Characterization of Crack Nucleation in Titanium Alloys with Metastable Microstructures

通过利用透射菊池衍射（TKD）和像差校正的扫描透射电子显微镜对纳米晶粒取向、晶粒旋转和原子尺度的晶格应变进行多尺度表征，提供了FGA内纳米结构化过程的有力证据。直接时效处理可用于通过热马氏体的分解和粗β晶粒内纳米层状α结构的沉淀来优化焊件的机械性能，从而增强裂纹成核阻力。氧化域和最顶部纳米晶粒的开裂界面促进表面退化，从而促进沿接触区域边界的应力失配沿着，并且在裂纹尖端处的氧吸收的帮助下导致其后的宏观裂纹的形成。

Definitive evidence for the nanostructured process within FGA is provided by a multi-scale characterization of the nanograin orientation, grain rotation, and the lattice strain up to atom scale by utilizing the transmission Kikuchi diffraction (TKD) and aberration-corrected scanning transmission electron microscope. Direct aging treatment can be used to optimize the mechanical properties of the weldment by decomposition of the thermal martensites and precipitation of the nano lamellar alpha structure within the coarse beta grain, thereby enhancing the crack nucleation resistance. Cracking interfaces of the oxidation domain and topmost nanograins facilitate surface degradation, thereby promoting the stress mismatch along the contact area boundary and giving rise to the formation of macrocrack thereafter with the assistance of oxygen pick-up at the crack tip.