Growth and properties of the monovariant (Fe, Cr)-(Cr, Fe) 7C 3 eutectic system

Abstract

The phase diagram of the ternary Fe-Cr-C system exhibits a eutectic curve, representing the compositions of melts that coexist with the solid phases γ(Fe, Cr) 7C 3 as a function of temperature at a given P. The mean composition of these solid phases, growing from such a melt under equilibrium conditions X̄ s( T), does not equal the melt composition. In order to be able to grow in situ (Fe, Cr) matrices of various compositions, reinforced with aligned (Fe, Cr) 7- C 3 rods and free from dendrites, the compositions X̄ s( T) were determined. Structural properties of the various (Fe, Cr)-(Cr, Fe) 7C 3 composites are discussed as a function of their growth rate and their composition. The carbide rods were found to be of the faceted type and the mean distance between the centres of two adjacent rods at a fixed composition of the material proved to be inversely proportional to the square root of the growth rate. The crystallographic relationship between the constituent phases and the growth direction is reported. It was found that when Cr is substituted for Fe, both in the matrix and in the carbide needles, the volume fraction and the diameter of the carbide needles decreases, while the number of carbide needles per unit volume increases at a constant growth rate. At the same time the resistance against corrosion by oxidation increases considerably.