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, .     

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.     

Effective biaxial modulus and strain energy density of ideally (h k l)-fiber-textured cubic polycrystalline films

The effective biaxial modulus (M_eff) and strain energy density (W) of cubic polycrystalline films with ideally (h k l) fiber textures are estimated using Vook-Witt (VW) grain interaction model and the data are compared with those derived from Voigt, Reuss and Voigt-Reuss-Hill (VRH) models. Numerical results show that the VW average of M_eff for ideally (1 0 0)- or (1 1 1)-fiber-textured films is identical to the VRH average of M_eff. For (1 1 0) and (1 1 2) planes, however, the VW average of M_eff for (1 1 0)-fiber-textured film is larger than that of (1 1 2)-fiber-textured film when the Zener anisotropic factor (A_R) is not equal to 1. Furthermore, M_eff and W exhibit incremental tendencies with the increase of the orientation factor (Γ_h k l) for the [h k l] axis when A_R > 1, implying that M_eff and W have the minimums on the (1 0 0) plane. Reversely, M_eff and W decrease with the increasing Γ_h k l when A_R < 1. This means that M_eff and W on (1 1 1) plane have the minimums.     

Spiral and curved periodic crack patterns in sol-gel films

Thin silicate sol-gel films with four different crack patterns were created reproducibly by controlling the film deposition parameters. The crack geometry, periodicity, and amplitude were studied experimentally as a function of the film thickness, curing time, and temperature. Direct evidence was found that the physical interplay between stress relief through film cracking and stress relief through film warping results in sawtooth, spiral, closed loop, or straight line crack trajectories. Received: 25 March 2002 / Accepted: 1 July 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. Fax: +1-941-359-4396, Email: sendova@ncf.edu     

The morphologies of fractured surfaces and fracture toughness in some As-Se-Sb-S-I glasses

As part of a general physical characterization of amorphous materials in the pseudobinary system (As₂Se₃)_100−x(SbSI)_x type, their indentation fracture toughness was determined. It is a system with the variable ratio of classical amorphous compound As₂Se₃ and the molecule of antimony sulfoiodide, SbSI, which in the monocrystal form is characterized as ferroelectrics. Because of chalcogenides are generally very brittle and under load they crack very easily, these glasses have been studied with the aim of examining the possibility of obtaining some new structures on the basis of the materials with amorphous internal network, the structures that will have a higher quality in respect of mechanical properties. The morphologies of fractured surfaces were investigated by scanning electron microscope.     

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.     

Structure and mechanical properties of reactive sputtering CrSiN films

CrSiN films with various Si contents were deposited by reactive magnetron sputtering using the co-deposition of Cr and Si targets in the presence of the reactive gas mixture. Comparative studies on microstructure and mechanical properties between CrN and CrSiN films with various Si contents were carried out. The structure of the CrSiN films was found to change from crystalline to amorphous structure as the Si contents increase. Amorphous phase of Si₃N₄ compound was suggested to exist in the CrSiN film. The growth of films has been observed from continuous columnar structure, granular structure to glassy-like appearance morphology with the increase of silicon content. The film fracture changed from continuous columnar structure, granular structure to glassy-like appearance morphology with the increase of silicon content. Two hardness peaks of the films as function of Si contents have been discussed.     

Effect of texture dispersion on the effective biaxial modulus of cubic polycrystalline films

The effective biaxial modulus of fiber-textured cubic polycrystalline films with texture dispersion is estimated using Voigt-Reuss-Hill and Vook-Witt grain-interaction models. The orientation distribution function with Gaussian distributions of two Euler angles θ and ? is adopted to analyze the effect of texture dispersion degree on the effective biaxial modulus. Numerical results based on Cu and Cr fiber-textured films show that the distribution of angle ? has slight influence on the effective biaxial modulus of (0 0 1)-fiber-textured films if and only if the distribution of out-of-plane angle θ is narrow. Enhanced θ and ? distributions destroy the perfect fiber textures and as a result the films represent an evolution of ideal (h k l) fiber textures to nonideal or random textures with varying full widths at half maximums of θ and ?.     

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.     

Macroscopic properties of fractured porous media

The macroscopic properties of fractured porous media locally governed by a Laplace equation are determined by several methods. The first one consists in discretizing the porous medium and the fractures and in solving the Laplace equation in the discretized structure. The other methods consist in successive upscalings. The first upscaling replaces the porous medium by a continuum with a given transport property. The second upscaling replaces the fractures by surfaces with equivalent properties. The results of the various methods give very close results. They suggest a simple approximation which is successful when the properties of the fluid and of the continuous porous medium are not too different.     

Growth of textured nonstoichiometric MoTe2 films from Mo/Te layers and their use as precursor in the synthesis of MoTe2−xSx films

Molybdenum ditelluride, MoTe₂, thin films have been prepared on a glass substrate by depositing first a layer of Mo of 1 μm thickness by rf sputtering and then layers of tellurium of 4 μm thickness followed by annealing, in a vacuum sealed Pyrex tube, at 823 K for one hour. The films have been studied by X-ray diffraction, electron microprobe analysis, scanning electron microscopy and for optical and electrical properties. This indicated that the films so prepared are well crystallized and textured with a hexagonal structure similar to 2H-MoS₂. However, the film is non stoichiometric due to the presence of small amounts of oxygen. The optical properties are in good agreement with those of single crystals and film deposited by other means. The room temperature conductivity is of the same order of magnitude as that of single crystals. Finally, MoTe₂ has been used as a precursor for the growth of MoTe_2−xS_x films.     

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     

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.     

Influence of a magnetic field on deposition of diamond-like carbon films

Received: 17 February 1998/Accepted: 25 May 1998     

Enhanced adhesion of diamond-like carbon films with a composition-graded intermediate layer

Received: 15 September 1998 / Accepted: 3 November 1998     

Dependence of the strain energies on grain orientations in HCP metal films

A thin polycrystalline film bonded tightly to a thick substrate of different thermal expansion coefficients will experience thermal strain when the temperature is changed. Calculations of the strain energies for grains having various crystallographic orientations (h k l) relative to the film surface were made for a polycrystalline film composed of the close-packed hexagonal (HCP) metal Be, Cd, Co, Hf, Mg, Re, Ru, Sc, Ti, Y, Zr and Zn, respectively. From strain energy minimization, the (0 0 1), (0 1 3), (0 4 5), (1 2 3), (2 5 8), (0 5 7), (0 3 5), (0 5 7), (0 1 0), (5 5 8), (1 4 7) and (0 0 1) textures should be favorable in Be, Cd, Co, Hf, Mg, Re, Ru, Sc, Ti, Y, Zr and Zn film, respectively.     

Mechanical properties of sol-gel derived lead zirconate titanate thin films by nanoindentation

Lead zirconate titanate (PZT) thin films are deposited on platinized silicon substrate by sol-gel process. The crystal structure and surface morphology of PZT thin films are characterized by X-ray diffraction and atomic force microscopy. Depth-sensing nanoindentation system is used to measure mechanical characteristics of PZT thin films. X-ray diffraction analyses confirm the single-phase perovskite structures of all PZT thin films. Nanoindentation measurements reveal that the indentation modulus and hardness of PZT thin films are related with the grain size and crystalline orientation. The increases of the indentation modulus and hardness with grain size are observed, indicating the reverse Hall-Petch effect. Furthermore, the indentation modulus of (1 1 1)-oriented PZT thin film is higher than those of (1 0 0)- and random-oriented films. The consistency between experimental data and numerical results of the effective indentation moduli for fiber-textured PZT thin films using Voigt-Reuss-Hill model is obtained.     

Magnetoresistance and magnetostriction of Co2Cr0.6Fe0.4Al Heusler alloy

We report a study of the magnetotransport properties of the Co₂Cr_0.6Fe_0.4Al Heusler alloy grown by means of the arc-melting technique. X-ray diffraction and energy dispersive X-ray spectroscopy have been used to analyze the crystallographic structure and the sample stoichiometry. Temperature-dependent magnetization measurements have been carried out in the range 300-850 K. Co₂Cr_0.6Fe_0.4Al is theoretically predicted to have full positive spin polarization at the Fermi level, and as a consequence its spin-dependent transport properties are being intensively studied. Low field magnetoresistance exceeding 30% has been observed very recently in Co₂Cr_0.6Fe_0.4Al compact pellets. We have performed magnetoresistance and magnetostriction measurements in both the as-grown alloy and compact pellets made of mechanically milled Co₂Cr_0.6Fe_0.4Al. The as-grown and the milled sample show negligible anisotropic magnetostriction (25 μst at saturation), whilst only the milled sample exhibits magnetoresistance (0.65% at 300 K). These results permit us to discard the magnetostrictive effects as the magnetoresistance source.     

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