Influence of Metallurgical Factors on the Fatigue Crack Growth Rate in Alpha-Beta Titanium Alloys.

摘要

Rsearch was conducted on two alpha-beta titanium alloys, Ti-6Al-4V and Ti-6Al-2Sn 4Zr-6Mo, as a function of alloy composition microstructure and strength level, environment, and fatigue loading conditions. A wide range of microstructural conditions, seven for Ti-6Al-4V and three for Ti-6Al-2Sn-4Zr-6Mo, was chosen to provide a full range of constituent types and morphologies and a range of mechanical properties. All the material for the program was obtained in the form of pancake forgings which were uniformly and weakly textured. The initial portion of the program consisted of material characterization and base line fatigue crack propagation (FCP) testing. Characterization included a full description of microstructures, as well as mechanical property testing, fracture toughness tests, S-N curve determination and measurement of other properties such as modulus and texture. Base line FCP testing in dry air at two stress ratios (R) of 0.1 and 0.3; and in a 3.5% NaCl solution at 1 and 20 Hz was conducted in this portion of the program. Results of this study included the determination that transformed beta microstructures generally exhibit the greatest fatigue crack propagation resistance and that increasing the load ration, R, can lead to significant increases in crack growth rate; the behavior in both cases was rationalized in terms of crack tip interaction with microstructural constituents.