Partition of Mn during the growth of proeutectoid ferrite allotriomorphs in an Fe-1.6 at. pct C-2.8 at. pct Mn alloy
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I.
INTRODUCTION
U N D E R S T A N D I N G the influence of a substitutional alloying element, X, upon the kinetics of the proeutectoid ferrite reaction in Fe-C-X alloys is the first step toward comprehending the role which these elements play in determining the hardenability of most commercial steels. However, even when interest is restricted to the growth stage of the proeutectoid ferrite reaction in high-purity Fe-C-X alloys, achieving a quantitative understanding of the effects of X upon the transformation has proved to be quite difficult. The central aspect of this problem is that in reaction temperature ranges of interest, the volume diffusivity of carbon in austenite is from four to six orders of magnitude higher than that of X. Hence the possibility arises that ferrite formation can occur with the partition of carbon between austenite and ferrite but without any partition of X, even though at equilibrium the solubility of X is normally quite different in these phases. Hultgren ~ has termed this type of transformation "paraequilibrium".* An X-ray study on Fe-C-Mo *More precisely, paraequdibrium is the situation in which the X/Fe ratm in ferrite is the same as that in austenite, carbon partitmn occurs between a and y to the extent required to equalize the partial molar free energy of carbon m the two phases.~ 45
alloys 2 and later an electron probe micro-analysis investigation of a number of Fe-C-X systems 3 have indicated that the growth made which accompanies no X partition is operative over wide ranges of reaction temperature in all of them. Now it has long been recognized that some undercooling below the equilibrium y / (a + y) temperature must be
MASATO ENOMOTO is Senior Researcher, National Research Institute for Metals, Tsukuba Laboratories, 1-2-1 Sengen, Sakura-Mura, Ibarakl 305, Japan. H. 1. AARONSON is R. E Mehl Professor, DeparTment of Metallurgical Engineering and Matermls Science, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted July 24, 1986.
METALLURGICAL TRANSACTIONS A
required in order to reach TVE, the paraequilibrium y~ (a + y) temperature. 3 The observation that when X = Mn, Ni, and Pt a sizable temperature range existed over which the amount of X partition was varied was attributed to particularly large differences between Ae3 and TvEin these alloys. 3'6 However, subsequent calculations utilizing different thermodynamic models of Fe-C-X systems, such as the Hillert-Staffanson regular solution model 7's and the central atoms model, 9'1~ invariably show that the temperature range in which paraferrite (no-partitioned ferrite) is experimentally observed lies much below TVE.12 On the other hand, it has been reported ~3'~4that the theory of local equilibrium 1~'~6'17can explain reasonably well both X partition and the growth kinetics of ferrite in Fe-C-Mn alloys. As described by Hillert ~5 and recently discussed by DeHoff,18 the theory provides a clear qualitative description of the gradual diminution in X partition between austenite and fen-Jte as the transformation
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