Initially, this article precisely determines that the “facet” on the fracture surface of the Ti-6Al-4V alloy during high cycle fatigue (HCF) loading under positive stress ratios, as determined by white light interferometry and transmission Kikuchi diffraction (TKD), originates from cracking along the slip planes that exhibit a grain orientated α grain in the loading direction, predominantly involving shear deformation along the crystallographic planes. The article emphasizes the crack initiation mechanisms in Ti-6Al-4V alloy related to internal microstructures under positive stress ratio HCF loading, elucidating relationship between texture-induced crack initiation and growth, and anomalous mean stress sensitivity. Under positive stress ratio fatigue loading, preferentially orientated texture regions tend to generate irreversible dislocations parallel to the basal plane, gradually accumulating within multiple locations to form persistent slip bands (PSBs), which, under the influence of internal stresses, sequentially collapse and rupture, leading to nucleation of microcracks. Upon encountering unfavorably oriented texture regions or grains, PSBs or existing microcracks experience dislocation accumulation, which in turn induce grain refinement, thereby causing further crack nucleation. Consequently, microcracks, through aggregation and coalescence during the fatigue loading sequence, form the main crack, ultimately precipitating fatigue failure and engendering the anomalous mean stress sensitivity observed in the Ti-6Al-4V alloy.

中文翻译：

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不同应力比下Ti-6Al-4V高周疲劳平均应力敏感性机制

首先，本文通过白光干涉仪和透射菊池衍射（TKD）精确确定了Ti-6Al-4V合金在正应力比高周疲劳（HCF）加载过程中断裂表面的“刻面”，起源于沿滑移面的裂纹，在加载方向上呈现晶粒取向的 α 晶粒，主要涉及沿晶面的剪切变形。本文强调了 Ti-6Al-4V 合金在正应力比 HCF 载荷下与内部微观结构相关的裂纹萌生机制，阐明了织构引起的裂纹萌生和扩展以及异常平均应力敏感性之间的关系。在正应力比疲劳载荷作用下，优先取向的织构区域往往会产生平行于基面的不可逆位错，并在多个位置逐渐积累形成持久滑移带（PSB），在内应力的影响下依次塌陷和断裂，导致微裂纹的成核。当遇到不利取向的织构区域或晶粒时，PSB 或现有的微裂纹会经历位错积累，进而引起晶粒细化，从而导致进一步的裂纹成核。因此，微裂纹在疲劳加载过程中通过聚集和合并形成主裂纹，最终导致疲劳失效并产生在 Ti-6Al-4V 合金中观察到的异常平均应力敏感性。