The transformation phenomenon in Fe-Mo-C alloys: A solute drag approach
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I.
INTRODUCTION
THE presence of a bay in the isothermal time-temperature-transformation (TTT) diagram, especially in the FeMo-C system, has been one of the interesting topics in the literature. Extensive investigations and reviews can be found in References 1 and 2. Figure 1 shows a TTT diagram for an alloy of Fe-0.19 wt pct C-1.81 wt pct Mo, reproduced from Reference 2. As can be seen, there is a so-called bay at temperatures around 625 7C, denoted by Tb. It was reported that, above the upper nose, ferrite nucleates first but is followed soon by carbide precipitation. Between the upper nose and Tb, ferrite and carbide form concurrently. Below Tb, a degenerate Widmansta¨tten morphology of ferrite was found. Various theoretical considerations to explain the presence of a bay were reviewed by Shiflet and Aaronson.[1] One of the theoretical explanations connected with the present work is the solute draglike effect (SDLE) proposed by Kinsman and Aaronson[3] and discussed further by Reynolds et al.[2] They argued that Mo adsorption at disordered areas of austenite:ferrite boundaries reduces the carbon activity in austenite in immediate contact with this boundary and, thus, the driving force for growth. However, no quantitative analysis of this model has been presented. On the other hand, various solute drag models for phase transformations were recently discussed by the present author.[4] Further development taking into account the interfacial segregation was made by Suehiro et al.[5] in order to understand the considerable depression of the ferrite formation temperature upon continuous cooling by very low Nb addition in ultra low carbon steels. The modeling was a success. In the present work, the same model was applied to the Fe-Mo-C system to consider the effect of interfacial segregation of Mo on the ferrite growth in austenite. The carbide precipitation was not taken into account in the present calculations. However, it should be mentioned that, as concluded by Shiflet and Aaronson,[3] the experimental obserZI-KUI LIU, Researcher, formerly with the Department of Materials Science and Engineering, The Royal Institute of Technology, S-10044, Stockholm, Sweden, is with the Department of Materials Science and Engineering, University of Wisconsin, Madison, WI 53706-1595. Manuscript submitted October 18, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
vations and theoretical explanations indicate that the ferrite growth plays an essential role in the presence of a bay.
II.
THE MODEL
The model is briefly summarized here by applying it to the Fe-Mo-C ternary system with the schematic concentration profile shown in Figure 2. The details of the model can be found in Reference 5. A ferrite (bcc, a) plate embedded in an infinitely sized austenite matrix (fcc, g) is considered. The edge radius of the ferrite plate is r, and the isotropic interfacial energy is s. The driving force for the phase transformation is dissipated by the interfacial energy, finite interfacial mobility, solute drag in the interface, and diffusion i
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