Fatigue crack propagation properties of Ti–6Al–4V in vacuum environments

Abstract

Abstract:To determine the effects of vacuum environment on fatigue crack propagations in a Ti-6Al-4V alloy, K-decreasing tests were conducted in air and vacuum. The fatigue crack propagation rate became slower and threshold stress intensity factor range became larger with decreasing vacuum pressure.The tendency cannot be fully explained by the crack closure. Based on fracture surface observations, granular region of a few micrometer size asperities was observed on the fracture surface only in high vacuum and ultra high vacuum. The high vacuum environment is one of the necessary conditions for the formation of the granular region, and the fraction of surface coverage of adsorbed gas on fracture surfaces relates to the phenomenon. The formation of the granular region represents the difference of the crack propagation mechanism between vacuum and air environments. A new mechanism for the formation of the granular region was proposed, and that is one of the phenomena which can explain the reduction of crack propagation rate in vacuum.

Faceted crack initiation characteristics for high-cycle and very-high-cycle fatigue of a titanium alloy under different stress ratios

“…While in the results by Morrissey et al [25,26], no facet was found for the similar microstructure of Ti-6Al-4V alloy. the case of positive stress ratios, crack initiation sites with facets were observed at fracture surfaces for Ti-6Al-4V alloy [27][28][29]. This suggested that the increase of stress ratio promoted the occurrence of faceted crack initiation.…”

Effects of stress ratio on high-cycle and very-high-cycle fatigue behavior of a Ti–6Al–4V alloy

“…The reason for crack initiation from the surface or from both the surface and the interior at R ¼ À1 is not clear. For the positive stress ratio, the results for a Ti-6Al-4V alloy [23,24] indicated that the fatigue strength exhibited a sharp decrease in the HCF and the VHCF regimes for the crack initiation from the interior with facets. It was also reported that cracks initiated occasionally from the surface with subsurface facets at the initiation region [25,26].…”

The influence of hydrogen on the very high cycle fatigue property and crack growth characteristics of bearing steels

“…Remarkable characteristic of the ODA is that the similar fracture surface morphology can be observed for different material systems, i.e. high strength steels and titanium alloys, and that the morphology is found on the fracture surface tested in high vacuum environment (Oguma and Nakamura, 2013). Recent study by Yoshioka and Nakamura, (2016), using synchrotron radiation CT imaging directly observed the initiation and growth of ODAs in Ti-6Al-4V alloy.…”