Effect of lamellar spacing on fatigue crack growth behaviour of a TiAl-based aluminide with lamellar microstructure

The fatigue crack growth resistance of a Ti–48Al–2Mn–2Nb (at.%) alloy with different lamellar spacings was investigated at room temperature in laboratory air. The crack growth resistance in the air-cooled specimen with a lamellar spacing of ∼0.9 μm was essentially higher than that in the furnace-cooled specimen with a lamellar spacing of ∼3.8 μm. The increased resistance is attributed to the zigzag crack growth across the fine lamellar plates in the air-cooled specimen. However, some furnace-cooled specimens actually exhibited superior fatigue crack growth resistance, comparable to that obtained by air cooling. In these specimens, evidence of crack branching and crack deflection, caused by interlamellar cracking away from the mode I crack growth direction, was frequently observed. Hence, the fatigue crack growth resistance can vary widely, especially for the furnace-cooled specimens of nominally similar fully lamellar microstructures.