Observations on interphase boundary structure

TLDR: In this article, a review is presented of experimental observations reported on the structure of interphase boundaries since 1968, showing that nearly all observed boundaries are of the partially or fully coherent type, and hence the principal structural features examined were misfit dislocations and ledges.

Abstract: SUMMARY A review is presented of experimental observations reported on the structure of interphase boundaries since 1968. Nearly all observed boundaries are of the partially or fully coherent type, and hence the principal structural features examined were misfit dislocations and ledges. Observations were drawn from studies of precipitation, spinodal decomposition, oriented overgrowths and eutectics. Interphase boundary structures were found to be little affected in the main by the type of reaction through which they were developed and were consistent with Van der Merwe theory and a theory of precipitate morphology. Observations of these structures have been considerably facilitated by the efforts of Weatherly and co-workers to define the visibility and the optimum TEM viewing conditions of misfit dislocations and ledges. Important new observations on misfit dislocations made since 1968 include: misfit dislocation spacing in the Cr(Mo)/NiAl eutectic in good agreement with theoretical expectation because misfit dislocations are also glide dislocations and the interface plane is also the glide plane; misfit dislocations on θ′ Al-Cu and η Al-Au plates with a Burgers vector (a/2 ) stable only at an interphase boundary; reduction of the interfacial energy of partially coherent boundaries on the same precipitates by dislocation interactions and rearrangements; reduction of interfacial energy of interfaces developed during spinodal decomposition through rotation of the interface itself from {100} to {110}; and misfit dislocation boundaries between f.c.c. (Cu-rich) and b.c.c. (Cr) crystals made possible with the assistance of ‘structural ledges’. Growth ledges have now been observed in a number of alloy systems which have undergone precipitation from solid solution or eutectic solidification; they appear to be confirmed as a customary feature of migrating partially or fully coherent interphase boundaries. Much new experimental information has been obtained on the sources (exceedingly diverse), heights (usually appreciably higher than monatomic), spacings (irregular) and migration kinetics (diffusion-controlled if their edges are disordered) of ledges. New observations continue to confirm the complexity of the processes through which misfit dislocations are acquired. The Ashby-Johnson and the Brown-Woolhouse theories of misfit dislocation nucleation or acquisition by spherical precipitates are in encouraging agreement with experiment.