A. P. Alur, F. Kertis, S. Kumar, Brown University, Providence, RI

Summary:

Multiphase, Mo-rich, Mo-Si-B alloys are being examined as possible next-generation aircraft engine materials. Two-phase alloys of interest consist of an elastic-plastic Mo-solid solution phase and a hard, brittle second phase, Mo₅SiB₂, often referred to as the T2 phase. In the alloy examined in this study, the Mo-solid solution is the continuous phase with the T2 being dispersed as discrete, angular ~20 mm particles and constituting forty volume percent. Fracture toughness of the two phase alloy was evaluated using fatigue precracked three point bend specimens, in the temperature range 20-1200oC. Resulting fracture surfaces were characterized and crack interaction with microstructure was understood using interrupted tests. Cyclic behavior of the two-phase alloy was characterized in terms of its S-N response as well as crack growth rate measurements using precracked compact tension specimens. These S-N tests were performed at ambient temperature and 1200C. Cyclic crack growth rates were measured in the temperature range of 20-600C, in air, and in a temperature range of 20-1400oC in vacuum. The interaction of the advancing crack tip with the microstructure on the specimen surface was examined, to obtain an appreciation of the role of the matrix and intermetallic phases in contributing to fatigue resistance. Fracture surfaces were characterized in all instances to help understand the measured properties. Results from these studies will be presented and analyzed in the context of crack growth kinetics under the combined influence of cyclic loading, environment and temperature.