Segregation and wetting transition at dislocations

We investigate solute segregation and wetting transition at dislocations and the corresponding drag effect on dislocation glide using a continuum model developed previously for grain boundary and based on gradient thermodynamics. The dislocation core structure and stress field are described by the newly developed phase field model. This study differs from much previous work because it takes into account not only the long-range elastic interactions but the short-range chemical interactions between solute atoms and dislocation core as well as among solute atoms themselves. The latter leads to the prediction of a wetting transition at the dislocation core with respect to varying temperature, solute concentration, or dislocation velocity. The transition temperatures obtained during heating and cooling are different from each other, leading to a hysteresis loop in the solute concentration-temperature plot and the solute concentration-velocity plot. These predictions could provide new insights into the phenomena of sharp yield point drop and strain aging observed in metal alloys. This article is based on a presentation made in the “Hillert Symposium on Thermodynamics & Kinetics of Migrating Interfaces in Steels and Other Complex Alloys,” December 2–3, 2004, organized by The Royal Institute of Technology in Stockholm, Sweden.     

Planar to equiaxed transition in the presence of an external wetting surface

First time experimental evidence is presented for the steady-state growth of equiaxed grains during directional solidification in the presence of a positive temperature gradient. The evidence is shown for transparent grains in the succinonitrile-acetone system, which are induced to grow on wetting surfaces (restraints). The conditions for equiaxed to dendrite transitions on the restraint are recorded, and a simple model is developed to explain this effect.     

Errata: Planar to equiaxed transition in the presence of an external wetting surface

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