Monotonic and cyclic crack growth response of a Mo-Si-B alloy

Fracture toughness and fatigue response in the temperature range 20-1400 °C of a two-phase Mo-Si-B alloy is compared to that of a TZM alloy. Fracture toughness of the Mo-Si-B alloy varies from ~25 MPa(square root)m at 1400 °C, the increase in toughness with temperature being steepest between 1200 and 1400 °C. S-N response at room temperature is shallow whereas at 1200 °C, a definitive fatigue response is observed. Fatigue crack growth in vacuum and air in the temperature interval 20-600 °C is similar for the Mo-Si-B alloy whereas significant deterioration is noted for TZM when it is tested in air. The difference in response is attributed to differences in the oxide scales formed in the two alloys. The Paris slopes for the two alloys is high at room temperature(~20-30) and decreases with increasing temperature to (~3 at 1400 °C. For the Mo-Si-B alloy, da/dN for a fixed value of (delta)K in the Paris regime in the 900-1400 °C range, increases with increasing temperature. Apparent activation energies extracted using an Arrhenius-type relationship illustrate grain-boundary diffusion dominance in the 900-1200 °C regime and volume diffusion dominance in the 1200-1400 °C regime.