Abstract
Build-up of a stress field in a matrix, as a result of a specific volume differential, affects the vacancy pattern near a moving transformation boundary; if the field is tensile extra vacancies are formed; if it is compressive they are annihilated. For a constant concentration of these vacancies in excess of the thermodynamic equilibrium value, the entropy term and the stress cross-effect term in the diffusion equation must be equal. Diffusion of the vacancies is facilitated by a Kirkendall mechanism involving the solute atoms of the matrix. The mass-balance equation, and the Anthony relationship between the concentration gradients of the solute and vacancies, are used for deriving a boundary condition relating the boundary velocity to the stress-field gradient. In accordance with the theory of thermally-activated transformation, an expression is derived for a system where a metastable phase gives way to a more stable one, through dissolution in the matrix of the major phase. Applicability of the model to graphitization of pearlitic grey cast iron at subcritical temperatures is discussed.
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