Anisotropy of fracture toughness in textured titanium-6Al-4V alloy

Abstract

Fracture toughness tests have been carried out on a wide range of specimen orientations from textured Ti-6A1-4V alloy plate. The tests were instrumented to allow measurement of crack extension as well as stress intensity factors and COD data. Preferred orientation of the dominant alpha phase was found to influence not only the toughness parameters but also the fracture surface and propensity to shear lip formation. The anisotropic nature of plasticity in the material was demonstrated by direct measurement of yield loci and by the variable shapes of plastic zones around crack tips. Results are discussed in terms of a qualitative analysis of slip systems at the crack tip and also in a semiquantitative anisotropic upper bound slip line treatment. It is found that stable crack extension starts at low values ofK and COD for specimens in which plastic flow can occur readily at the crack tip but toughness, assessed as resistance to unstable crack propagation, is greatest in these cases. The magnitude of microstructure induced anisotropy is shown to be much smaller than that originating in preferred crystallographic orientation. Troughlike features on some fracture surfaces were found to be associated with regions of particularly strong local texture.