三种组织TC17合金的疲劳裂纹扩展行为

Three typical microstructures for TC17 titanium alloy, i.e. equiaxed microstructure, basket-waved microstructure and globularized microstructure, were obtained by conventional forging, β forging and "secondary forging", respectively, and the fatigue crack growth rates of TC17 samples with the three microstructures were tested in order to evaluate the influence of different microstructures. The results reveal a significant dependence of microstructure on fatigue crack growth rate for this material. Basket-waved microstructure has an excellent fatigue crack growth resistance, followed by globularized microstructure, while the equiaxed microstructure shows the poorest propagation resistance. Fracture analysis shows that fatigue crack growth resistance is closely related to the crack deflection and bifurcation. On the basis of fracture mechanics, prediction models for fatigue crack growth life based on Paris equation were developed for the three different microstructures. The models suggest that the propagation life for the large crack in the three microstructure accounts for only a very small proportion of the total fatigue life, hence the control of crack initiation is more important. Superior propagation resistance means a longer critical crack length, which is easily testable in practice.