Stress-Corrosion and Corrosion Fatigue Cracking in Ti-5111 and VLI Ti-6Al-4V

摘要

The stress-corrosion cracking (SCC) and corrosion-fatigue cracking resistance of two alpha-beta titanium alloys, Ti-5Al-1Sn-1Zr-1V-0.8Mo-0.1Si (Ti-5111) and very-low-interstitial (VLI) grade Ti-6Al-4V, both in the 700 MPa yield strength class, were investigated. Results to date indicate that both Ti-5111 and VLI grade Ti-6Al-4V titanium alloys exhibit good subcritical cracking resistance in a natural seawater under SCC conditions with threshold stress intensities about 100 MPa m~(1/2) that are significantly superior to that of the regular grade Ti-6Al-4V titanium alloy (60 MPa m~(1/2)). Fractographic analyses, however, suggested that SCC did not occur in the rising-step-load tests of VLI Ti-6Al-4V and Ti-5111 in a natural seawater and that the probable cause of subcritical cracking was sustained load cracking. In addition, both Ti-5111 and VLI Ti-6Al-4V demonstrate good corrosion-fatigue cracking resistance in a 3.5 wt% NaCl solution at stress ratios of 0.1 and 0.9, as their fatigue crack growth rates and fatigue crack growth thresholds are similar to those obtained in an ambient air environment. Finally, just as both Ti-5111 and VLI Ti-6Al-4V have similar subcritical cracking threshold factors, K_(th), under SCC conditions in seawater, these two 700 MPa-yield-strength class Ti alloys also have comparable fatigue crack growth kinetics.