Microstructural effects on fatigue crack growth of cast and forged TiAl alloys

Fatigue crack growth and tensile experiments were conducted on three TiAl alloys in the cast duplex 47-XD, cast nearly fully lamelar 45-XD and wrought fully lamellar 47-5 at 25 and 650 degrees C. The microstructures were fully characterized defining the extent of lamelar colony size, lath spacing presence of other phases. The tensile experiments showed the two lamellar structures to be stronger while the duplex alloy maintained a significantly higher fracture toughness at elevated temperatures. Fatigue crack growth threshold tests showed negligible difference between the three alloys and microstructures, having a K(th)=6 to 8 ksi root in. The need is exposed for an alternate failure design criteria to take advantage of gamma-TiAl, when low densities and high threshold stress intensities hold advantage it light weight and durability not attainable in conventional nickel base alloys.