Strain softening and strain localization in copper based Cu-Al single crystals

The paper reports the results of the study of highly effective relaxation mechanisms of plastic deformation in Cu-Al single crystals and their effect on mechanical properties and tendency toward strain localization. The experiments concern a wide range of aluminium content (0–16 at. % Al) in copper based single crystals, tested in tension. These were oriented in the centre of stereographic triangle. Slip markings, changes of crystal shape, strain hardening rate and strain rate sensitivity provided independent information permitting to identify the mechanism of relaxation and to determine its kinetic features. It was found that selfinduced entry of critical slip system causes sudden decrease of hardening rate, changes the strain rate sensitivity and leads to strain localization. The effect occurs regardless the crystal composition but its extent is dependent on Al content. The model of the relaxation mechanism has been proposed.Presented at the 4^th International Symposium on Plasticity of Metals and Alloys, September 7–11, 1987, Srní, Czechoslovakia.     

Temperature dependence of the yield stress of metallic nano-sized crystals

It is shown that the temperature effect on the variance of local shear stresses is the main factor pre-determining the temperature law of the yield stress of nano-sized crystals. The results of molecular dynamics simulations of uniaxial tension of Mo, α-Fe and W nanowires in three crystallographic directions ([100], [110] and [111]) over the temperature range 100–1000 K are presented. It is found that within this temperature range, the yield stress of nano-sized crystals varies not exponentially, as for bulk single crystals, but is a parabolic function of temperature.     

Anisotropy and temperature dependence of the spin-wave attenuation parameter in ferrite single crystals

Results are presented for ferromagnetic resonance in Mg-Mn and Li ferrites. The anisotropic part H_k of the spin-wave line width is defined as the difference between the spin-wave widths H_k as measured on a spherical specimen magnetized along [111] and [100] by parametric excitation of the spin waves by a longitudinal UHF magnetic field. The results are compared with the theory.Read at the All-Union Conference on Ferrites, Leningrad, 25 October 1963.     

Uniaxial stress dependence of the dielectric properties of barium titanate single crystals

BaTiO₃ single crystals were grown by the melt-grown method. The effect of uniaxial pressure (0–1700 bar) on the dielectric properties of these crystals has been systematically studied. The external stress showed obvious effects on these properties. An increase in the difference between the Curie T_c and Curie–Weiss T₀ temperatures induced by the applied pressure is observed. This could be ascribed to the inducing of non-ferroelectric cubic islands in the tetragonal phase by the applied compressive stress. On the other hand, the pressure behavior of thermal hysteresis and the ??/?T vs. T plot strongly suggests that the phase transition changes to second-order type with increasing pressure. The Curie–Weiss constant obtained from a modified Curie–Weiss law strongly decreases with increasing pressure, suggesting that the mechanism of phase transition is going to order–disorder type. An increase in the difference between the Curie T_c and Burn's T_B temperatures with increasing pressure is observed. This could be ascribed to the narrowing of the temperature range on which the Curie–Weiss law is valid. In general, the obtained results are in good agreement with hydrostatic pressure data. Some kind of relaxation near T_c, which is strongly coupled with strain caused by applied compressive stress, is postulated.     

The temperature dependence of dislocation charge in calcium doped single crystals of potassium chloride

Abstract

Using the ultrasonic piezoelectric composite oscillator technique we have measured the temperature dependence of dislocation charge in bent KCl single crystals as a function of Ca impurity concentration. In each case the charge changes sign at an extrinsic isoelectric temperature, T_e , and the shift in Te with impurity concentration allows the calculation of the individual thermodynamic vacancy formation parameters. We obtain h₊ = 1.244±0.014 eV, s₊ = 4.50±0.19 k, h- = 1.346? 0.014 eV and s- = 5.11?0.19K.     

Strain aging in rock salt single crystals

Aging of plastically deformed rock salt single crystals resulted in an increased room temperature yield stress. This effect, which was found to be transient, was investigated as a function of aging time and temperature, prestrain and critical shear stress. The results suggest an interpretation in terms of Cottrell's theory of strain aging, the dislocation obstacles being represented by impurities, vacancies and their agglomerations. An estimation of the possible jump frequencies of the effective lattice defects from aging experiments yielded migration energies between 0.7 and 1.1eV.     

Orientation dependence of the incoherent radiation yield in crystals

The yield of incoherent bremsstrahlung due to 1.2-GeV ultrarelativistic electrons moving near atomic chains in crystals is investigated by numerical simulation. It is shown that the electron distribution over the number of close collisions with crystal atoms deviates significantly from the Poisson distribution. The simulation results are compared with experimental data.     

Orientation dependence of relativistic-positron annihilation in single crystals

An effect of the orientation dependence of the cross section for the single-photon annihilation of relativistic positrons with atomic electrons in a crystal is predicted. It is shown that the probability for the single-photon annihilation of a channeled positron in a crystal may be either suppressed in a crystal in relation to a homogeneous medium or, on the contrary, enhanced. The reason is that, depending on their incidence angle, the positrons may be either in the vicinity of ion planes of the crystal, where the electron density is higher, or far away from them, where the electron density is lower.     

Effect of heat transfer mode on the temperature and thermal stress fields in single crystals

Different modes of conjugate heat transfer in a crystal environment–growth chamber walls system with geometry similar to the upper part of a Czochralski thermal unit are investigated numerically. Calculations are performed for an argon Prandtl number of 0.68 and a Grashof number of 16000. The thermal stress fields in the crystal are calculated using the resulting temperature fields.     

The temperature dependence of the edge emission of GaSe single crystals at high excitation levels

The temperature dependence of five edge emission lines of GaSe at high excitation levels has been investigated by using the second harmonic of a neodymium-glass laser. The following lines were indentified at 77.3K:hv_B = 2.102 ± 0.002 eV, annihilation of free excitons; hv_C = 2.082 ± 0.003 eV, Auger recombination of free excitons; hv_D = 2.072 ± 0.003 eV and hv_E = 2.055 ± 0.004 eV, annihilation of free excitons with emission of optical phonons (LO₁ = 31 meVandLO₂ = 45 meV, respectively); hv_G = 2.036 ± 0.004 eV, radiative recombination at indirect transition with emission of LO₁ phonons.     

Shear localization in dynamic deformation of copper single crystals

Dynamic deformation of copper single crystals, especially of fatigued copper single crystals with different orientations, was conducted on a split-Hopkinson pressure bar apparatus. The strain rates were in the range 2???9?×?10³?s^?1. After dynamic deformation, the adiabatic shear bands (ASBs) were examined in a light microscope and SEM. The width and spacing of ASBs formed under different strain rates in a fatigued copper single crystal were measured and the spacing of ASBs is one-order of magnitude smaller than the theoretical predictions. The possible reasons for the discrepancy were discussed. The critical strains for the ASB formation in four different orientated single crystals at stain rate of about 4?×?10³?s^?1 were determined by examining the post-deformation specimens and dynamic stress–strain curves. It is clearly indicated that the critical strains for the ASB formation are orientation-dependent in copper single crystals. A simple microscopic mechanism for ASB formation in fatigued single crystals was proposed.     

Calculation of thermal stresses in profiled single crystals taking into account the temperature dependence of thermal expansion

Approximate expressions for stress tensor components in cylindrical and ribbon crystals are obtained taking into account the temperature dependence of the thermal expansion coefficient. These expressions are used for estimating the effect of the thermal expansion nonlinearity factor on the stress level. It is shown that the stresses emerging in linear temperature fields due to this factor are comparable with critical stresses for defect formation.     

Concentration dependence of the critical resolved shear stress of cadmium-silver alloy single crystals at low temperatures

The increase of the critical resolved shear stress of cadmium single crystals by addition of silver has been investigated in the temperature range of 77 K to 199 K. At all temperatures the critical resolved shear stress increases withc ^2/3, wherec is the atomic concentration of silver as solute, and it decreases with increasing temperature. The concentration dependence of the critical resolved shear stress is explained according to the theory ofLabusch (phys. stat. sol.41 (1970), 659).     

Crystal orientation dependence of ferromagnetism in Fe-implanted MgO single crystals

Single crystals of MgO with (100), (110) and (111) orientations were implanted with 64 keV Fe ions at a dose of 1.9×10¹⁷ ions/cm² by using metal-vapor vacuum arc ion source (MEVVA). The magnetic properties were investigated by a superconducting quantum interference device magnetometer and Rutherford backscattering spectrometry (RBS) was used to analyze the Fe concentration and distribution. The presence of Fe nanoparticles in MgO matrix was verified by magnetization measurements. Results show that all the samples behave with ferromagnetism at 5 K and 300 K, and the coercive field, H_C, follow well the relation: at measured temperatures. The orientation dependence of the coercive field may result from the different distribution of Fe nanoparticle size.     

Orientation dependence in molecular dynamics simulations of shocked single crystals

We use multimillion-atom molecular dynamics simulations to study shock wave propagation in fcc crystals. As shown recently, shock waves along the <100> direction form intersecting stacking faults by slippage along 111 close-packed planes at sufficiently high shock strengths. We find even more interesting behavior of shocks propagating in other low-index directions: for the <111> case, an elastic precursor separates the shock front from the slipped (plastic) region. Shock waves along the <110> direction generate a leading solitary wave train, followed (at sufficiently high shock speeds) by an elastic precursor, and then a region of complex plastic deformation.