Atom probe field ion microscopy study of the partitioning of substitutional elements during tempering of a low-alloy ste
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INTRODUCTION
THE kinetic models of reconstructive growth in lowalloy steels aim to predict the interface velocity, volume fraction, and composition of product phases as a function of alloy composition, transformation temperature, and time. Theories of reconstructive transformations in ternary steels are already available. 1~-8] These theories have been extensively used to model the reconstructive transformation of austenite to ferrite in low-alloy steels, tg'l~ Nevertheless, the theories are general and can be applied to any situation involving reconstructive growth of a precipitate phase. In these kinetic models, local equilibrium at the interphase interface is assumed for predicting velocities of transformation interfaces. With the local equilibrium assumption, three transformation modes were proposed for the transformations in steels involving the diffusion of both substitutional and interstitial elements. In low supersaturation and low interface velocity, the transformation is controlled by the slow diffuser (substitutional elements) and the local equilibrium at interface exists. This mode is known as partitioning local equilibrium (PLE). In high supersaturation and high interface velocity, the transformation is controlled by the fast diffuser (interstitial atoms) and the local equilibrium at the interface exists. This mode is known as negligible partitioning local equilibrium (NPLE). In NPLE mode, a steep concentration gradient of substitutional element, which is known as a diffusion spike, is assumed in front of the growing interface, taj The estimated width of these diffusion spikes reduces as a function of undercooling. In very high undercoolings, the diffusion-spike width reduces to the order of atomic spacing. In this condition, it is assumed that the local equilibrium fails and the transformation mode switches to paraequilibrium.ia'7-111 In the paraequilibrium S.S. BABU, formerly Research Associate, Institute for Materials Research, Tohoku University, is Postdoctoral Research Associate, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830. K. HONO, Research Associate, and T. S A K U R A I , Professor, are with the Institute for Materials Research, Tohoku University, Sendal 980, Japan. Manuscript submitted April 12, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
condition, there is no redistribution of substitutional elements between the precipitated phase and the matrix. The ratio of Fe/X (X = substitutional elements) concentration remains constant at both sides of the interface. Paraequilibrium growth is possible if C reaches its equilibrium concentration in both parent and product phases, t4-81 In this case, the growth of the product phase is controlled only by C diffusion. Several experimental works were conducted to determine the partitioning of substitutional elements under the paraequilibrium reaction by employing electron probe microanalysis and analytical electron microscopy, t9'l~ However, because of the insufficient spatial resolution of the conventional anal
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