-scale characterization. This is the main reason why the formation mechanism of fine granular area in very high cycle fatigue has been unknown and controversial. Therefore, on the basis of fracture analysis method to invert the fine granular area formation, we further put forward an experimental proposal whether a microcrack in the fine granular area formation stage can be prepared to observe the critical event of early damage evolution. Here, we selected selective laser melting Ti-6Al-4 V alloy with inherent defects as the model material to obtain significant defect-initiating fine granular areas, found two secondary microcracks after dissecting along the defects, and then carried out multiscale characterization and quantitative analysis of main cracks and secondary cracks. We found that the fine grains originate from the severe plastic deformation in local of crack tip plastic zone and lowered the cracking threshold by the grain boundary sliding between the hard-oriented grains, which results in grain refinement, cavitation and cracking. This work systematically describes damage evolution mechanism, which has guiding significance for the reliability evaluation and fatigue resistance design of materials.

中文翻译：

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基于选择性激光熔化Ti-6Al-4 V合金中细晶粒区域起始和演化的VHCF跨尺度早期损伤机制


由于材料内部细颗粒区域的开始和纳米级颗粒的形成，很难进行原位观察和大尺度表征。这是极高周疲劳中细粒区形成机制一直未知且存在争议的主要原因。因此，在裂缝分析方法反演细粒区形成的基础上，进一步提出了是否可以制备细粒区形成阶段的微裂纹来观察早期损伤演化临界事件的实验建议。本文选取具有固有缺陷的选择性激光熔化Ti-6Al-4 V合金作为模型材料，获得显著的缺陷起始细颗粒区域，沿缺陷解剖后发现两个次生微裂纹，然后对主裂纹和次生裂纹进行多尺度表征和定量分析。研究发现，细晶粒源于裂纹尖端塑性区局部的严重塑性变形，并通过硬取向晶粒之间的晶界滑动降低了开裂阈值，导致晶粒细化、空化和开裂。本工作系统地描述了损伤演化机制，对材料的可靠性评价和抗疲劳设计具有指导意义。