Dislocation structures and cyclic behaviour of [011] and [formula omitted]-oriented nickel single crystals

Abstract

[011] and [ 1 ̄ 11] -oriented nickel single crystals were cycled at room temperature with constant plastic strain amplitudes ( ε pa) ranging from 1×10 −4 to 1×10 −2. The paper describes the cyclic stress–strain curves (CSSC), the surface features and the saturated dislocation structures for both crystals orientations. The CSSC of the [011]-crystals shows a very pronounced plateau in the range of 1×10 −4 to 5×10 −3 in which the saturation stress amounts to 123 MPa. The occurrence of persistent deformation bands is the most typical feature of the surfaces of cycled [011]-crystals. A deformation band is characterised by a stack of densely distributed and strongly localised slip markings which belong to a certain slip system. The dislocation structures of [011]-crystals are typically grouped in structure bands which consist of short ladder-like substructures (short persistent slip bands), wall-like dislocation configurations and patch dislocation patterns in the plateau region. At ε pa≥5×10 −3 elliptical cells exist in the entire crystal. The CSSC of [ 1 ̄ 11] -crystals shows apparently no plateau. The surfaces of cycled [ 1 ̄ 11] -crystals are typically covered by two sets of slip traces crossing one another. Deformation bands with the characteristics of conventional kink bands were also found but as a minor case. The dislocation structures of [ 1 ̄ 11] -crystals consist of long “condensed” wall-like dislocation configurations with the longest extension lying perpendicular to the tensile axis. The observed structure types fall in line with those found in corresponding oriented grains of cycled nickel polycrystals.