Creep effects on crack growth in a Mo-Si-B alloy

Crack growth behavior during monotonic and cyclic loading at elevated temperature is affected by creep and/or by environment and, therefore, high-temperature toughness and fatigue response can be temperature and loading-rate dependent. This paper reports on the effect of loading rate on fracture toughness at high temperatures for a two-phase Mo-Si-B alloy; the observed response is understood by examining the interaction of the advancing crack with the microstructure, and the evolution of microstructure ahead of the crack tip as a consequence of the crack-tip field. Parallel studies were also performed under cyclic loading conditions by subjecting compact tension specimens to sinusoidal and trapezoidal loading waveforms. In certain cases, the microstructure ahead of the crack tip revealed several instabilities (recrystallization, grain growth and creep cavitation). Finite element analysis revealed strain localization 'pockets' ahead of the crack tip, which are thought to provide the driving force for the observed microstructural instabilities.