Microstructural evolution in nickel during rolling and torsion

Microstructural evolution in high purity nickel was investigated by comparing the dislocation substructures formed following torsion deformation with those following rolling. Results from transmission electron microscopy observations and measurements of microdiffraction show a common evolutionary path in rolling and torsion from (i) dislocation tangles to (ii) equiaxed cells bounded by dense dislocation walls (DDWs) which form cell blocks, then to (iii) microbands which, together with DDWs, bound smaller and smaller cell blocks, and, finally, to (iv) subgrains. During this evolution, grains are continually subdivided by the formation of new cell blocks. New cell blocks arise through the formation of new DDWs between cell blocks or of first generation microbands by the subdivision of a single DDW Since the dislocation content of a microband is principally derived from that of the parent DDW, microbands can take various forms, such as a string of small pancake shaped cells in highly recovered DDWs, short double walls in discontinuous DDWs, or long paired dislocation sheets from continuous DDWs which can be very straight if formed close to {111} planes. At moderate strains and subsequent to microband formation, localised shear and shear offsets were observed in some first generation microbands. A few second generation microbands, in which localised shear is part of the formation process, were also observed. Localised shear was much more prevalent in rolling than in torsion. These results provide evidence for the theory that grains are divided into regions in which slightly different slip systems operate. Collectively, these regions provide evidence for strain accommodation, although in individual regions the number of slip systems is less than that required by the Taylor criterion.