As an extension of a general dislocation description of interphase boundary structure in which the transformation lattice deformation is accomplished by discrete coherency dislocations, criteria are developed for determining the relative physical significance of interfacial coherency dislocations based on boundary microtopography. A step vector is defined such that the relation between the Burgers vector and step height of a dislocation is established. The step structure of an interphase boundary then serves as the basis for determining the dislocation structure. Two parameters are defined to express the relative topographical significance of coherency dislocations, taking into account the magnitude of the dislocation Burgers vector. The number of dislocation core atoms per unit length, ρL, establishes the absolute strength of a dislocation line. The dimensionless parameter β, which is the product of ρL and the separation of the dislocation lines in the boundary, expresses the local step character of a set of dislocations and reflects the extent to which such dislocations are resolvable as fully discrete line defects. The dependence of both parameters on interplanar spacing indicates that the fine structure of interphase boundaries will be strongly dominated by planes of closest packing.
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