Crystallography of Fatigue Crack Initiation and Growth in Fully Lamellar Ti-6Al-4V

Abstract

Fatigue crack initiation in titanium alloys is typically accompanied by the formation of planar, faceted features on the fracture surface. In the present study, quantitative tilt fractography, electron backscatter diffraction (EBSD), and the focused ion beam (FIB) have been used to provide a direct link between facet topography and the underlying microstructure, including the crystallographic orientation. In contrast to previous studies, which have focused mainly on the a-phase crystal orientation and the spatial orientation of the facets, the present analysis concentrates on the features that lie in the plane of the facet and how they relate to the underlying constituent phases and their crystallographic orientations. In addition, due to the anisotropic deformation behavior of the three basal slip systems, the orientation of the b phase as it relates to facet crystallography was investigated for the first time. The implication of the b-phase orientation on fatigue crack initiation was discussed in terms of its effect on slip behavior in lamellar microstructures. The effect of the local crystallographic orientation on fatigue crack initiation was also investigated by studying cracks that initiated naturally in the earliest stages of growth, which were revealed by FIB milling. The results indicate that boundaries that are crystallographically suited for slip transfer tend to initiate fatigue cracks. Several observations on the effect of the crystallographic orientation on the propagation of long fatigue cracks were also reported.

On the mechanistic basis of deformation at the microscale in hexagonal close-packed metals

“…Crack growth occurred over differently oriented α lamellae if the colonies shared a common basal plane orientation. Subsequent work [142] addressing non-dwell fatigue in fully lamellar Ti-6Al-4V found facets to be parallel, or near-parallel, to basal planes. FIB, EBSD and tilt fractography were employed to show that all cracks were found to nucleate at colony boundaries which were favourably oriented for slip transfer from (0001) 1120 to {1011} 1123 systems in the adjacent grain.…”

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

“…1,2) The subsurface fatigue crack initiation sites in near-α and α-β type titanium alloys commonly appear crystallographic transgranular facet or facets. [1][2][3][4][5][6][7][8] Mostly the facets formed on or near the {0001} basal plane regardless their inclination to the principal stress axis. Dislocation movement in α phase is restricted on the primary slip plane and fairly planar so that dislocation arrays on {01 1 0} < 11 2 0 > are piled-up in the vicinity of grain boundaries and a local stress concentration generates near α grain boundary.…”

Effect of microstructure on the fatigue properties of Ti–6Al–4V titanium alloys

“…Depending on the thermomechanical treatment or heat treatment of the (α + β) titanium alloy, such as Ti-6Al-4V, the microstructure and mechanical properties can vary in a wide range [3,6]. Such influences have been documented in numerous reports in the literature [4,[7][8][9][10][11]. However, due to experimentation limitations, experimental results are not always reproducible, and thus it is difficult to compare among fatigue properties obtained from different tests, even for a same microstructure.…”