The evolution of persistent slip bands in copper single crystals

The transformation of the so-called matrix structure into persistent slip bands (PSBs) during the fatigue of copper single crystals has been investigated by transmission electron microscopy (TEM). By cyclic pre-deformation a saturated, hard matrix structure was established which is not capable of further hardening. A sudden increase of the applied amplitude of the resolved plastic shear strain initiated the transformation of the matrix structure into PSBs. The number of deformation cycles with enlarged amplitude of resolved plastic shear strain was increased from experiment to experiment in order to obtain crystals with PSBs in consecutive stages of evolution. Surface observations indicated strain localization well before first fragments of the typical ladder-like dislocation pattern of PSBs could be identified in the bulk. From our experiments, we conclude that the transformation from the matrix structure into PSBs very likely starts from the centers of the veins which exhibit small dislocation-poor, soft areas. These areas are enclosed by a harder shell, where a high dislocation density is maintained and which may develop into first dislocation walls. During the evolution of PSBs the frequency distribution of the wall spacings narrows. This indicates that a shift of dislocation walls (1–2 nm/cycle) plays an important rôle in establishing the typical regular ladder-like dislocation pattern of well-developed PSBs.     

Electron Backscattered Diffraction of Transgranular Crack Propagation in Soft Steel

We consider the crack propagation in a soft steel sheet during the formation. The drawability is considered in relation with the structural anisotropy, the mechanical behaviour is related to both the grain morphology and the texture. The structure heterogeneity could lead to the apparition of micro-cracks. The results show the texture effect on the crack propagation and on the crack arrest in soft steel during the formation. The EBSD technique allows to show that the adjustment of the grain orientation from the initial main component {111}（112） towards the deformation orientation {111}（110） incites a trans-granular crack through a grain with initial {111}（112） orientation in a globally ductile material. It is the presence of grains with {111}（110） orientation which permits the closing of micro-cracks.     

Estimating the sizes of surface cracks based on Hall element measurements of the leakage magnetic field and a dipole model of a crack

A method is proposed for estimating the sizes of surface cracks in magnetic materials. The method is based on applying a magnetic field, then determining the leakage magnetic field in the vicinity of a crack by moving a Hall element on the surface of the material along one or two scanning lines crossing the crack, and measuring the corresponding Hall voltage distribution. A dipole model of a crack is utilized, in which a surface crack is considered as being full of magnetic dipoles aligned parallel to the applied field, and whose density varies linearly along the depth of the crack. Analytical expressions are derived for the z-component of the intensity of the leakage magnetic field, and for the measured Hall voltage in the vicinity of a crack with an arbitrary cross-section along its long axis when it is perpendicular to the applied field. The crack sizes and the parameters of the distribution of magnetic dipoles along the crack depth are computed by crack inversion, which represents a regression for the Hall voltage distribution. A variable theoretical Hall voltage distribution is fitted to the measured Hall voltage distribution by minimizing the corresponding RMS error, which gives the unknown parameters at the end of the minimization. Hall voltage distributions are measured on ferromagnetic steel samples containing one artificial surface crack. Some crack inversions are performed for estimating the maximum crack depth and the crack width of cracks with rectangular and isosceles triangular cross-sections along the long crack axis. The accuracy of these crack inversions increases by utilizing either Hall voltage distributions measured along only one of the scanning lines, instead of along both scanning lines, or by using more precisely measured Hall voltage distributions. The fast and accurate estimation of the maximum crack depth and the crack width by such crack inversions could be important for pipeline inspection. Other crack inversions are performed for determining the cross-section along the long axis of the investigated cracks with satisfactory results. Received: 2 March 2001 / Accepted: 10 April 2001 / Published online: 25 July 2001     

On the foundations of stochastic dislocation dynamics

A stochastic approach to dislocation dynamics is proposed that starts off from considering the geometrically necessary fluctuations of the local stress and strain rate caused by long-range dislocation interactions during plastic flow. On a mesoscopic scale, a crystal undergoing plastic deformation is thus considered an effective fluctuating medium. The auto- and cross-correlation functions of the effective stress and the plastic strain rate are derived. The influences of dislocation multiplication, storage and cross slip on the correlation functions are discussed. Various analogies and fundamental differences to the statistical mechanics of thermodynamic equilibrium are outlined. Application of the theory of noise-induced transitions to dislocation dynamics gives new insight into the physical origin of the spontaneous formation of dislocation structures during plastic deformation. The results demonstrate the importance of the strain-rate sensitivity in dislocation patterning.     

Spatio-temporal aspects of low-temperature thermomechanical instabilities: A model based on dislocation dynamics

The occurrence of plastic instabilities which are accompanied by a significant heat release is a typical feature of the plastic behaviour of metals deformed at sufficiently low temperature. This phenomenon may be studied within the framework of a dislocation-dynamical model. The influence of the heat which is released by the deformation process on the dislocation velocity, and thus on the deformation dynamics, is taken into account. In particular, the influence of the spatial coupling which arises from heat conduction on the spatio-temporal behaviour of the deformation process is studied.     

Magnetic studies of the dislocation structure of iron single crystals deformed at 195K and 77K

Measurements of the coercive field, the initial susceptibility and the reversible susceptibility in the approach to ferromagnetic saturation show that during low-temperature deformation of iron single crystals mainly screw dislocations are created. Long-range internal stresses are found to be significantly smaller than in crystals deformed at room temperature. Macroscopic slip occurs on several slip systems. In the parabolic region of the work-hardening curves at 195 K the relation is valid, where τ isthe shear stress andN is the dislocation density. In the region of saturation of the shear stress the dislocation density further increases. After room-temperature prestrain the relation appears to hold for 77K-deformation also. Exhaustion hardening of edge dislocation is found at the beginning of the low-temperature deformation.     

AFM tip-induced ripple pattern on AIII-BV semiconductor surfaces

Modification of c(8x2) InSb(0 0 1) surface induced by prolonged scanning with an atomic force microscope tip has been investigated. The experiment performed with loads of few tens of nanoNewtons resulted in creation of ripples perpendicular to the fast scan direction. It was found that terrace edges are acting as initial instabilities leading to development of the ripple pattern. As a result, information about initial surface topography is preserved in the ripple amplitude, even so the final height of the ripples and their periodicity are determined by the tip curvature.     

In-situ NMR study of dislocation motion in Ca-doped NaCl crystals

Using a fast multi-window NMR technique, we have measured in-situ the mean jump width x of mobile dislocations during plastic deformation in a series of NaCl single crystals with varying Ca⁺⁺ content. Aside from immobile forest dislocations, the Ca⁺⁺ impurities form additional obstacles for the moving dislocations thus lowering x. We found that the Ca⁺⁺-related obstacles exhibit a pronounced non-random distribution which results in a corresponding broad distribution of x. We show that the data can be evaluated by means of an appropriate distribution function g(1/x) with an uncommon dependence of the observed fitting parameters on the Ca⁺⁺ content. As expected, quenching of a sample leads to a more uniform distribution of the Ca⁺⁺-related obstacles resulting in a corresponding narrowing of g(1/x).     

Dislocation dynamics in cyclic plastic deformation

The paper presents a theoretical investigation of the slip avalanches (so-called strain bursts) which occur in single-glide-orientated face-centered cubic or hexagonal close-packed metals during stress-amplitude-controlled cyclic plastic deformation. The study is based on a model of the dynamics of dislocations that has been developed in a companion paper (Part I). It is shown that this model allows for a quantitative treatment of the strain-burst phenomenon. In particular, the scaling relations between different strain-burst-characteristic parameters which have been found by experiment are connected to the evolution of the dislocation microstructure and thus find a natural explanation.     

Spectroscopic investigation of the physicochemical origin of the spontaneous delamination of the sputtered amorphous carbon nitride films

We present in this study a spectroscopic investigation of the delamination of the amorphous carbon nitride (a-CN_x) films deposited by RF magnetron sputtering of a graphite target in Ar/N₂ gas mixture. The microstructure of the studied films have been analysed prior and after their delamination. The origin of the observed spontaneous delamination have been elucidated in terms of chemical reactions between water and CN bonds at the a-CN_x/Si interface, which support delamination crack advance.     

Defect structure in EuS single crystals grown from the melt

The defect structure in EuS single crystals grown form the melt is studied by etch pitting, scanning and high-voltage electron microscopy. Circular and square etch pits and a second phase in the shape of thin hexagonal platelets are observed by etching. Microprobe analysis indicates the platelets to consist of Eu metal. In the transmission electron microscope, smoothly curved dislocations and helical dislocations, small dislocation loops and inclusions associated with dislocations are observed. The possible origin of the detected dislocation structure is considered with reference to climb and glide processes occurring during cooling down the grown crystals. The results corroborate the glide geometry of the NaCl lattice for EuS. On leave from Institute of Physics, Academic Sinica, Peking, VR China     

Temporal dissipative structures in cyclically deformed metallic alloys

cyclically deformed metallic alloys. The model employs quasi-chemical reactions of multiplication, annihilation and positive feedback among the populations of mobile, immobile, and Cottrell-type dislocations [1]. Three major types of loading have been simulated, namely, pure sinusoidal, “creep fatigue”, and ramp loading. Computer movies of the temporal evolution of stress serrations and dislocation densities have been produced as an aide for analysis and illustration. It has been demonstrated that the model successfully reproduces strain bursts and stress serrations in fatigued metallic alloys in terms of the underlying dislocations mechanisms, thus establishing the fundamental connection between micro- and macromechanics of cyclic deformation. Received: 20 June 1996/Accepted: 6 October 1996     

Crystal quality boosts responsiveness of magnetic shape memory single crystals

Magnetic shape memory alloys are promising materials to replace giant magnetostrictive materials and piezoelectrical ceramics in actuating devices due to the large magnetically induced strains. Ni-Mn-Ga is the most intense studied system due to its relatively high operational temperatures and the huge magnetically induced strains reported. Up to now the application of these materials is still limited by the operational temperature range. Additionally twin boundary mobility suffers from structural defects increasing the magnetic fields needed for significant and reproducible strains. The sample quality is affected by crystal inhomogeneity, porosity and impurities. Here new results are reported for the Ni-Mn-Ga class based on a set of single crystals grown by the SLARE method, recently developed by Mecklenburg et al. Single crystalline samples of Ni_49.7Mn_29.3Ga₂₁ of tetragonal martensitic structure exhibit a magnetic field induced strain of more than 4% below 170 mT and 6.5% at only 340 mT. Furthermore the operational temperature regimen could be expanded up to 65 °C.     

Improved magnetic properties and fracture strength of NdFeB by dehydrogenation

Effects of the dehydrogenation of the hydrogen decrepitated (HD) powders on the magnetic properties and the fracture strength of sintered NdFeB magnets were studied. It was found that the lattice parameters and the crystal phase of NdFeB changed significantly with the various hydrogen contents of the resultant HD powders due to the different degrees of dehydrogenation. The magnetic properties and fracture strength increased with decreasing hydrogen content, reaching the maximum increases of 200% for both intrinsic coercivity and bending strength, which can be ascribed to the improved microstructure of the sintered NdFeB magnets. The hydrogen remaining in the HD powders diffused out and affected drastically the grain and grain boundaries by the hydrogen out-take channel during the subsequent sintering process.     

X-ray topography study of the cleaved Si-(111)-face

Cleaved silicon (111) faces were studied with reflection x-ray topography using a high resolving double-crystal-diffractometer. Three types of defects due to cleavage process could be observed: long-range strain due to overlap cracks along macroscopic cleavage steps, a homogeneous decrease of intensity within the inclined portion of step mainly due to optical refraction of x-rays and point-like defects along some macroscopic cleavage steps, which develop only after heat treatment. The findings are important with respect to interpretation of electrical measurements, since all types of defects may influence the surfaces state density decisively.     

An experimental test of the critical behaviour of fracture precursors

The statistical properties and the localization of fracture precursors on heteregeneous materials is studied by recording their acoustic emission as a function of the applied load. It's found that the microcrack cluster together as the load increases and the instantaneous acoustic energy has an invariant power law distribution. The integrated acoustic energy presents a critical divergency close to the failure load for the sample. These result support the idea that fracture can be viewed as a critical phenomenon. Finally a measure of the concentration of microcraks, which allows us to predict the failure load, is introduced. These properties are studied also when a periodic load is applied to the sample. It's found that the Kaiser effect is not strictly satisfied in heteregeneous materials. Received: 23 January 1998 / Revised: 20 April 1998 / Accepted: 17 June 1998