REACTIVE MELT INFILTRATION OF SILICON-NIOBIUM ALLOYS IN MICROPOROUS CARBONS

Studies of the reactive melt infiltration of silicon-niobium alloys in microporous carbon preforms prepared by the pyrolysis of a polymer precursor have been carried out using modeling, DTA, and melt infiltration. Mercury porosimetry results indicate a very narrow pore size distribution with virtually all the porosity within the carbon preforms open to infiltrants. The morphology and amount of the residual phases (niobium disilicide and silicon) in the infiltrated material can be tailored according to requirements by careful control of the properties (pore size and pore volume) of the porous carbon preforms and alloy composition. The average room temperature four-point flexural strength of a reaction-formed silicon carbide material (made by the infiltration of medium pore size carbon preform with Si-5 at. % Nb alloy) is 290 +/- 40 MPa (42 +/- 6 ksi) and the fracture toughness is 3.7 +/- 0.3 MPa square-root m. The flexural strength decreases at high temperatures due to relaxation of residual thermal stresses and the presence of free silicon in the material.