The role of surface condition, residual stress and microstructure on pre-yield cracking in Ti44Al8Nb1B

Fully lamellar and duplex samples of Ti44Al8Nb1B, with a range of surface treatments, have been tested in tension using acoustic emission to detect the onset of pre-yield cracking. The magnitudes and the senses of the stresses left in the surface regions of machined, polished and annealed samples have been measured using synchrotron radiation and these measurements correlated with the minimum level of the applied stress at which cracking was detected. The observations have shown that the surface condition is far less significant than the microstructure in determining when cracking is first observed and that, whatever the surface condition, the duplex structure never shows detectable pre-yield cracking. These results are discussed in terms of the mechanism which leads to pre-yield cracking and its role in limiting fatigue life.