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 a-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 a-phase microstructures, i.e., recovered a grains and recrystallized a grains. Untested samples contained mobile dislocations in recovered a grains, but in recrystallized a grains, any dislocations were observed. Internal crack initiation sites were formed transgranularly and were related to the recrystallized a grain region, judging from their morphology, size, and chemistry. Dislocations in recovered a grains were rearranged after cyclic loading in either {0110} 2 ^1120& planar arrays or subgrain structures due to dislocation annihilation. Few dislocations were seen in recrystallized a grains. We discuss the relationship between localized strain incompatibility due to coplanar arrays in recovered a grains and transgranular cracking in recrystallized a grains, and propose a model for fatigue crack generation.