Temperature-induced rearrangement of the dislocation pattern of Persistent Slip Bands in copper single crystals

Experimental investigations are reported on mechanisms by which dislocation arrangements of Persistent Slip Bands (PSBs) respond to changes of the deformation temperature. Copper single crystals orientated for single slip were cyclically deformed well into saturation at 300 K at an applied resolved plastic shear-strain amplitude, , such that the plastic strain became localized in PSBs. The spacings of the dislocation walls in these PSBs are about 1.4 m. After the temperature had been lowered to 77 K, cyclic deformation was continued with unchanged . A transformation of the dislocation pattern started. A certain fraction of the PSBs produced at 300 K finally showed a mean wall spacing of about 0.7 m which is typical for PSBs formed at 77 K. The remaining PSBs did not finish the transformation and became obviously inactive. In the state of cyclic saturation reattained at 77 K 50% of the PSBs, which had been formed at 300 K, show the dislocation pattern characteristic of 77 K. It is concluded that the amplitude of the resolved plastic shear strain localized in a PSB, , must be twice as large at 77 K as at 300 K. In an additional series of experiments crystals were cyclically deformed at constant temperatures of 430 K, 300 K, 190 K, and 77 K. In the temperature range covered by these experiments, the amplitude of the saturation flow stress, _S, appears to be proportional to the intrinsic amplitude of the 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.     

Annihilation of persistent slip bands and its effect on the fatigue life of copper single crystals

In single-slip-oriented copper single crystals, which were cyclically deformed under constant plastic shear strain amplitude control, a persistent slip band (PSB) was initiated on the persistent slip line (PSL) that first formed in the matrix vein dislocation structure. During intermediate temperature recovery (ITR) treatment in vacuum at 245 to 400°C, the variations in dislocation configurations and other defects of fatigued specimens were studied by using the scanning electron microscopy (SEM) electron channelling contrast (ECC) and the positron annihilation life (PAL) techniques. The results show that the ladders in the PSBs became curved and broken, or escaped locally until the PSBs disappeared completely during ITR treatment. The annihilation of the PSBs and other dislocation structures in the fatigued specimens prolonged the fatigue life to a certain extent.     

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     

Statistical theory of slip channels in neutron-irradiated metals

Accepted: 6 January 1997     

Statistical theory of slip channels in body-centered cubic metals

Received: 26 July 1996/Accepted: 7 October 1996     

Nucleation of persistent slip bands

The slip-unslip phenomenon has been used to examine the depth of penetration of persistent slip bands in a low-strain amplitude fatigue specimen. The conclusion is that the concentration of strain may well be nucleated in the bulk of the specimen.I would like to thank the Czechoslovak Academy of Sciences for their hospitality. The research described in this paper was done at McMaster University under the direction of I. D. Embury and R. K. Ham and was supported by the National Research Council of Canada.     

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.     

Improvement of Plasticity of Ti-Based Bulk Metallic Glasses by Addition of Nb

Ti-based bulk metallic glasses （Ti40Zr25Cu9NisBe18 ）100-x Nbx with x = 0 to 5 at. % are prepared by copper-mold casting. The glass formation ability is almost unchanged by addition of Nb. The compression plasticity is, however, apparently changed, from 3% at x = 0 to 13% at x = 3, about 330% increases at the strain rate of 1 × 10^-4 8^-1. The increment of the plasticity can be attributed to the segregation of Nb in the area of shear bands during the compression processing. An effective way to increase the plasticity of Ti-based bulk metallic glasses is thus proposed.     

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.     

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     

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     

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.     

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.     

The influence of shear bands on final structure and magnetic properties of 3% Si non-oriented silicon steel

The presence of shear bands in the deformed material before final annealing is very important for Goss and Cube textures formation in silicon steel [S.C. Paolinelli, M.A. Cunha, J. Magn. Magn. Mater. 255 (2003) pp. 379. [1]; J.T. Park, J.A. Szpunar, Acta Mater., 51 (2003) 3037. [2]]. The increase of the hot-band grain size can increase the number of shear bands, which favor the nucleation of these orientations. In this work, the effect of the hot band grain size variation, promoted by varying the hot rolling finishing temperature, on final structure and magnetic properties was investigated for 3% Si alloy. It was found that the increase of the hot-band grain size increases the occurrence of shear bands and promotes an increase of η fiber fraction and a reduction of γ fiber fraction, improving the magnetic induction. On the other hand, the final grain size is reduced when the hot-band grain size is larger than 190 μm, deteriorating the core loss values in spite of the texture benefits. The reduction of final grain size was explained by the increase of the number of nuclei at the beginning of the recrystallization caused by the increase of shear bands in the deformed material.     

The microstructure of local strain nuclei observed for Zr alloy in the stage of parabolic work hardening

The dislocation structure of the deforming Zr+1% Nb alloy in the stage of parabolic work hardening was examined by the technique of transmission electron microscopy (TEM). The faulted structure of the material is found to vary both qualitatively and quantitatively in the regions corresponding with the zones of local strain maxima and minima. The data on the density of different types of defect in the above zones have been analyzed. Received: 19 December 2000 / Accepted: 20 December 2000 / Published online: 23 March 2001     

The distribution of dislocations in slip bands of Fe-3% Si alloy single crystals deformed by bending

The distribution of dislocations at the ends of slip bands was studied by etching on surfaces parallel to the slip plane. In these places the slip band is formed by groups of asymmetric dislocation loops several hundred microns wide. The long mixed-type parts of these loops running nearly equidistantly and lying in near planes, are the equilibrium arrangement of dislocations of the same sign in the shear stress gradient. From the results we can judge that the dislocation sources are at larger distances from the ends of the slip bands and that the dislocation groups at the ends of the slip bands are sources of large stress fields.     

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.