Observations of Facet Formation in Near-α Titanium and Comments on the Role of Hydrogen

Abstract

Faceted features are frequently observed on the fracture surfaces of titanium alloys that have failed by static loading, continuous cycling, dwell fatigue loading, and stress corrosion cracking (SCC). Although the facets formed under different loading conditions seem qualitatively similar, there are significant differences in the spatial and crystallographic orientations of the facets as well as subtle differences in facet surface topography. The current study compares and contrasts facets for various loading conditions (cyclic, creep, SCC, and dwell) in the Ti-8Al-1Mo-1V alloy with the primary motivation being to understand the mechanisms of crack initiation and faceted growth during dwell fatigue. The spatial and crystallographic orientations of the facets were determined using quantitative tilt fractography and electron backscatter diffraction, whereas facet topography was examined using ultra-high-resolution scanning electron microscopy. Collectively, the experimental observations suggest that hydrogen may play an important role in facet formation and accelerating small crack growth rates during dwell fatigue loading.

Effect of the build orientation on the mechanical properties and fracture modes of SLM Ti–6Al–4V

“…In addition, the fracture surface profiles reveal that the predominant fracture is inter granular where cracks have propagated mainly along the grain boundaries present in the microstructure as shown in the insets of the same figure. This result is consistent with research on fatigue crack propagation of Ti alloys with fine microstructure, where it has been reported that crack propagation is highly depended on the crystallographic orientation of the α grains containing the crack tip and the number of grain boundaries in the microstructure [40,47,48]. It has been shown that in fine microstructures the crack tip deflects at the grain boundaries generating predominantly inter granular fracture as the neighbouring α grains (or colonies) have multiple distinct crystallographic orientations (i.e.…”

“…The analysis of the orientation of the α laths (Fig. 12d) indicates the fracture has occurred in none of the basal or prismatic planes that are instead generally associated with α cleavage fracture [30,40,51]. In addition, the reconstruction of the parent β phase shows that all these α laths have originated from one unique grain.…”

“…Although the volume fraction of β phase is limited, the β phase contributes to improve the ductility of the stress relieved samples thanks to slip transfer at the interface of the two phases [39,40]. Two primary slip systems of the α phase are, in fact, precisely aligned with the 110 111 and 112 111 systems of the β phase, with a third set misoriented only ~ 10.5° and thus, slip transfer across the two phase can occur [39,40]. Table 4 shows the stress tensor calculated from the frontal plane of an as-built sample.…”

“…weak texture) [31,46,49]. Ti-6Al-4V with microstructure consisting of α colonies will, on the other hand, give intragranular fatigue fracture [31,40].…”

The Influence on Microstructure and Microtexture on Fatigue Crack Initiation and Growth in Alpha + Beta Titanium

“…Although multiple cracks form during dwell fatigue loading, the critical crack seems to grow from the largest of these regions 5) . The sample volume and shape of microtextured regions 6) have recently been implicated in dwell fatigue life variability in laboratory specimens making the development of transfer functions for components difficult. Modeling efforts [7][8][9][10] have revealed the effects of microtexture at the grain level in creating non-uniform stress-strain distributions, the accumulation of plastic slip near grain boundaries and the propensity for crack nucleation.…”