Microstructural evolution in nickel during rolling from intermediate to large strains
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I.
INTRODUCTION
DEFORMATION microstructures in high-purity nickel during rolling at small to intermediate effective strains, t:vm = 0.10 to 0.50, develop within an evolutionary framework common to medium and high stacking fault energy fcc polycrystals.[1-5] Within this framework, deforming grains are subdivided into rotated volume elements at two levels for all deformation modes. [5,6] The smallest subdivision is by incidental boundaries that arise from the statistical trapping of glide dislocations together with forest dislocations, [6] These incidental boundaries form dislocation cells whose walls contain small lattice misorientations, On a larger scale of subdivision, geometrically necessary boundaries (GNBs) surround groups of cells.[5,6] These GNBs incorporate the lattice rotations that arise from the geometrical requirements of strain accommodation. Consequently, there is a larger misorientation across 'heir boundaries than across cell walls, These GNBs in~lude dense dislocation walls (DDWs) and microbands MBs). Grain subdivision into misoriented regions occurs at II strain levels, but there is a change in both the type . structures and the macroscopic orientation of those uctures as deformation continues from low to high ains. The structures at low to medium strains consist '1inly of DDWs, MBs, and ordinary cells.[2A,7,8] Other Jestigators have also observed MBs and cells in demed nickel at intermediate strains,[9-12] All of those ,erved structures have been discussed earlier within evolutionary framework established for a number of polycrystalline materials, [2] The large strain strucs developed in nickel have been separately described composite microstructure containing equiaxed cells, .A. HUGHES is with the Materials and Processes Research 'lftment, Sandia National Laboratories. Livermore. CA 94550. ,-IANSEN. Materials Department Head. is with Ris0 National >ratory. Roskilde. Denmark. lanuscript submitted lanuary 25. 1993. fALLURGICAL TRANSACTIONS A
subgrains, and lamellar dislocation boundaries. [8,91 This large strain structure has not been placed within the context of grain subdivision and the evolutionary framework described previously;[5] nor have the microstructural components at intermeoiate strains, which make the structural transition between the differing structures at small and large strains, been documented. This article will describe the microstructural and textural evolution as observed by transmission electron microscopy (TEM) and neutron diffraction from intermediate to large strains in nickel deformed by rolling. The mechanisms that affect the structural transition will be discussed. A new geometric and slip model for one of the transition mechanisms from small to large strain microstructures is developed and presented based on these observations.
II.
EXPERIMENTAL PROCEDURES
High-purity nickel (99.99 pct) with a recrystallized grain size of 80 to 100 JLm was deformed by unidirectional rolling to the conditions outlined in Table I. The rolling deformation was done using 75-mm-diamete
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