Nucleation kinetics of proeutectoid ferrite at austenite grain boundaries in Fe-C-X alloys
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I.
INTRODUCTION
SEVERAL studies have been reported on the heterogeneous nucleation kinetics of proeutectoid ferrite at austenite grain boundaries in alloy steels.l-4 However, most of these studies are seriously compromised by inadequate methods of making the nucleation rate measurements, by analyses based on semi-quantitative rate equations, and by ancillary parameter data not accurately applicable to the complex commercial-type steels employed. In the recently-completed study of the nucleation kinetics of grain boundary allotriomorphs of proeutectoid ferrite at austenite grain boundaries in high-purity Fe-C alloys by Lange et al. ,5 several important improvements in the experimental techniques for conducting such measurements were made: (1)nucleation at austenite grain faces was distinguished from that at grain edges by careful delineation of the former austenite grain boundaries; (2)the SchwartzSaltykov analysis 6'7 was used to convert the number and the size distribution of ferrite crystals on the plane of polish to the actual size distribution and the total number per unit area of untransformed and compositionally unaffected (hereafter simply unreacted) grain boundary; (3)a correction was made for the effects of the carbon diffusion field surrounding previously nucleated ferrite allotriomorphs on subsequent nucleation at the grain boundaries. The measured nucleation rates were shown to be far too rapid to be explained by critical nuclei based upon high M. ENOMOTO, formerly Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, is Senior Researcher, Tsukuba Laboratories, National Research Institute for Metals, 1-2-1 Sengen, Sakura-Mura, Niihari-Gun, Ibaraki 305, Japan. 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. METALLURGICAL TRANSACTIONS A
interfacial energy spherical caps. Pillbox-type nuclei of the type shown in Figure 1 were accordingly proposed. The model of Figure 1(a) is taken to have very low energy edges and low energy broad faces. The models of Figures l(b) and l(c) replace one low energy, coherent broad face with a high energy, disordered-type spherical cap; this is permissible
I ~ (a) o'ay
(b)
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(c) Fig. 1 - - T h r e e variants of pillbox critical nucleus modelr (a) both top and bottom surfaces are low energy a : y interfaces; (b) top broad surface and (c) bottom surface are high energy a : 3~ interfaces. VOLUME 17A, AUGUST 1986-- 1385
when the other broad face has a sufficiently low energy; these two models yield kinetics little different from that of Figure l(a). Although the latter two are crystallographically less restrictive, nonetheless accurate parallelism of a low energy broad face to one of the conjugate planes forming the austenite grain boundary is required, thus providing a basis for sharply restricting the proportion of the grain boundary area at which nucleation can tak
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