Microstructure and the development of fatigue cracks at notches

The paper focuses on the α/β titanium alloy, Ti–6Al–4V, in a range of microstructural conditions from extensive equiaxed alpha with retained beta or transformed beta through bimodal morphologies consisting of primary alpha and transformed beta to coarse fully transformed forms produced by heat treatment above the beta transus. The material is taken from several processing routes encompassing cross-rolled plate, hot-rolled bar stock and small disc forgings. The work brings together strain control low cycle fatigue behaviour obtained on plain testpieces and data obtained on a variety of notch geometries with Kt factors in the range 1.4–4.7. A correlation technique based on the parameter KtΔσ for notches and Δσ for plain specimens, with Δσ the appropriate stress range, is used to relate the datasets. Notch behaviour that is not consistent with this correlation is related to the microstructural condition, the cyclic stress–strain behaviour of that condition and the presence of ‘fatigue sensitive’ features. Finally, the titanium characteristics are compared with a competing particle reinforced aluminium based metal matrix system, AMC 225.