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

Fracture toughness and fatigue response in the temperature range 20-1400 degrees 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 similar to 8 MPa root m at room temperature to similar to 25 MPa root m at 1400 degrees C, the increase in toughness with temperature being steepest between 1200 and 1400 degrees C. S-N response at room temperature is shallow whereas at 1200 degrees C, a definitive fatigue response is observed. Fatigue crack growth in vacuum and air in the temperature interval 20-600 degrees 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 (similar to 20-30) and decreases with increasing temperature to similar to 3 at 1400 degrees C. For the Mo-Si-B alloy, da/dN for a fixed value of Delta K in the Paris regime in the 900-1400 degrees C range, increases with increasing temperature. Apparent activation energies extracted using an Arrhenius-type relationship illustrate grain-boundary diffusion dominance in the 900-1200 degrees C regime and volume diffusion dominance ill the 12001400 degrees C regime. (c) 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.