Synergistic Toughening of MoSi2

Abstract

Abstract : Research during the past three years has focused on the study of individual transformation toughening and crack bridging mechanisms in molybdenum disilicide composites reinforced with partially stabilized zirconia and thermodynamically compatible Nb, Mo and W phases. The primary objective in the initial studies was to study the fatigue and fracture mechanisms in model composite systems prior to a focused research effort aimed at developing synergistically toughened hybrid composites reinforced with zirconia particles and ductile/brittle bridging reinforcements. The hybrid composites were described as synergistically toughened composites due to anticipated interactions between crack bridging and stress-induced phase transformations. Such interactions may promote higher toughening levels than those expected from the sum of the individual toughening components. Room temperature fatigue and fracture behavior was studied in the transformation toughened and ductile phase toughened composites during the first and second year of the project. Crack-tip scanning and transmission electron microscopy studies were performed to determine the failure mechanisms at elevated temperature. The studies showed clearly that elevated temperature crack growth in MoSi2 and MoSi2 composite occurs by a combination of mechanical fatigue and creep crack growth (viscous flow of amorphous glass phase and microvoid nucleation) mechanisms. A model was also proposed for creep-fatigue crack growth at elevated temperature. Relatively fast elevated temperature cyclic crack growth rates were thus explained by considering the combined effects of creep and fatigue crack growth components. Finally, the possible synergistic interactions between crack bridging and transformation toughening were studied during the fourth year of the program.