The influence of micro structural scale on the creep resistance of high volume fraction ceramic-metal composites made from aluminum oxide and niobium

High temperature deformation behavior of two-phase microstructures, made from nominally pure niobium and alumina, was studied in the temperature range 1300–1500 °C. One of these alloys, containing 65 vol% ceramic, not only has excellent ductility but has a flow stress that is more than ten times the flow stress of nominally pure niobium. The composite has a directional microstructure consisting of elongated, bamboo-like niobium crystals surrounded by oxide particles. The flow stress appears to vary inversely with the transverse dimension of the niobium grains. This leads us to propose that the strengthening is due to the pinning of metal dislocations at the niobium/sapphire interfaces. The applied stress is related to a local criterion for flow in terms of the angle formed by an extended metal dislocation held at a metal-ceramic interface.