C. Holste

Dislocation structures in cyclically deformed nickel polycrystals

Abstract The effect of the grain orientation and the plastic strain amplitude e pa on the saturated dislocation structure was studied on individual grains of cyclically deformed nickel polycrystals by means of scanning electron microscopy using the electron back scattering pattern technique and the channelling contrast of back scattered electrons. The main features of the dislocation configuration in a grain were found to be essentially determined by the crystallographic axial orientation of the grain. A labyrinth-like dislocation pattern is typical for grains with axial orientations near [001], a patch pattern exists in grains with a loading axis (LA) near [011] and fragmented dislocation walls are dominant in grains with LA near [111]. Grains with axial orientations in the central part of the stereographic standard triangle contain a bundle arrangement of dislocation structures. All four types of dislocation structures, but mostly the bundle type, can occur together with the ladder structure of persistent slip bands. Cell patterns were found to be a result of a modification of the bundle and patch configuration at high deformation amplitudes. The mesoscopic dimensions of the dislocation patterns turned out to depend on e pa in the same way for all grain orientations: while the thickness of regions with high dislocation density is reduced with increasing e pa , the width of regions with low dislocation density remains roughly constant.

Cyclic plasticity of nickel single crystals at elevated temperatures

Abstract Single slip oriented nickel crystals have been cyclically deformed at deformation temperatures between 293 K and 900 K until saturation of the stress amplitude. Persistent slip bands (PSBs) with a typical ladder-like dislocation substructure occur at temperatures up to 800 K. At 600 K and 750 K the measured cyclic stress-strain curves (CSSCs) exhibit extended ranges in which the saturation stress amplitude is nearly independent of the applied plastic strain amplitude. The plastic strain amplitdes at the upper and the lower borders of these “plateaus” in the CSSCs decrease with increasing temperature. Most of the measured structure lengths of the wall/channel arrangement in the PSB ladders and of the dipole arrangement in the PSB walls have been found to become larger at elevated temperatures. The only exception observed is the wall thickness which remains constant, independent of the temperature. Analysis of the measured data on the basis of current modelling approaches shows that the local flow stresses and the residual stresses are reduced at elevated temperatures.

CYCLIC STRESS-STRAIN RESPONSE AND STRAIN LOCALIZATION EFFECTS UNDER STRESS-CONTROL CONDITIONS

Abstract Polycrystalline nickel was cyclically deformed under stress control. Experiments with different maximum and mean stresses were performed, always applying the maximum stress in the first quarter cycle. During the tests the cyclic mechanical behaviour was studied and the occurrence of strain localization was investigated by surface relief observation and by transmission electron microscopy of bulk foils. The cyclic stress-strain response was shown to be affected characteristically by the mean and maximum stresses. On the surface, persistent slip markings were found not only for zero mean stress but also for wide range of non-vanishing mean stresses. Persistent slip bands with the typical ladder-like dislocation structure appeared in the bulk material only at symmetrical or nearly symmetrical cycling.

Mechanical behaviour and development of dislocation arrangements of f.c.c. single crystals fatigued at 77 K

Abstract The influence of low test temperatures on strain localization effects was studied on nickel and copper single crystals cycled at plastic strain amplitudes corresponding to the low end of the plateau in the cyclic stress-strain curve. For both materials the occurrence of extrusion-type surface slip markings after cyclic deformation at 77 K indicates that the cyclic strain becomes localized in layers parallel to the primary glide plane. In contrast to the situation at room temperature, there are no characteristic arrangements in the microstructure of dislocation walls which can be correlated with the extrusions on the surface. Nearly the entire volume of the saturated dislocation structure of crystals deformed at 77 K without predeformation consists of extended wall structures with irregularly arranged dislocation-dense regions. In particular, no regular persistent slip band ladders were found in nickel and copper crystals. There is a transition temperature below which no ladder-like dislocation arrangements are formed. Both the evolution of the dislocation arrangements and the mechanical behavior suggest that the structure at low temperatures develops in two stages: stage I in which the hardening behavior is qualitatively the same as at room temperature, followed by stage II in which a second hardening occurs and the extended wall structure is formed.

Effect of grain size and deformation temperature on the dislocation structure in cyclically deformed polycrystalline nickel

Abstract The effect of grain size and deformation temperature on the stabilised dislocation structure was studied in individual grains of fine- and coarse-grained nickel polycrystals after cyclic deformation at room temperature and at 77 K. It has been found that, independent of the grain size and deformation temperature, the dislocation patterns in the individual grains are substantially determined by the axial orientation of the grains. A typical feature of the structure in the coarse-grained material fatigued at room temperature is a fragmentation of the grain dislocation configuration into structure sections with different mesoscopic structure parameters. In fine-grained material after cycling at room temperature and in coarse-grained material cycled at 77 K, no subdivision of the grain structure could be detected. Low-temperature deformation of coarse-grained nickel leads to the formation of a ‘two-phase’ structure, that is characteristically different to the extended wall structure observed in single-slip oriented nickel monocrystals cyclically deformed at 77 K.

Prediction of the cyclic stress–strain curve of polycrystalline nickel

Abstract The cyclic stress–strain curve (CSSC) of polycrystalline nickel is predicted in the framework of Taylor type models. Particular attention is paid to a modified Taylor model (MTM) which takes into account recent experimental findings: (i) the dislocation patterns in the individual grains are essentially determined by the axial grain orientation, (ii) the observed structures resemble the dislocation patterns formed in single crystals with corresponding axial orientations. The proposed MTM uses the CSSCs of [001], [ 1 11] and [011] monocrystals and the CSSC of crystals oriented for single slip as hardening relations. In a more elaborated MTM more than four hardening laws are taken into account. For comparison a self-consistent Sachs model is considered which treats each individual grain as an inclusion in a homogeneously deformed matrix. The more detailed MTM is shown to provide the best agreement between the predicted and measured polycrystal CSSC.

X-ray diffraction analysis of internal stresses in the dislocation structure of cyclically deformed nickel single crystals

Typical dislocation configurations appearing after cyclic deformation of nickel crystals oriented for single slip are the ladder-like arrangements in persistent slip bands (PSBs) and the bundle structure in the matrix. Long-range internal stresses are expected to arise in the PSBs. The change of these stresses within the deformation cycle is investigated by an analysis of asymmetric X-ray line profiles. For crystals with a pure matrix structure or with a low volume fraction of PSBs, no significant long-range stresses occur. In dislocation structures with a PSB volume fraction above 20%, long-range stresses could be resolved changing their sign within each half-cycle. The saturation of the internal stresses within each half-cycle indicates macroyielding which may be interpreted as plastic yielding of the PSB walls at a local yield stress of about 120 MPa.

Analysis of local variations of internal stresses in cyclically deformed nickel crystals by Barkhausen noise measurements

A new method for producing and monitoring Barkhausen noise (BN) was applied to analyse local variations of internal stresses in the surface layer of nickel crystals which have been cyclically deformed at room temperature into the saturation stage. For different sites on the crystal surface both the integral BN signal and the BN signal height distribution along the magnetizing half-cycle were mapped. The evaluation of the spatial variation of the integral BN leads to the conclusion that macroscopic compressive internal stresses exist in the persistent slip bands (PSBs) which do not reverse their sign during the loading half-cycle. The observed cyclic variation of the BN signal distribution stemming from PSB surface regions shows that cyclic changes of the internal stresses also occur. With the growth of the microcrack population the cyclic variation of the BN signal distribution, and hence of the internal stresses in the surface layer, becomes smaller and finally fades completely.

Investigation of the tensor character of mesoscopic internal stresses in tensile-deformed nickel single crystals by X-ray diffraction

Abstract X-ray diffraction profiles of tensile-deformed nickel single crystals with two different orientations of the loading axis were investigated. To characterize the internal elastic strain state on a mesoscopic scale, the profile shapes of different Bragg reflections were analysed. The asymmetric profile broadening is interpreted on the basis of a two-component model for internal stresses in a dislocation cell structure. A least-squares method was applied to determine the stress tensor for the cell interior from the asymmetry parameters of at least six independent reflections. Furthermore, this method was advanced to evaluate the distribution of the plastic deformation across the slip systems. Taking into account an inclusion approximation, by this way the calculated internal stresses were correlated with the plastic strain in the cells. The orientation of the principal axes of the stress tensor and the heterogeneity of slip in the slip systems found agree with model assumptions for the tensile deformation.

Influence of Thermal Treatment and Cyclic Plastic Deformation on the Defect Structure in Ultrafine-Grained Nickel

Ultrafine-grained (UFG) high purity nickel samples produced by equichannel angular pressing were submitted to thermal treatment and cyclic plastic deformation at different temperatures in order to investigate the stability of the defect structure. Recrystallization was observed already after annealing above 425K. Cyclic plastic deformation at 300K and 425K leads to a coarsening of grains and to a dynamic recrystallization, respectively. Investigations performed by means of synchrotron radiation diffraction revealed that the mean volume expansion, long-range and short-range internal strains are diminished in consequence of the cyclic plastic deformation.