The Effects of Load Ratio,Interstitial Content,and Grain Size on Low-Stress Fatigue-Crack Propagation in α-Titanium

Abstract

Low-stress fatigue-crack-propagation tests have been carried out on three commercially pure α-titaniumalloys. It was found that decreasing load ratio (R), increasing grain size, and increasing interstitial alloying content could all produce significant reductions in growth rate over the ∆K range studied (4–20 MN/m²). The conclusion was reached that the fatigue-fracture process comprised two stages: (1) the formation of relatively planar facets (primarily ∆K-controlled) and (2) their interconnection by a mechanism involving plastic tearing (primarily K _max-controlled). Scanning electron microscope examination of the fatigue-fracture surfaces revealed that the orientations of individual grains exerted a considerable influence on fracture-surface morphology. This effect occurred when the scale of reversed plasticity at the crack tip was of the order of, or less than, the grain size. A transition in fracturesurface appearance occurred in all specimens at an approximately constant value of growth rate (2.5 × 10–5 mm/cycle). This was associated with an environmentally controlled change in the extent to which a faceted fracture surface was developed in individual grains.