Fracture mechanism of full lamellar TiAl under high-temperature creep and fatigue conditions based on the in situ observational tests

In this paper, crack growth mechanism of full lamellar TiAl intermetallic compound under high temperature creep and fatigue conditions were investigated using in situ observational high temperature creep-fatigue testing machine. Under high temperature creep condition, inter-granular voids were found to be originated and to coalesce, which results in inter-granular cracking. Furthermore, since the grain size is large (1.5-3.0 mm), inter-granular cracks distribute not only around a notch but also the sites apart from the notch. Since the creep crack mechanism is coalescence of distributed inter-granular cracks, the resistance against creep crack growth becomes higher. Under high temperature fatigue condition, a trans-granular crack was found to initially grow along the direction of lamellar structure, which is different from that under creep condition. After that, the crack incubates and the crack opening displacement increases until it takes some critical value. After that, final unstable crack growth occurs along the perpendicular direction of applied load due to the delamination between matrix and lamellar structure. The characteristic of load frequency for fatigue fracture life is found to be dominated by cyclic dependent mechanism and it is not affected by time dependent mechanism even under low frequency.