Fatigue crack growth and toughness of niobium silicide composites

Niobium based In Situ composites are desired for high temperature structural materials for aerospace applications in order to gain a better understanding of the mechanical properties is desired. The microstructure and mechanical properties of several Niobium based In Situ composites were analyzed. The microstructure was analyzed using SEM and optical microscopy. The materials consisted of a niobium solid solution matrix and a large volume fraction (∼45-55%) of hexagonal silicide particles and smaller Laves phase particles.;Mechanical properties were evaluated at both room temperature and elevated temperature at 550°C. The main purpose of the present research was to characterize the room temperature and high temperature fatigue crack growth behavior. The fatigue threshold, Paris slope, and overload toughness obtained during fatigue was determined through direct current potential drop method (DCPD) at room temperature and elevated temperature (550°C). It was observed that the majority of alloys had fatigue threshold of 5-8 MPa√m at room temperature and elevated temperature. The majority of alloys tested had room temperature notch toughness, room temperature toughness obtained during fatigue overload and elevated temperature toughness acquired during fatigue overload of 10-20 MPa√m. It was also observed that testing at high temperature (550°C) in air produced time dependent crack growth behavior in some compositions. The toughness and hardness was also determined at room temperature.;The fatigue crack path was quantified at different DeltaK levels after reaching threshold at room temperature and high temperature. Several extrinsic toughening mechanisms were observed to occur in the composites. The toughening mechanisms and crack path behavior were qualitatively and quantitatively evaluated. Preferential crack growth was observed to occur in the silicide particles while largely avoiding the Laves particles.