The kinetics of ferrite nucleation at austenite grain edges in Fe-C and Fe-C-X alloys
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INTRODUCTION
THE spherical-cap based nucleus models originally due to Gibbs I and analyzed by Clemm and Fisher 2 have been used by Cahn3 to show that as long as the ratio of the nucleus :matrix interfacial energy to the grain boundary energy (both of which are assumed to be isotropic) exceeds a certain threshold, nucleation will first predominate at grain corners, then at grain edges, and finally at grain faces as undercooling below the solvus temperature is progressively increased. Qualitative support for this prediction has been available for some time. 4'5 In a more recent study, the ratio of the number of edge-nucleated ferrite allotriomorphs to the number nucleated at faces was reported as a function of temperature and composition in Fe-C alloys. 6 This appears to be the only quantitative documentation available on the subject in the literature. The present investigation appears to be the first in any alloy system in which data on the rate of nucleation at matrix grain edges was obtained. In current investigations of nucleation kinetics of ferrite allotriomorphs at austenite grain faces in Fe-C 7 and Fe-C-X s alloys, grain face- and grain edge-nucleated ferrite allotriomorphs were distinguished by revealing the former austenite boundaries with specially selected etching solutions. Measurements of nucleation rates at grain edges become feasible, however, only if the aspect ratio, i.e., the ratio of the (maximum) thickness to the length, of allotriomorphs growing along grain edges can be measured. Fortunately, coarse-grained Fe-C-X alloys containing - 0 . 4 wt pct C which have been largely transformed to martensite will fracture along austenite grain boundaries. 9 This circumstance permits determination of the aspect ratio of grain edge allotriomorphs and thus allows conversion of the number of edge-nucleated allotriomorphs on a plane of polish (perpendicular to the edges) to the number per unit length of grain edge through direct application of the Schwartz-Saltykov analysis. 10,..12 M. ENOMOTO, formerly Graduate Student with the Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, has now returned, as Senior Researcher, to the Tsukuba Laboratories, National Research Institute for Metals, 1-2-1 Sengen, Sakura-Mura, Niihari-Gun, Ibaraki 305, Japan. W.F. LANGE, III, is Manager of the Advanced Packaging Technology Group, IBM, Boulder, CO 80301. H. I. AARONSON is R. F. Mehl Professor, Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15213. Manuscript submitted September 11, 1985.
METALLURGICALTRANSACTIONSA
The present study was undertaken in order to compare nucleation kinetics at grain edges with those previously determined in the same alloys and specimens at grain faces in Fe-C 7 and Fe-C-X 8 alloys. Questions of special interest included: (i) is a low interfacial energy nucleus also required at grain edges, (ii) is the number of viable nucleation sites as severely restricted as it is at grain faces, 7'8 and (iii
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