Dependence of Fatigue Macrocrack Propagation Resistance of Titanium Beta 21S on Precipitate Size and Distribution

The purpose of this study was to investigate the influence of microstructure on the macrocrack propagation resistance of the titanium alloy Beta 21S. In this work, single-step aging treatments were used to create different microstructures by altering variables such as aging temperature and time. The influence of the α-precipitate size and distribution in the fatigue crack macrocrack propagation resistance of the resulting microstructures was investigated. The main microstructural feature of β titanium alloys is the preferential precipitation of the hardening α-phase at β grain boundaries forming a continuous α-layer. The resulting PFZ （precipitate-free zone） adjacent to this continuous α-layer does not contain any precipitates, and is therefore soft with respect to the surrounding age-hardened matrix. Preferential plastic deformation in the soft zones can have a negative effect on some properties. Two microstructures were evaluated and the results obtained showed that, at low stress intensity （delta K） levels, the cracks propagated transcrystalline and the propagation rate ofmacrocracks depends on the size and distribution ofα-plates in the β matrix, while with increasing plastic zone size （high delta K levels） the macrocracks propagated intercrystalline and the propagation rate is influenced by the PFZ path in the microstructure.