Surface relief effects as evidence for coherency of interphase boundaries

Exceptional difficulties are currently being encountered in determining the structure of certain interphase boundaries, especially those across which an irrational orientation relationship obtains and at which the conjugate habit planes are irrat ona. Th s paper explores the possibilities of determining at least whether or not particular boundaries are incoherent, or alternatively, either partly or fully coherent, using the presence or absence of an Invariant Plane Strain or Tent-shaped surface relief effect (SRE). Both of these effects are now accepted as associated with nucleated diffusional phase transformations involving a change in stacking sequence across their interphase boundaries. The particular problem engaged is whether the absence of such a relief can be assigned to an incoherent boundary structure or to a partly/fully coherent boundary of a type that either does not produce a SRE, or does so in a manner difficult to ascertain experimentally. Partly/fully coherent interphase boundaries which should not exhibit a SRE include those across which there is no stacking sequence change, an ideal O-lattice interface is present or when diffusion during the transformation involves atoms not part of a lattice correspondence. The proposed decay of a SRE as rapidly as it forms has been disproved experimentally. Canceling of successive shears on a near-atomic scale could be troublesome experimentally but appears not to occur when transformation takes place in contact with a free surface. The questions as to whether or not a SRE will be produced, and if so what type of relief would be expected, remain to be answered for displacement of micro-facets on grain boundary allotriomorphs, and for migration of non-invariant plane interfaces normal to any product phase morphology.