Geometrically necessary, incidental and subgrain boundaries

During the deformation of polycrystals, the grains break up into domains within which the selection of operative slip systems differs. The domains then subdivide into “cell blocks”. Locally, each group of cell blocks comes near to fulfilling the Taylor criterion when taken collectively, but the number of active glide systems in any one cell block is fewer than predicted.

The boundaries between cell blocks and/or domains accommodate the lattice misorientations which result from glide on the different slip system combinations. They are therefore named “geometrically necessary bpundaries”. They, like all boundaries capable of accommodating variable lattice misorientations, are composed of dislocations.

Microscopically, such boundaries appear as “dense dislocation walls” and “microbands”. Geometrically necessary boundaries are distinguished from ordinary dislocation cell boundaries by the absence of a change of glide systems across the latter. In materials deforming with a cell structure, ordinary dislocation cell boundaries as well as traditional “deformation bands” arise from the mutual trapping of dislocations into low-energy configurations.

Such cell boundaries or walls are therefore named “incidental dislocation boundaries”. The misorientation across incidental boundaries is typically much smaller than for geometrically necessary boundaries. A further distinction is their respective on the flow stress. The average spacing of dislocation cell walls is inverse proportional to the flow stress whereas geometrically necessary boundaries obey the Hall-Petch relationship.

Since they tend to occur more frequently the incidental boundaries typically control the flow stress. At increasing strain the angles between dislocation cells increase and different slip system combinations can operate in neighbouring cells. Cell walls are then no longer incidental boundaries but geometrically necessary boundaries.

Such boundaries are termed “subgrain boundaries”.