Elevated temperature mechanical behavior of multiphase molybdenum alloys

Mo-Si-B alloys are being considered as possible candidates for high-temperature applications. In this study, the high-temperature compression response, monotonic and cyclic crack growth behavior (as a function of temperature) of a two-phase Mo-Si-B alloy is discussed and compared to the TZM alloy.; The compression response was examined as a function of strain rate in the 1000°C-1400°C range. A limited number of tests were also conducted on a three-phase alloy. These compression studies confirmed that deformation in the temperature-strain rate space evaluated is matrix-dominated. As a consequence, the response of the three-phase material overlaps that of the two-phase material. In all instances evaluated, the Mo-Si-13 alloys exhibit superior flow stress relative to their TZM counterpart. Finite element analysis assuming an elastic-plastic matrix and an elastic second phase illustrates strain localization in the matrix. The interplay between matrix and T2 properties is used to explain the observed deformed microstructure.; Fracture toughness of the Mo-Si-B alloy varies from ∼8MPa√m at room temperature to ∼25 MPa√m at 1400°C, the increase in toughness with temperature being steepest between 1200°C 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°C-600°C is similar for the Mo-Si-13 alloy whereas significant deterioration is noted far TZM when it is tested in air. The Paris slopes for the two alloys is high at room temperature (∼20-30) and decreases with increasing temperature to ∼3 at 1400°C. Apparent activation energies extracted using an Arrhenius-type relationship illustrate grain-boundary diffusion dominance in the 900°C-1200°C regime and volume diffusion dominance in the 1200°C-1400°C regime.; Creep contributes in a significant way to monotonic and cyclic crack growth at elevated temperatures (1200°C-1400°C). Clack tip stresses are effective in producing localized microstructural instabilities including recrystallization and creep voiding. Whereas primary creep appears beneficial to toughness and fatigue crack growth resistance, the formation of creep voids and their linkage results in a significant loss of these properties. From these studies, a "fail-safe" regime can be identified for applications where tension-tension cyclic loading is relevant.