EFFECT OF MICROSTRUCTURE ON DWELL FATIGUE BEHAVIOR OF TI-6242

摘要

The dwell effect in beta heat treated near-alpha titanium alloys is a well established phenomenon with internal fatigue crack initiation at aligned alpha colonies resulting in a reduction of fatigue life. Limited work has been conducted on titanium alloys with an alpha-beta structure since they are not beta heat treated and, therefore, do not contain aligned alpha colonies from the heat treatment. However, some research indicates that there is a dwell fatigue reduction for alpha-beta titanium alloys, although there is no evidence relating microstructural features to the early failures. This work details the relationship between microstructure and dwell low cycle fatigue life and fracture morphology in alpha-beta processed Ti-6242. It has been found that aligned alpha colonies that form during a beta quench in the billet cycle can survive subsequent alpha-beta billet, forge and heat treat processing even though the alpha plates become spheroidized. The surviving primary alpha colonies in forgings often cannot be recognized using conventional metallographic procedures and a new experimental technique for observing these features has been developed. This technique uses electron back scattered patterns generated from a sample in a scanning electron microscope and combines crystallographic information from these patterns to form an image of the microstructure based on the crystallographic orientation. A method for quantifying the microstructure in terms of aligned primary alpha colony/primary alpha grain parameters has been developed and a model constructed which relates the microstructural parameters to dwell fatigue life. The influence of macrotexture on dwell fatigue has been shown, and the importance of neighboring colony orientations on dwell fatigue crack initiation has been confirmed.