Growth kinetics and morphology of grain boundary ferrite allotriomorphs in an Fe-C-V alloy

Grain boundary ferrite allotriomorphs in an Fe-0.12 pct C-0.11 pct V alloy reacted in the temperature range 800‡ to 870 ‡C develop in two morphologies: conventional allotriomorphs, with an aspect ratio of about one-half, and “snakes≓, whose aspect ratio approaches zero. Interphase boundary vanadium carbides (VC) precipitate densely in association with conventional allotriomorphs but not with “snakes≓. Thickening kinetics of “snakes≓ are about an order of magnitude slower than predicted by the paraequilibrium model. Conventional allotriomorphs, on the other hand, thicken with the kinetics predicted by paraequilibrium. These observations are considered in terms of the solute drag-like effect and of “wetting≓ of austenite grain boundaries by ferrite in the presence of V. Absence of a distinct change in the thickening kinetics of conventional allotriomorphs in the vicinity of 845 ‡C, the approximate upper temperature limit of interphase boundary carbide precipitation, strongly supports the view that such carbides can precipitate only on immobile,i.e., partially coherent areas of austenite : ferrite boundaries. Formerly Visiting Research Associate, Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931 Formerly Republic Steel Corporation Fellow, Michigan Technological University, Houghton, MI 49931 Formerly Professor, Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931