Cyclic deformation, dislocation structure, and internal fatigue crack generation in a Ti-Fe-O alloy at liquid nitrogen temperature

To clarify the internal fatigue crack generation in a Ti-Fe-O (near α-type) alloy, microstructures, internal fatigue crack initiation sites, and dislocation structures in samples fractured during high-cycle fatigue tests at liquid nitrogen temperature were studied. The alloy contained two kinds of elongated α-phase microstructures, i.e., recovered α grains and recrystallized α grains. Untested samples contained mobile dislocations in recovered α grains, but in recrystallized α grains, any dislocations were observed. Internal crack initiation sites were formed transgranularly and were related to the recrystallized α grain region, judging from their morphology, size, and chemistry. Dislocations in recovered α grains were rearranged after cyclic loading in either \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document} $$\left\{ {01\bar 10} \right\} - \left\langle {11\bar 20} \right\rangle $$ \end{document} planar arrays or subgrain structures due to dislocation annihilation. Few dislocations were seen in recrystallized α grains. We discuss the relationship between localized strain incompatibility due to coplanar arrays in recovered α, grains and transgranular cracking in recrystallized α grains, and propose a model for fatigue crack generation.