Effects of Al content and α2 precipitation on the fatigue crack growth behaviors of binary Ti–Al alloys

Abstract

The influence of Al content and α2 precipitation on the fatigue crack growth (FCG) behaviors of binary Ti - xAl (x = 2, 4, 6, 8 wt %) alloys was investigated. Ti–Al binary alloys with different Al contents were solution-treated and followed by aging treatment to obtain Widmanstatten microstructure and promote α2 precipitation at higher Al content. Results show that the increase of Al content dramatically reduces the FCG rate. The enhanced resistance to FCG propagation is related to the increase of the critical resolved shear stress (CRSS) for dislocation slipping that acts as crack initiation sites, basing on solution strengthening with the addition of Al. Furthermore, roughness-induced crack closure with increasing the Al content also contributes to the decrease of FCG rate. With the increment of the Al content, the early fatigue crack propagation stage, in which the crack propagation significantly affected by microstructural factors, will be enlarged due to the decrease of crack tip plastic zone, leading to crack deflection and the reduction of FCG rate. In Ti–8Al alloy, the presence of α2 precipitates and the increment of its size and volume fraction after aging treatment will result in significant increase of FCG rate. Moreover, the activation of large-scale slipping in relatively larger colonies promotes the crack to propagate along the large-scale slipping that induces smoother crack path and higher FCG rate.