M. Niewczas

Deformation of copper single crystals to large strains at 4.2K I. Mechanical response and electrical resistivity

Abstract The deformation of Cu single crystals at 4.2 K was studied by simultaneous measurements of mechanical and electrical properties. The aim was to extend the study of low-temperature deformation to higher strains than are usually employed; thus most of the crystals were stretched to failure. The deformation can be divided into three distinct regions. In region A (which includes stages I and II of plastic deformation) the crystal deforms by slip. In region B there is twinning but no slip; at the end of region B the specimen is 70% twinned. The crystal enters region C as a fine layer structure consisting of twin and parent lamellae; in this region there is no more twinning and deformation proceeds by slip. Examination of the deformation-induced resistivity and resistivity annealing confirms the conclusion that flow stress in region A is due to dislocation accumulation and indicates that in region C other obstacles, such as twin-parent interfaces, are also important. The annealing data, together with other evidence, suggest that elimination of short vacancy dipoles and loops by pipe diffusion accounts for the recoverable component of the resistivity.

Work-hardening behaviour of Mg single crystals oriented for basal slip

Work-hardening behaviour of Mg single crystals oriented for basal slip was studied by means of tensile tests carried out at 4, 78 and 295 K. The crystals show critical resolved shear stress values (CRSS) for a {0001} basal slip system in the range 1–1.5 MPa. The samples exhibit two-stage work hardening characteristics consisting of a long easy glide stage and a stage of rapid hardening terminated by failure. The onset of the plastic flow up to the point of fracture is accompanied by a low work-hardening rate in the range 5 × 10−5–5 × 10−4 µ, corresponding to the hardening rate in Stage I of copper single crystals. The analysis of thermally activated glide parameters suggests that forest interactions are rate-controlling processes. The very low value of the activation distance found at 4 K, ∼0.047 b, is attributed to zero-point energy effects. The failure of crystals occurs well before their hardening capacity is exhausted by mechanisms which are characteristic of deformation temperature.

Low-temperature dynamic precipitation in a supersaturated Al± Zn± Mg alloy and related strain hardening

Abstract Dynamic precipitation during low-temperature deformation of a supersaturated Al-Zn-Mg alloy has been studied by small-angle X-ray scattering and in-situ resistivity in the temperature range 4.2–300 K. At 300 K, dynamic precipitation is observed both for the material strained in the form of a solid solution and for the material strained in the partially aged state, that is containing Guinier-Preston (GP) zones. In the first case, dynamic precipitation occurs by nucleation of precipitates 5 A in diameter, and in the second case it occurs by coarsening of existing precipitates. In all cases the dynamic precipitation results in an unusual-strain hardening behaviour in which the work-hardening rate is very high and essentially temperature independent.

Twinning nucleation in Cu-8 at.% Al single crystals

Abstract Transmission electron microscopy observations of the dislocation substructure in Cu-8at.% Al single crystals deformed in tension have been carried out to elucidate the origin of the twinning dislocation which produces conjugate twinning in this alloy. It is argued that the ‘correct’ Heidenreich-Shockley dislocation is obtained in the reaction of the primary dislocation with the faulted (Frank) dipole. These interactions are expected to occur frequently during the deformation. Different configurations of the twinning sources are analysed with respect to the nucleation and the growth of the twin. Geometrical issues related to the pinning of the component dislocations at the nodes formed, the motion of the single twinning dislocation and the non-coherent twinning front and the passing barriers encountered during twin growth are analysed. It is shown that there are no geometrical and energetic obstacles to the development of a macroscopic twin by the pole mechanism from the source proposed here.

Deformation of copper single crystals to large strains at 4.2 K II. Transmission electron microscopy observations of defect structure

Abstract Transmission electron microscopy (TEM) observations of the dislocation substructure developed in high-purity single crystals of Cu deformed at 4.2 K have been carried out in order to relate the detailed defect structures to the mechanical and electrical properties discussed in part I. The results based on weak-beam TEM show that the dislocation substructure contains a very high density of narrow dislocation dipoles of vacancy character. These dipoles become progressively refined in scale as deformation continues. In-situ annealing experiments carried out in the transmission electron microscope allow the stability of these structures against annealing at room temperature and elevated temperatures to be studied. The observations suggest that fine dislocation dipoles can be annealed by processes such as pipe diffusion and that these defects represent the recoverable component of electrical resistivity. For comparison some studies were undertaken in Cu-5 at.% Ni single crystals which indicate that the recoverable component of resistivity in this alloy is smaller owing to the influence of the Ni on the pipe diffusion process. In addition, TEM studies indicate the complexity of processes occurring at the twin-parent interfaces produced in Cu deformed at 4.2 K. These interfaces have an important role in debris storage and in processes that can occur after large plastic strains at 4.2 K.

Chapter 75 Dislocations and Twinning in Face Centred Cubic Crystals

Publisher Summary This chapter describes the dislocation substructure in highly strained copper and copper alloy crystals initially oriented for single glide and have analyzed the consequences of passage through this complex structure of a twinning front. The relation among various types of dislocations in the parent and the twinned crystals and the nature of new lattice defects resulting from twinning is analyzed. Dislocation transformations can be classified into five groups, depending upon the character of the parent defect and its transformation product. The first group includes transformation of dislocations which are glissile in the matrix but sessile in the twin. The second group includes those dislocations, which are sessile in the matrix but become glissile in the twin. The next group, glissile to glissile transformations, includes all secondary dislocations having Burgers vectors in planes K1 and K2, which are undistorted by passage of the twinning front. These dislocations are important in the development of the three-dimensional dislocation substructure in the twin lattice. Next are the sessile to sessile transformations represented by faulted dipoles and their products. These defects have Burgers vectors normal to the K1 and K2 planes; they may or may not inherit similar structures in the twin lattice.

Transmission electron microscopy observations of debris structure in deformed copper single crystals

Abstract The debris component of the dislocation substructure in copper single crystals deformed at 78 K was studied by weak-beam transmission electron microscopy. The following defects were identified: primary unfaulted dipoles, faulted dipoles, stacking-fault tetrahedra and defect clusters. It is shown that all these defects can be derived from primary dislocations. The distribution of dipole sizes as a function of flow stress suggests that they are refined by chopping as deformation proceeds; so just before fracture their lengths vary from 30 nm down to vanishingly small loops. The size distribution of stacking-fault tetrahedra shows a small dependence upon the degree of deformation and suggests that some equilibrium size of these defects is established in the presence of the other elements of the substructure. Small dot-like clusters of dimension below 2nm are most probably the remains of larger entities that have shrunk during warm-up to room temperature. Rearrangement of the structure, which occurs between 78 K and room temperature, may to a large degree explain the observed changes in electrical resistivity described in previous work by Niewczas et al.

Magnetic flux jumps in textured Bi 2 Sr 2 CaCu 2 O 8 + δ

Magnetic flux jumps in textured Bi 2 Sr 2 CaCu 2 O 8 + δ have been studied by means of magnetization measurements in the temperature range between 1.95 K and T c , in an external magnetic field up to 9 T. Flux jumps were found in the temperature range 1.95-6 K. with the external magnetic field parallel to the c axis of the investigated sample. The effect of sample history on magnetic flux jumping was studied and it was found to be well accounted for by the available theoretical models. The magnetic-field sweep rate strongly influences the flux jumping and this effect was interpreted in terms of the influence of both flux creep and the thermal environment of the sample. Strong flux creep was found in the temperature and magnetic-field range where flux jumps occur suggesting a relationship between the two. The heat exchange conditions between the sample and the experimental environment also influence the flux jumping behavior. Both these effects stabilize the sample against flux instabilities, and this stabilizing effect increases with decreasing magnetic-field sweep rate. Demagnetizing effects are also shown to have a significant influence on flux jumping.

The deformation of copper single crystals at 4.2 °K

Abstract The evolution of the defect structure in copper single crystals during tensile tests at 4.2 °K, was investigated by means of in situ measurements of resistivity, mechanical response and TEM observations. The results suggest that dipoles created during deformation represent an early annealable component of the overall resistivity. Twinning produces both a transformation of the dislocation structure and effectively converts the material into a composite. Final shear instability occurs on the twin-matrix interface.

Dislocation microstructures and surface morphology in fatigued fine-grained copper polycrystals

Transmission electron microscopy observations of the dislocation substructure in fine-grained copper polycrystals fatigued at room temperature were carried out to investigate the correlation between surface morphology and the underlying dislocation substructure. The results indicate that samples fatigued under constant plastic strain amplitude well into the saturation region develop a small-scale surface roughness. The volume fraction of persistent slip band (PSB)-type structures is extremely low. PSB structures formed by the dominant slip system are observed only within narrow twin grains and are associated with an extrusion-like surface profile. The results are interpreted in terms of the size effect that allows one dominant slip system to operate in the plane parallel to a coherent twin boundary.