Fatigue cracking in gamma titanium aluminide

Abstract

Fatigue crack initiation and growth were examined in cast and HIP'ed \textgamma-TiAl 4522XD. It was found that fatigue crack growth rates were higher at $750\celsius$ than $400\celsius$, but that $\Delta K_\mathrm{th}$ was also higher. Temperature excursions between $400$ and $750\celsius$ during fatigue crack growth resulted in retardation of the crack growth rate, both on heating and cooling; however heating from 400 to $750\celsius$ at a $\Delta K$ that would then be below threshold did not result in complete crack arrest. It was also found that for notches $0.6$~mm in length and smaller, initiation from the microstructure could instead be observed at stresses similar to the material failure stress; a microstructural initiation site exists. Secondary cracking around borides could also be observed. A change from trans- to mixed trans-, inter- and intra-lamellar cracking could be observed where the estimated size of the crack tip plastic zone exceeded the colony size. Changes in fracture surface morphology could not be related to the temperature of fatigue crack growth, although this could be observed from the oxide scale colouration. Compressive pre-loading of a crack results in retardation of the crack, which could also be observed from the oxide.