Fractographic Analysis of Ti-7A1-2Cb-1Ta and Ti-6A-4V Fractures Developed in “Wet” Fatigue

摘要

Crack-growth-rate studies on Ti-7Al-2Cb-lTa and TÍ-6A1-4V alloys in low-cycle fatigue were performed in air and in a 3.5-percent salt-water solution. The Ti-7Al-2Cb-lTa alloy was additionally tested in dis¬tilled water. An increase in crack-growth rate at all strain ranges was observed for the Ti-7Al-2Cb-lTa in the "wet" environments over that in the "dry," whereas the TÍ-6A1-4V alloys did not appear to be sensitive to a "wet" environment. At a strain range of 8000 to 9000 microinches/inch, corresponding to about 60-percent yield strength, the crack-growth rate of Ti-7Al-2Cb-lTa alloy accelerated and became erratic.nFractographic studies show that the increase in crack-growth rate is due to a change from the ductile mode of fatigue fracture, usually found in "dry" low-cycle fatigue tests of titanium alloys, to the brittle quasi-cleavage mode of fracture — a fracture mechanism of stress-corrosion cracking. The amount of quasi-cleavage seen on the fatigued fracture surface generally corresponds to the crack-growth-rate increase; thus, it is indicative of the degree of stress-corrosion cracking sensitivity.