MECHANISMS OF AMBIENT-TEMPERATURE FATIGUE-CRACK GROWTH IN TI-46.5AL-3NB-2CR-0.2W

Fatigue crack growth studies have been conducted on a two-phase alloy with a nominal composition of Ti-46.5Al-3Nb-2Cr-0.2W (at. pct), heat t reated to produce duplex and lamellar microstructures. Fatigue crack g rowth tests were conducted at 23 degrees C using computer-controlled s ervohydraulic loading at a cyclic frequency of 20 Hz. Several test met hods were used to obtain fatigue crack growth rate data, including dec reasing-load-range-threshold, constant-load-range, and constant-K-max increasing-load-ratio crack growth control. The lamellar microstructur e showed substantial improvement in crack growth resistance and an inc rease in the threshold stress intensity factor range, Delta K-th, when compared with the behavior of the duplex microstructure. The stress r atio had a significant influence on crack growth behavior in both micr ostructures, which appeared to be a result of roughness-induced crack closure mechanisms. Fractographic characterization of fatigue crack pr opagation modes indicated a highly tortuous crack path in the fully la mellar microstructure, compared to the duplex microstructure. In addit ion, limited shear ligament bridging and secondary cracking parallel t o the lamellar interfaces were observed in the fully lamellar microstr ucture during fatigue crack propagation. These observations were incor porated into a model that analyzes the contribution of intrinsic vs ex trinsic mechanisms, such as shear ligament bridging and roughness-indu ced crack closure, to the increased fatigue crack growth resistance ob served for the fully lamellar microstructure.