A new theory and kinetic modeling of strain-induced precipitation of Nb(CN) in microalloyed austenite
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INTRODUCTION
F O R its industrial importance, the kinetics of strain-induced precipitation in microalloyed austenite has been studied extensively in the past 2 decades. It has frequently been observed that deformation can significantly increase the precipitation rate by up to two orders of magnitude.V~ Some experimental studies also demonstrated that deformation not only accelerates but also enhances precipitation. For example, studies carried out by Yamamoto et al. [2} showed that the measured amount of precipitated Nb in a low-carbon Nb steel deformed at 1050 ~ is about two times greater than that calculated by equilibrium thermodynamics. Such a phenomenon, i.e., second-phase precipitation in excess of its thermodynamic limit in an alloy system, is called "overprecipitation" in the present article. The effect of deformation on precipitation kinetics has often been attributed to the increase of dislocation density which increases both the nucleation site density and diffusivities of precipitate-forming elements in the material. Based on such an understanding and on classical nucleation theory, Dutta and Sellars, in 1987, proposed a model which relates the precipitation start time to strain, strain rate, temperature, and steel composition.t3] Although this model has widely been adopted, the results are often unsatisfactory,t3m In order to improve the accuracy of the prediction, Liu and Jonas proposed a more detailed formulation which is coupled with a thermodynamic model.iS] Unfortunately, the accuracy has not been significantly improved despite the consideration of the detailed thermodynamic properties of the multicomponent austenite system. The limitations of the conventional theory and models, especially their failure to explain and to predict the enhancement effect of deformation on precipitation (overprecipitation), have recently been recognized by a number of researchers, t4,61They have all attempted to correct the theory by taking into account an excess chemical driving force for nucleation associated with excess vacancies. However, such an excess driving force is unlikely to exist under the con-
W.J. LIU, Research Scientist, is with the Metals Technology Laboratories, Canada Centre for Mineral and Energy Technology, Ottawa, ON, Canada K1A 0G1. Manuscript submitted September 16, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
dition of strain-induced precipitation for the following reasons. (1) Nb(CN) nuclei energetically prefer to form at edge rather than screw dislocations.This is because the lattice parameter difference and the cube-cube orientation relationship between a Nb(CN) nucleus and the austenite create a hydrostatic distortion which can only be canceled out by the stress field around an edge dislocation. An edge dislocation can also be directly converted into a particle/matrix interface dislocation array, releasing additional energy to assist nucleation. However, since edge dislocations are excellent vacancy sinks, the excess vacancy level and, in turn, the excess driving force at edge di
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