PHASE FORMATION DURING ANNEAL OF SUPERSATURATED TIB2-CRB2-WB2 SOLID SOLUTIONS

摘要

Numerous transition metal diborides of AlB2 structure type reveal large mutual solid solubility, Temperature-depending homogeneity ranges bordering immiscibility gaps within the system TiB2-CrB2-WB2 provide the opportunity to tailor an in situ grown typical platelet-shaped phase emerging from a supersaturated host matrix, Results are presented where variations in composition, manufacturing parameters, and heat treatment lead to precipitates of a ''W2B5'' structure accompanied by the formation of another tungsten boride less rich in boron in a (Ti, W, Cr)B-2 matrix. All phases are characterized by high melting points and hardness, The enhanced formation of the matrix as well as the precipitation mechanism were activated by the addition of 6 to 20 mole% CrB2 to the system TiB2-WB2. Within the ternary solid solution, a particular composition range exists exhibiting a thermal expansion behavior similar to boron carbide, Attempts are made to toughen boron and silicon carbide ceramics by processing this in situ growth of platelets out of dispersed transition metal diboride particles within a carbide matrix. (C) 1997 Academic Press. [References: 10]