Microstructural influence on low cycle fatigue dwell effect of TC11 wrought titanium alloy: Kou, C. and Cao, C. J. Aeronaut. Mater. (China) 1988 8, (2), 39–45 (in Chinese)

Abstract

Cylindrical specimens of 12%CrMoV steel have been tested under high temperature low cycle fatigue (HTLCF) conditions using a symmetric triangular waveform under total strain control in both air and vacuum conditions. An investigation of the internal microstructure which develops under HTLCF conditions in this steel was performed using transmission electron microscopy. Features typically associated with fatigue of f.c.c. metals, such as crystallographic slip bands and veins of high dislocation density, were observed in addition to regions of equiaxed subgrains.The development of surface damage was monitored using scanning electron microscopy and a semiautomatic image analysis system by periodically interrupting the tests and replicating the entire gauge section. By testing in air and vacuum environments, it is possible to determine the effect of the environment on the nucleation and growth of fatigue microcracks. In air, surface oxide attack at former austenite grain boundaries greatly enhances the initiation of intergranular microcracks, which tend to be perpendicular to the stress axis. This effect is reduced in vacuum conditions, resulting in an increase in the number of cycles to failure by over a factor of two. In either environment, the crack initiation process tends to take place on grain boundary facets which were perpendicular to the applied stress direction. The influence of crystallographic slip bands on the initiation of the intergranular microcracks is also discussed.