Growth kinetics and morphology of grain boundary ferrite allotriomorphs in an Fe-C-V alloy
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INTRODUCTION
INTERPHASE boundary carbides nucleate at austenite: ferrite boundaries and appear as "sheets" of discrete particles approximately parallel to the advancing interfaces of grain boundary ferrite allotriomorphs in Fe-C-X alloys. 1-4 Fe-C-V has been identified as an alloy system in which interphase boundary carbide precipitation is particularly intense and occurs on an especially fine scale. 5-s A principal objective of this investigation was to examine more directly than heretofore the much debated 9 ~4 influence of such precipitation upon the growth mechanism and kinetics of ferrite allotriomorphs by measuring these kinetics at temperatures both above and below the highest temperature at which interphase boundary carbide precipitation occurs in an Fe-C-V alloy. The second main objective was to understand why vanadium shifts the TTT-curve for the initiation of the proeutectoid ferrite reaction so little compared with other strong carbide-forming alloying elements such as Cr and MO. 6'15'16 The position of this TTT-curve is much more heavily influenced by the kinetics of growth than by those of nucleation. 17'~8 The solute drag-like effect t9'2~ has recently been assessed as exerting a particularly powerful influence upon allotriomorph growth kinetics in Fe-C-X alloys in which X markedly decreases the activity of carbon in austenite and exhibits an appreciable size misfit with respect to iron. 21'22This effect is considered to result from X segregation to disordered areas of austenite:ferrite boundaries; when X decreases the activity of carbon in austenite, T. ABE, formerly Visiting Research Associate, Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931, is now Chief Technical Consultant and Manager of the Metals and Machinery Section, Government Industrial Research Institute, Tohoku, Agency of Industrial Science and Technology, Nigatake, Haranomachi, Sendal 983, Japan. G.J. SHIFLET, formerly Republic Steel Corporation Fellow, Michigan Technological University, Houghton, MI 49931, is now Assistant Professor, Department of Materials Science, University of Virginia, Charlottesville, VA 22901. H. I. AARONSON, formerly Professor, Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931, is now R. E Mehl Professor, Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15213. Manuscript submitted March 14, 1984. METALLURGICALTRANSACTIONSA
the driving force for ferrite growth can be diminished if segregation is of a nonequilibrium type. 23 Since V does possess in good measure both of the characteristics needed to induce a significant solute drag-like effect, its rather small influence upon the location of the TTT-curve for the beginning of transformation is clearly in need of further consideration. Measurements of the thickening kinetics of grain boundary ferrite allotriomorphs were accordingly the primary data sought during this investigation. Studies of the morphology of fer
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