Unidirectional solidification of Co−Cr−C monovariant eutectic alloys

Compositions on the eutectic liquidus line between Co-25 wt pct Cr-3.5 wt pct C and Co-45 wt pct Cr-2.2 wt pct C wherein the simultaneous freezing of a metal matrix and a carbide occur have been solidified unidirectionally. The composite structures formed consist of a cobalt matrix containing substantial amounts of soluble chromium with the carbide phase, (Cr, Co)₇C₃, as an aligned fibrous dispersion. The structure of one monovariant eutectic composition has been examined as a function of solidification velocity. Cellular growth was observed at rates above approximately 4 cm per hr. However, the cell cusps only gradually deepen with increasing growth rate and the carbide phase remains essentially aligned even at 50 cm per hr. The concentration of carbide fibers in the transverse section was measured at various growth rates and was found to be approximately a linear function of rate. The structure of the alloy was also examined by transmission electron microscopy. These results indicate that the preferred growth direction of the carbide is more important than the solid-solid surface energy in controlling the growth.