Simulation of ferrite growth in continuously cooled low-carbon iron alloys
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TION-INVARIANT (or partitionless) massive transformation is usually considered to occur when alloys are quenched into a single-phase field. It was reported in several alloy systems that the massive transformation occurred in two-phase fields near the solvus of the product phase.[1–5] Hillert[6] proposed that a limit of the phase field in which the massive transformation occurs could be related to the achievement of local equilibrium at the moving boundary. Thus, one of the most important characteristics that is related to the limit of partitionless transformation may be the extent of the solute diffusion zone in front of the boundary. The width of a diffusion spike depends strongly on the transformation conditions, e.g., the amount of undercooling in an isothermal condition and the cooling rate in continuouscooling transformations. Under isothermal conditions, the growth is usually time dependent in the two-phase field because a diffusion spike has developed, although a steadystate growth can occur with Widmansta¨tten plates. On the other hand, in continuous cooling, a diffusion spike does not necessarily increase in width, because the solute concentration at the boundary is not constant, even if local equilibrium is achieved, and, thus, the boundary velocity may well increase with time. In this study, the austenite-to-ferrite transformation in continuously cooled low-carbon iron alloys was simulated. A Green-function method was employed to simulate the diffusion-controlled growth of ferrite in the (␣ ⫹ ␥) two-phase field. The primary interest in Fe-C alloys is whether local equilibrium of carbon is achieved and how much the loss of driving force due to boundary friction and carbon diffusion in austenite affects the growth rate of proeutectoid and massive ferrite. The characteristics of the ferrite transformation in low-carbon iron and iron binary alloys during continuous M. ENOMOTO, Professor, is with the Department of Materials Science, Ibaraki University, Hitachi 316-8511, Japan. This article is based on a presentation made at the symposium entitled “The Mechanisms of the Massive Transformation,” a part of the Fall 2000 TMS Meeting held October 16–19, 2000, in St. Louis, Missouri, under the auspices of the ASM Phase Transformations Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
cooling have been studied by experiments, i.e., rapid cooling,[7] and, recently, by computer simulation.[8,9] In this study, simulation was also conducted in Fe-C-Mn alloys, incorporating the solute-drag effects due to an alloying element. The solute drag is considered to have a marked influence on the growth of precipitates and massive transformation.[10–15] II. CALCULATION METHOD A. Growth of Ferrite in the (␣ ⫹ ␥) Two-Phase Field Simulations will be conducted in the three alloys shown in Table I. Alloys A and C are cooled through the (␣ ⫹ ␥) two-phase field and the ␣ single-phase field and alloy B is cooled through the (␣ ⫹ ␥) two-phase field. In alloy B, the influence of the intrinsic ␣:␥ boundary mobility on the growth of ferri
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