Short crack growth behavior and its transitional interaction with 3D microstructure in Ti-6Al-4V

Abstract

Currently, short crack growth behavior and rate variations are not well understood in the literature. This is due to lack of studies regarding the interaction between 3D short crack and microstructure and its effect on crack growth. In order to study this interaction, in situ computed tomography was performed to measure crack growth at sub-grain level (every 5 µm) during a fatigue test in a bimodal Ti-6Al-4V alloy for two crack front regions. This was followed by serial sectioning coupled with electron backscattering diffraction (EBSD) to identify the short crack growth in the microstructure, i.e. α, α + β phases and interface. Results show that crack growth has the highest rate in α phase as compared to the α + β phase and the interface in both regions. The crack grows preferably into α phase when compared to the average microstructural fraction in the first region, but it decreases below this fraction in the second region. The crack grows mainly close to crystallographic planes in α grains with the maximum shear stress (favorable planes) in the first region. As the short crack grows into the second region, there is an increase in number of grains enclosed in the plastic zone size. As a result, there is a decrease in the mismatch angle between neighboring cracked grains, which leads to higher deviation from favorable planes causing a local variation in crack growth rate.