ON THE THERMODYNAMICS OF PRECIPITATE PLATE FORMATION BY SHEAR ABOVE T-O ASSISTED BY SOLUTE ADSORPTION EFFECTS

To first-order, the solute-depleted or solute-enriched regions adjacent to intragranular dislocations or to grain boundaries should have no effect upon the driving force for nucleation once sufficient local diffusion has taken place to make the partial molar free energies of solutes in these regions equal to those in the bulk. Before such diffusion has taken place, the average T0 temperature of the matrix in the immediate vicinity of solute-poor precipitate plates, e.g., ferrite/bainite plates in steel and α1 plates in CulZn alloys, will be lower than that in the bulk alloy. Hence formation of such plates by shear at temperatures above the martensite range will then become even more difficult. Thus attempts which have been recently made to circumvent the T0 restriction upon the temperature range of shear transformations by means of solute segregation to dislocations and to grain boundaries lead to either very little advantage (due to possible but weak second-order effects) or to further disadvantage thermodynamically for such transformations. © 1995.