Effects of Microstructure and Frequency on Corrosion-Fatigue Crack Growth in Ti-8Al-1Mo-1V and Ti-6Al-4V

摘要

Fatigue crack growth studies were conducted on Ti-8Al-1Mo-1V and Ti-6Al-4V alloys in 3.5% NaCl aqueous solution. Each alloy was studied in two microstructural conditions and at two cyclic frequencies. The Ti-8Al-1Mo-1V was heat treated to produce a fine-grained duplex anneal microstructure and a coarse-grained Widmanstatten microstructure resulting from a beta anneal. The two microstructural conditions for the Ti-6Al-4V were an as-received mill anneal and a beta anneal. The two cyclic frequencies were 01. and 5.0 Hz. Each of the four alloy/microstructure combinations studied has been the subject of prior investigation regarding fatigue crack growth rate/microstructure interactions in ambient air environments. For both alloys, crack growth rates in air were found to be significantly reduced as a result of microstructural modifications associated with the beta anneal heat treatment. Although the salt water environment significantly accelerated crack growth rates for both alloys, this same ranking of fatigue crack growth resistance persisted in the present study. Both microstructures of the Ti-6Al-4V alloy exhibited a frequency crossover effect; in contrast, no significant frequency effects were observed in the Ti-8Al-1Mo-1V, in either microstructure. Similar frequency effects were seen in separate specimens cycled at a single constant frequency or in single specimens cycled at two alternating frequencies. Out-of-plane cracking was observed in both alloys in the beta annealed condition. The effects of varying degrees of out-of-plane cracking on apparent crack growth rates are noted. (Author)