Processing by equal-channel angular pressing: Applications to grain boundary engineering

Equal-channel angular pressing (ECAP) is a processing technique in which a sample is pressed through a die constrained within a channel so that an intense strain is imposed without incurring any change in the cross-sectional dimensions of the sample. This procedure may be used to achieve considerable grain refinement in pure metals and metallic alloys with as-pressed grain sizes lying typically within the submicrometer range. Careful experiments reveal only a minor change in the grain size with increasing numbers of passes through an ECAP die but there is a significant change in the distribution of grain boundary misorientations as a function of the total imposed strain. In practice, the microstructure evolves with increasing strain from an array of grains where the boundaries are predominantly in low-angle misorientations to an array of grains where a high fraction (typically ≥60%) is in high-angle misorientations. This evolution has a significant effect on the characteristics of the as-pressed materials including the high temperature mechanical properties and the measured rates of diffusion. In addition, the evolution provides an opportunity to use Grain Boundary Engineering in order to optimize the behavior of the material.