Micromechanisms of fatigue crack propagation in Ti₃Al based alloys

In order to understand micromechanisms of fatigue crack growth in titanium aluminides, direct monitoring of the influence of the α₂ and β phases on a growing crack was performed using an FEG SEM for Ti-23Al-9Nb-2Mo-1Zr-1·2Si (at.-%) and Ti-23Al-11Nb-0·9Si (at.-%) Ti₃Al based alloys. Crack growth rates are observed to be faster across individual α₂ laths than across β laths and/or along α₂/β lath interfaces. It is found that fatigue cracks propagate incrementally through the α₂ phase by decohesion of a favoured slip band rather than crossing it catastrophically in one cycle. The formation of intersecting slip bands can lead to a tortuous crack path and a decreased average crack growth rate in the α₂ phase. When a crack meets the β phase, the most common phenomenon observed is crack deflection. The fatigue crack then extends continuously along α₂/β interfaces under the effect of a mixed mode local stress intensity factor range. The basket weave microstructure achieves the maximum fatigue crack growth resistance from α₂/β interfaces. Bridging and blunting can reduce fatigue crack growth rate remarkably. However, crack bridging happens only with larger β laths and blunting is mainly seen only for secondary cracks. The efficiency of bridging and blunting thus appears to depend on the ratio of load bearing capability of the β laths involved over the local effective ΔK range. Mechanisms operating during fatigue crack propagation are also compared with those observed during monotonic fracture.