Local crack growth evaluation in a fully lamellar Ti-6Al-4V alloy using an electrothermal Crack-Tip marking technique

Abstract

Local crack growth properties within microstructural components are often anisotropic and difficult to measure, especially in materials like titanium alloys, that do not respond well to conventional, stress-ratio-based crack tip marking techniques. A method is described to extract a 3D anisotropic crack growth model from a common triplet colony type in titanium alloys, consisting of three grain-orientation variants of alpha-phase sharing a common 1¯21¯0 plane, parallel to a 111 (octahedral) plane from a parent beta grain. A fatigue crack growth test was run using a novel axisymmetric specimen (Pettit, 2021) of Ti-6Al-4V with an extremely coarse, fully lamellar microstructure. The microstructure consisted of ∼ 10 mm diameter beta grains, one of which had spanned nearly the entire specimen cross section of crack growth, and had decomposed to multiple regions of triplet basketweave microstructure during colony from above the beta transus. During pauses in the test, a succession of crack fronts was recorded on the fracture surface using a novel electrothermal crack tip marking technique (Pettit, 2021; Pettit, 2019) as the crack propagated through these colonies. Crack growth data was then extracted from post-test microscopy to obtain local crack growth rates. The position of the crack front on successive cycles and the fracture surface topography were correlated to local microstructural features using electron backscattered diffraction analysis following minimal polishing of the fracture surface. A numerical model was developed in Franc3D to quantify local crack driving forces. Local triplet-colony crack growth data was fit to a 3D anisotropic crack growth model. Both the crack growth rate and the fracture morphology in the triplets were strongly dependent on the crack orientation relative to the triplet axis. Crack propagation in a direction normal to the octahedral plane was estimated to grow 2.88x faster than crack growth within the octahedral plane at the same crack driving force. The methodology described appears promising for local crack growth evaluation.