The twisting mechanism of subsurface fatigue cracking in Ti–6Al–2Sn–4Zr–2Mo–0.1Si alloy

The initiation phenomenon of a subsurface fatigue crack was examined by monitoring of the acoustic emission (AE) effects revealed by the slightly surface-hardened specimens of titanium-based VT3-1 (Ti–6Al–2Sn–4Zr–2Mo–0.1Si) alloy under fatigue tests. The circumferentially notched round-bar specimens were cyclically stretched in the stress range of 720–680 MPa, with the stress ratio varying as 0.31–0.36, at 35-Hz loading frequency. The AE monitoring has shown the subsurface cracking events as concomitant with the unloading (compressive) portions of the cyclic load trajectory. In the several fractographically examined areas of subsurface fatigue fracture, mode III (rotational or twist-like) way of crack opening was found to dominate on the short-crack path; during the damage-accumulation period, creative of the first smooth crack-origin facets, such a crack-opening pattern appeared mainly related to the unloading portions of the loading trajectory. Internal residual stresses appear to control such a deformation mode in the locally compressed material. Stress-release effects should follow from the occurrence of the first crack facet favoured by gas diffusion. Therefore, the subsurface crack origination is actually a synergetic problem. The dominant mode-III deformation is also creative of a plastic zone around the now stress-released area of the first fracture facet. This plastic zone is to be involved in the mode-I (tension) opening of the subsurface crack propagating from the first subsurface crack facet. The subsurface crack opening occurs as facilitated by the dissolved-gas diffusion toward a material discontinuity. Consequently, a short crack can expand around the first facet through a combined modes III and I opening to form a growing subsurface-fracture area. Such a model of the twist-controlled subsurface cracking is proposed and discussed here in terms of the well-known numerical data on the subsurface stress-state evolution as well as on the progress of plastic mesoscopic-scale level deformation in tensile-loaded metals.