Dislocation-disclination model of heterogeneous martensite nucleation in transformation-induced-plasticity steels
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I. INTRODUCTION
THE unusual combinations of strength, ductility, and toughness exhibited by the metastable austenitic steels, some of which are termed transformation-induced-plasticity (TRIP) steels,[1] have called attention to the potential benefits of deformation-induced martensitic transformations. Lowtemperature plastic flow in TRIP steels has been found to be controlled by a stress-assisted isothermal martensitic transformation.[2,3] For these conditions, the thermodynamics and kinetic theory of martensitic transformations lead directly to constitutive relations predicting the dependence of flow stress on temperature, strain, strain rate, and stress state, consistent with the observed behavior of TRIP steels.[3] Due to the rapid growth of a martensitic particle, the kinetics of isothermal martensitic transformations in steels is nucleation controlled. Thus, the problem of the initial growth of a martensitic embryo is traditionally considered as the key question in the theory of martensitic transformations.[4,5,6] The most clear and comprehensive theoretical consideration of the initial growth of a martensitic embryo was done by Olson and Cohen,[7,8,9] who proposed, within the classical thermodynamic theory of nucleation, dislocation models to describe embryo generation and development. They postulated that the first step in martensitic nucleation is faulting on planes of closest packing. It was further postulated that the faulting displacements are derived from an existing defect, while matrix constraints cause all subsequent processes to occur so as to leave the fault plane unrotated, thus accounting for the observed general orientation relations. Using basic concepts of classical nucleation theory, the M.Yu. GUTKIN, Leading Researcher, and K.N. MIKAELYAN, Researcher, are with the Institute of Problems of Mechanical Engineering, Russian Academy of Sciences, St. Petersburg 199 178, Russia. Contact email: [email protected] V.E. VERIJENKO, Professor of Solid Mechanics, is with the School of Mechanical Engineering, University of Natal, Durban 4041, Republic of South Africa. L.D. THOMPSON, Professor and Chair, is with the Department of Mechanical Engineering, San Diego State University, San Diego, CA 92182-1323. Manuscript submitted April 5, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
stacking-fault energy was shown[7] to consist of both volumeenergy and surface-energy contributions, as follows: W ⫽ n (⌬G ⫹ E ) ⫹ 2␥
[1]
where W is the fault energy, expressed per unit area of fault (in the fault plane); n is the number of atomic planes (in thickness) composing the fault; is the density of atoms in a close-packed plane in moles per unit area; ⌬G is the chemical free-energy difference between the parent and product phases; E is the strain energy; and ␥ is the free energy per unit area of the particle/matrix interface. The terms ⌬G and E are defined in Eq. [1] as molar quantities. When the volume-energy contribution (⌬G ⫹ E ) is negative, the fault energy decreases with increasing fault thickness (once th
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