In recent years, (0001) twist grain boundaries (BTGBs) located in primary α grain clusters were identified as fatigue crack nucleation sites in different Ti alloys. In the present study, crack initiation was investigated in a bimodal Ti-5Al-4 V alloy subjected to low-cycle fatigue and dwell-fatigue loadings at room temperature. The low fraction of primary α grains was not associated with a lack of sensitivity to BTGB cracking. Transmission electron microscopy and electron back-scattered diffraction were used to characterize BTGBs in the initial microstructure. The fatigue mechanisms were then analyzed with a focus on dislocation activity. αp grains adjacent to cracked BTGBs contained a high dislocation density. It was primarily composed of planar slip bands of 〈a〉 dislocations. In addition, 〈c + a〉 dislocations were noticed in the vicinity of cracked BTGBs. They supposedly pertain to crack tip plasticity during growth, and no evidence of a role of an incoming slip event in crack nucleation was obtained. Also, basal slip bands extending across adjacent grains were found to emerge from BTGBs. This feature provides an easier path for crack extension when growth along the grain boundary becomes difficult owing to a deviation from the basal plane. Atom probe tomography analyses evidenced V and Fe segregation at a grain boundary with a significant deviation from the BTGB configuration. This suggests a possible contribution of local solute segregation to the high cracking resistance of general αp / αp grain boundaries. This work provides new insights into the mechanisms involved in cracking of BTGB in Ti alloys subjected to cyclic loadings.

中文翻译：

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双峰 Ti-5Al-4V 合金基面扭曲晶界 （BTGB） 的优先疲劳开裂：位错活性和裂纹萌生


近年来，位于初级晶粒簇中的（0001）扭曲晶界 （BTGBs） α 被确定为不同 Ti 合金的疲劳裂纹成核位点。在本研究中，研究了在室温下承受低周疲劳和驻留疲劳载荷的双峰 Ti-5Al-4 V 合金的裂纹萌生。初级α粒的低分数与对 BTGB 开裂缺乏敏感性无关。透射电子显微镜和电子背散射衍射用于表征初始微观结构中的 BTGB。然后分析疲劳机制，重点关注位错活动。与裂解 BTGBs 相邻的 αp 晶粒含有高位错密度。它主要由 〈a〉 位错的平面滑带组成。此外，在破裂的 BTGB 附近观察到 〈c + a〉 位错。据推测，它们与裂纹尖端生长过程中的可塑性有关，并且没有证据表明传入的滑移事件在裂纹成核中的作用。此外，还发现延伸到相邻颗粒的基底滑带从 BTGBs 中出现。当由于偏离基平面而难以沿晶界生长时，这一特征为裂纹扩展提供了更容易的路径。原子探针断层扫描分析证明 V 和 Fe 在晶界处偏析，与 BTGB 构型有显着偏差。这表明局部溶质偏析可能对一般 αp / αp 晶界的高抗裂性有贡献。这项工作为在循环载荷作用下 Ti 合金中 BTGB 开裂的机制提供了新的见解。