Relationship between fatigue life, changing of mechanical properties and dislocation structure during fatigue in pure titanium

Mechanical properties, dislocation structure, fatigue strength and fatigue crack initiation characteristics were investigated at various fatigue steps in pure titanium having equiaxed α and Windmanstatten α structures. Relationship between fatigue life and the above mentioned factors was also studied. In pure titanium with both structures, long fatigue crack propagation life strongly affects fatigue strength and this life occupies total fatigue life as compared with small fatigue crack initiation and propagation life. The mechanical properties, that is, 0.2% proof stress and hardness tend to increase obviously according to the fatigue steps particularly at the early stage of the low cycle fatigue (LCF) region, whereas elongation shows the reverse trend. The hardness far from the specimen surface is smaller than that near the specimen surface at the early stage. Hardness of both areas becomes however nearly equal each other at the later stage. The reason why those phenomena occurred would be that increment of dislocation density near the specimen surface is larger than that far from the specimen surface at early stage and the dislocation density of both areas saturates at later stage, becoming similar. Dislocations in pure titanium with both structures form equiaxial dislocation cell structure. The dislocation structure at each fatigue step has a correlation with the fatigue life under constant maximum cyclic stress.