Abstract
A computer model has been developed to simulate non-equilibrium grain boundary segregation. The model is based on simultaneous diffusion of free vacancies, free solute atoms and vacancy-solute complexes while maintaining local equilibrium between these three entities. The driving force for segregation is the change in equilibrium vacancy concentration during cooling. A sufficient condition for grain boundary enrichment to occur is that complexes are more mobile than free solute atoms. The simulations showed that the amount and width of segregation should be strongly dependent on 1. (i) the heat treatment temperature and cooling rate, 2. (ii) the solute concentration, 3. (iii) the binding energy of complexes 4. (iv) the vacancy formation energy and 5. (v) the mobility of solute atoms and complexes. The maximum boundary enrichment occurs at some intermediate cooling rate. Higher mobilities, higher starting temperatures and smaller grain sizes require higher cooling rates to give maximum enrichment. The model was applied to the system boron-austenite. Good agreement between experimental results and simulations was achieved. The diffusion coefficients for boron and vacancy-boron complexes in austenite were estimated at D B = 2.10 −7 exp [ −1.15( ev kT ] and D VB = 2.10 −7 exp [ −1.15( ev kT ] ( m 2/s ) respectively.
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