Analysis of Subsurface Fatigue Crack Generation in Ti–Fe–O Alloy at Low Temperature

Abstract

Subsurface microcracks developed in the Ti-Fe-O cross-rolled material were characterized in order to clarify the subsurface fatigue crack initiation. A considerable amount of microcracks in β platelets were detected. The {0001} transgraular microcracks in α grains predominantly generated at the β grain boundary between recovered α grain and recrystallized α grain, and grew into the recrystallized α grain along the basal plane, although the microcracks at the twist boundary of α grains and at the α-β interface were occasionally detected. Stress concentration around the microcrack in β platelets may assist the microcrack initiation on basal plane (transgranular facet) in neighboring α grain due to a combination of the shear stress and tensile stress normal to the basal plane at α-β boundary. The {0001} transgraular microcracks growth and/or coalescence occurred with the assistance of shear stress on prismatic plane such as {1010} steps.

Effects of Crystallographic Texture on Subsurface Fatigue Crack Generation in Ti–Fe–O Alloy at Low Temperature

“…The stress concentration by the microcracks developed at the β-grain boundary may trigger (0001) microcracking in the neighboring α grain. Although (0001) α microcracks constituted the main crack in the cross-rolled material, 25) few (0001) α microcracks were detected in the present material. The subsurface crack initiation sites (particularly at 77 K) consisted mostly of 10 10 { } α facets and were rather devoid of (0001) α facets.…”

“…It developed strain incompatibility between a recovered α (soft) grain and a recrystallized α (hard) grain in the cross-rolled Ti-Fe-O alloy after cyclic deformation. 25) The strain incompatibility between recrystallized α grains and β grains could have induced microcrack (or void) nucleation, and the rearrangement of dislocations in the neighboring recovered α-grains enhanced the strain incompatibility. In the present study, a considerable number of microcracks in β grains were detected in highly strained regions such as near the fracture surface.…”

“…7 was higher than that in the α grains. 33) Because the thickness of β grains was larger than that of the cross-rolled material, 25) the longer mean free slip length in the β phase could have induced a higher local stress concentration at the α-β interface, particularly at the triple point of α-β1-β2 (see Fig. 9).…”

“…22,23) The near-α type Ti-Fe-O alloy which provided a characteristic structure of α phase with dispersed small amount of β phase clearly exhibited subsurface fatigue crack initiation at 77 K, where the facet was detected as a (0001) α crack. 10,24,25) Cryogenic temperature fatigue could be most favorable to characterize the subsurface crack generation, because the subsurface crack initiation and the strain incompatibility prevail more at lower temperatures with higher the critical resolved shear stresses (CRSSs) of individual slip systems and their differences. 26) Such heterogeneous slip deformation manner revealed that the internal stress in the α grain had accumulated normal to (0001) α by the full constraints model analysis, in which resulted in (0001) α cracking.…”

Fractographical Analyses of Crack Initiation Site in High-cycle Fatigue for Ti–Fe–O Alloy at Low Temperature

“…The RD-split component is overlapped on the TD-split one. The CR provided microstructure with rather clearly divided into the two regions because of higher reduction in the rolling process, 20) where the rotation in the TD-split texture 21) and the basal texture in the recrystallized α grains were promoted. Furthermore, these rolled plates contain a macrozone (microtexture) such as recovered α grain region as the previous study.…”