MICROSTRUCTURAL EVOLUTION IN NICKEL DURING ROLLING FROM INTERMEDIATE TO LARGE STRAINS

High-purity nickel (99.99 pct) with a grain size of 80 to 100 Am was deformed by cold-rolling from 37 to 98 pct reductions (von Mises effective strains of epsilon(vm) = 0.5 to 4.5). The deformation microstructures and texture at five strain levels were observed and characterized using transmission electron microscopy (TEM) and neutron diffraction. The microstructures evolved within a framework common to medium and high stacking fault energy fcc polycrystals. This framework consists of structural subdivision by higher angle boundaries (geometrically necessary boundaries) at one volume scale and at a smaller volume scale by lower angle cell boundaries (incidental boundaries) for all strain levels. We have characterized the dislocation boundaries, including dense dislocation walls (DDWs), microbands (MBs), and lamellar boundaries (LBs) in terms of crystallographic and macroscopic orientations, morphology, and frequency of occurrence. The microstructural evolution is discussed with special emphasis on factors that contribute to the transition from structures characteristic of small and medium strain microstructures to those characteristic of large strain microstructures.