Altering the time cycle of heat treatment by controlling grain boundary and subboundary structure

Abstract

The spectrum of grain and subgrain boundary structures in the matrix phase can frequently be changed by processes such as texturing and polygonization. These changes can markedly alter the morphology, kinetics of nucleation and/or growth and even the identity of the precipitate formed during subsequent heat treatment. The numerous effects of grain and subgrain boundary structure upon precipitation processes are summarized and discussed. When the boundary structure is of the dislocation type many of the noncrystallographic effects are qualitatively explicable in terms of Cahn’s theory of nucleation at isolated dislocations or by simple extensions of this theory; but a more detailed study of the strain energy interactions between a plate-like nucleus and a dislocation boundary is needed for a quantitative understanding of these effects. Some anomalies which are presently unexplained are noted. At general high-angle grain boundaries, on the other hand, the effects of boundary structure are simpler and better understood. Grain boundary allotriomorphs are normally the only morphology present. A question remains about the detailed mechanism of nucleation, but the relative roles of volume diffusion directly to allotriomorphs and of volume diffusion aided by interfacial diffusion in the growth kinetics of allotriomorphs are becoming increasingly well established.