Changes Produced by Deformation in Grains and Grain Boundaries of Nickel

Abstract

Polycrystalline nickel (grain size 0.04 mm) has been examined with the electron microscope after annealing and then after plastically straining at room temperature to between 0.4 and 12% elongation. In addition to measuring the grain-interior dislocation density (ρ, cm/cm³), the number (n) per cm³ of three-fold nodes was counted and the arrangement and density (p, cm/cm²) of dislocations in the grain boundaries was examined. The applied tensile stress σ = 1.4 Gb ρ½ = 1.3 Gb n⅓ (where G is the shear modulus, b the Burgers vector). The node density was high and the dislocation network tightly linked at all strains, which is a necessary feature if attractive junctions are to contribute substantially to the flow stress. Straining evidently does not change the shape of the network so much as refine it, and the refinement is presumably the essential feature of strain-hardening. Superimposed on the refinement there developed the normal cellular structure. The grain boundaries trapped dislocations and held them immobile. The plastic strain thus produced at grain boundaries (pb) was ∼ ¼ × macrostrain. The spacing of dislocations in the grain boundaries was ∼ 6 times less than that in the grains, from which it is concluded that the average length in a grain boundary is 6 interior spacings (∼ 1 μm), and that such a length at least moved through the grain as a unit.