The atkermal stress component in neutron irradiated mild steel

Abstract

The temperature dependence of the tensile lower yield stress of an annealed aluminium grain size controlled mild steel has been investigated in the range 23–250 °C and at a strain rate of 1.67 × 10^?4 sec^?l before and after neutron irradiation to 2.3 × 10¹⁸ n/cm² (fission). The yield stress of the irradiated steel decreases with increasing temperature due to thermal activation of the radiation damage and is predicted to reach asymptotically that of the unirradiated steel at ～285 °C; the maximum test temperature was below that at which thermal annealing of the damage occurs. This implies that the athermal stress component due to irradiation is zero and hence there is negligible long range interaction between dislocations and radiation-induced defects.     

Plastic deformation of decagonal Al73Ni10Co17 single quasicrystals

Uniaxial compression tests were performed on decagonal Al₇₃Ni₁₀Co₁₇ single quasicrystals at a constant strain rate of 10^?5?s^?1 in the temperature range between 800 and 890°C. The compression axis was chosen in three different orientations: parallel to, inclined by 45° to and perpendicular to the tenfold symmetry axis. During the deformation experiments stress relaxation tests and temperature changes were carried out to determine the strain-rate and temperature sensitivities of the flow stress. Highly anisotropic plastic behaviour is observed in both the flow stress and its strain and temperature dependences. Additionally, repeated stress relaxations were performed during the deformations, giving evidence that only minor microstructural changes take place during the relaxation tests.     

Molecular dynamic simulation of stress evolution analysis in Cu nanowire under ultra-high strain-rate simple tension

This study analyses the behaviour of atoms associated with the propagation of stress waves in Cu nanowires (NWs) during uniaxial tensile deformation using molecular dynamic simulation. Maximum local stress (MLS) and virial stress (VS) methods are adopted to express dynamic stress in ?100? Cu NWs under tension. Simulation results indicated that the VS method enhances the averaging effect at ultra-high strain rates (above 10¹⁰ s^?1), leading to serious undervaluation of yield stress. However, the MLS method provides superior prediction results for the dynamic mechanical responses of NWs under tension at the ultra-high strain rate than does the VS. At a strain rate of 7 × 10¹⁰ s^?1, the double-peak stress phenomenon was observed in the stress–strain curve using the MLS method. The response time (T_rs) to wave propagation, observed at an ultra-high strain rate, is responsible for the accumulation of the elastic stress that is applied at the beginning of tensile loading in a short period, producing the first stress peak. Following plastic deformation, the encounter of the wavefronts with the reduced tensile stress causes the fully constructive interference effect in the middle of the tensile NWs, producing the second stress peak. The results explain the dynamic mechanical behaviour of NWs, contributing to future applications of subsonic manufacturing.     

Shock-wave compression of x-cut quartz as determined by electrical response measurements

The mechanical response of x-cut quartz in the vicinity of the Hugoniot elastic limit is determined from measurements of the piezoelectric current from samples impact loaded from 26 to 130 kbar. The Hugoniot elastic limit is determined to be 60_?1·5⁺³ kbar at a compression of 0·066_?0·002^+0·004 This Hugoniot elastic limit corresponds to a shear strength of 5·5 per cent of the C₄₄ shear modulus. For stresses well above the Hugoniot elastic limit the electrical current measurements show that the material exhibits a substantial reduction of shear strength. The pressure derivative of the bulk modulus is determined to be 4·5, substantially less than the ultrasonic value. The experimental records show evidence for a time delay for reduction of shear strength which varies from about 10^?7 sec immediately above the 60 kbar Hugoniot elastic limit to about 10^?8 sec for stresses well above the Hugoniot elastic limit. The measurements also show stress relaxation below the Hugoniot elastic limit between 40 and 60 kbar.     

Structure and strength of Al-Si alloys obtained by the Stepanov method

Al-Si alloy samples with a silicon content from 8 to 15 wt % were obtained by the Stepanov method at solidification rates of 10² and 10³ µm s^?1. Tensile and bending strain diagrams were studied at a strain rate of about 10^?4 s^?1. The microstructure of the samples was investigated. It was found that the silicon content in the eutectic structure of the alloy grows as the solidification rate increases. The yield stress and the tensile strength increase as the silicon content grows.     

Diffusion von Protonen (Tritonen) in Eiskristallen

The diffusion coefficient of tritons in ice crystals has been measured at different temperatures between 0°C and ?35°C. The applied method excludes any surface diffusion. The absolute value of the diffusion coefficient at ?7°C was determined as 2·10^?11 cm²/sec (±10%) and the activation energy as 13,5 kcal/mole (±8%). The activation energy of proton (triton)-diffusion is therefore consistent with the activation energy found for the dielectric and mechanical relaxation of protons in ice. The importance of this consistency relative to the diffusion mechanism is discussed.     

Isotopeneffekt in der paraelastischen relaxation

The kinetics of the reorientation of the molecular impurities¹⁶O₂ ^? and¹⁸O₂ ^? in the host lattices KCl, KBr and KJ has been investigated by means of ESR as a function of uniaxial stress in the temperature range between 1,3°K and 4,5°K. Below 4°K the reorientation rate is proportional to the temperature, indicating that one-phonon processes dominate at low temperatures. ForT>4°K the reorientation through excited librational levels becomes significant. The large isotope effect in the reorientation rate of¹⁶O₂ ^? and¹⁸O₂ ^? confirms that tunneling plays an important role. The analogy between the diffusion of polarons and the reorientation of molecules in solids is discussed in detail. The evaluation and interpretation of the experimental results in terms of the theory of Gosar and Pire yields values for the tunneling matrix elements, the potential barrier between two equilibrium orientations, the libration frequencies and an effective moment of inertia. The theory accounts well for the contribution of the one-phonon processes to the relaxation rate for stress along [001]. For stress along [111] the agreement is only qualitative.     

Determination of the energy relaxation time in GaSb from microwave harmonic mixing and transport phenomena

The energy relaxation time τ_? of GaSb has been determined independently from microwave harmonic mixing experiments in the range of 160–200 K and from the current-voltage characteristics at 300 and 77 K. From measurements of the i.r. Faraday effect at 300 K valley populations as a function of the electric field strength are obtained which are in agreement with values calculated with help of the energy relaxation time obtained by methods mentioned before. For the evaluation of the data a two-band model has been used. The results obtained from the microwave experiments are extrapolated to 300 and 77 K assuming polar optical mode scattering. Good agreement of these values and the results obtained from the current-voltage characteristics and the Faraday measurements was found. The values for 300 and 77 K are 4 × 10⁻¹² sec and approximately 1·6 × 10⁻¹¹ sec, respectively.     

Intramolecular relaxation processes of some aromatic esters

Dielectric absorption measurements have been made in the frequency range 10²?10⁵ Hz over a variety of temperatures on dimethyl terephthalate, dimethyl 2,6-naphthalene dicarboxylate, methyl and ethyl benzoates, ethyl-p-chlorobenzoate, methyl and ethyl cinnamates, and methyl salicylate. The solutes have been dispersed in a polystyrene matrix and in four cases also in a polyphenyl ether medium. One compound, methyl salicylate, was also examined as a pure compressed solid. The Eyring parameters have been determined and employed to characterize the relaxation processes. The results are discussed in terms of molecular and intramolecular processes involving the whole ester grouping or the methoxy group or an overlap of both processes. In the case of methyl salicylate the barrier for the methoxy group relaxation has been determined in the compressed solid state.     

Messung der Verweilzeit von He-Atomen an Glasoberflächen

Mean sticking times of helium on a glass surface are determined at very low pressures from nonstationary molecular flow through glass capillaries. The temperature range covered is 13.8 °K to 20.4 °K. Resulting sticking times are of the order of 10^?7 to 10^?5 sec. They show a characteristic dependence on temperature and pressure. These measurements can be interpreted by means of a simple model: He-atoms mostly are bound to the surface with an adsorption energyE of 229 cal/mol?0.01 eV (±20%). However with a probability of 10^?4 the energy is 530 cal/mol?0.023 eV (±6%). In both cases sticking times τ follow the equation τ=τ₀exp(E/RT) where τ₀ is about 10^?9 to 10^?10 sec.     

Temporal evolution of exciton resonant secondary emission spectrum

The exciton secondary emission spectrum of CdS crystals has been studied at 2 K under $\text{λ =}\text{4765}\text{A}\text{?}\text{Ar}{}^{\mathrm{+}}$ laser excitation in the samples with different exciton lifetime. In these conditions three distinct components of secondary emission — Raman scattering, hot and equilibrium luminescence were spectrally resolved. It has been shown that the exciton lifetime acts like a very fast optical shutter and permits one to observe in stationary conditions the temporal evolution of the exciton secondary emission spectrum. The energy relaxation of excitons is accompanied by its “phase” relaxation, the measure of the latter being the polarization degree of emission. With the aid of the Hanle effect the acoustic phonon relaxation time 3.5 × 10^-12 sec has been estimated. The exciton lifetime has been found to vary from 3 × 10^-12 to 1.6 × 10^-9 sec in the samples studied.     

Implant redistribution in high-dose ion implanted and annealed silicon

Abstract

Diffusion constants of helium in gold, silver and aluminium are determined from thermal desorption experiments, giving:

Au: D ₀ = 10^?1.0 cm²/sec, ΔH = 1.70 eV

Ag: D ₀ = 10^?1.2 cm²/sec, ΔH = 1.50 eV

Al: D ₀ = 10^+0.1 cm²/sec, ΔH = 1.35 eV

The results are compared to self-diffusion and to the diffusion of other light elements in metals. Possible diffusion mechanisms are discussed.     

Optical pumping of ions in buffer gases

Transient signals measured with a pulsed rf-optical pumping method are used to determine longitudinal relaxation rates for Sr⁺ ions (even isotopes) in noble gas buffers. Depolarization cross sections of the electronic spin in the Sr⁺5² S _1/2 ground state for binary collisions with rare gas atoms are deduced. The results for σ(Sr⁺5² S _1/2) in Ų are (at temperatures between 374 and 449 °K): 2·10^?5(He),4·10^?5(Ne), 5.7·10^?3(Ar), 1.8·10^?2(Kr), and 4.0·10^?2(Xe). These cross sections for the Sr⁺ ion are about two to three orders of magnitude larger than the corresponding ones for the isoelectronic neutral Rb atom. The large increase of the Sr⁺ relaxation rates is explained with the relaxation mechanism of spin-orbit coupling, taking into account two “indirect” effects of the ionic charge: the increase in the gas kinetic cross sections and the more intimate collisions of the Sr⁺ ion with the noble gas atoms. The depolarization is shown to be predominantly due to short-range interactions. A contribution to the relaxation of the Sr⁺ ion from Sr⁺-noble gas molecule formation, induced by three-body or resonant two-body collisions, could not be established for applied pressuresp between 1.5 and 15 Torr of Ar, Kr, and Xe.     

Structural and physicomechanical properties of directionally crystallized aluminum-silicon alloys

Aluminum-silicon alloys (from 8 to 25 wt % Si) have been prepared by directional crystallization of shaped samples by the Stepanov growth at a solidification rate of 10³ μm s^?1. The dependences of the microhardness, Young’s modulus, internal friction, yield stress, and ultimate tensile stress of the alloys on the silicon content have been studied. It has been shown that the ultimate tensile stress has a maximum, and the yield stress has a kink at 15 wt % Si; the composition corresponds to the eutectic composition at the solidification rate used. The silicon content in the eutectics increases with an increase in the solidification rate. The increase in the ultimate tensile stress is explained by an increase in the volume fraction of the more strength fine-crystalline structure of the eutectics as a result of the decrease in the volume fraction of more plastic dendrites of the primary crystals of the α-Al solid solution. The decrease in the ultimate tensile stress of the hypereutectic alloy is determined by the increase in the volume fraction of brittle primary silicon crystals of various shapes.     

Microwave absorption of alkylammonium carboxylate surfactants and their micelles in benzene solution

The microwave absorption of a series of solutions of n-octylammonium carboxylate surfactants [CH₃(CH₂)₇⁺NH^?₃O₂C(CH₂)_n CH₃; n=12, 14, 16 and 18] and n-octylammonium formate in benzene at concentrations below, at, and above the c.m.c. was studied at frequencies of 9.314, 16.2, 23.98, and 35.11 GHz, and in a few cases in the range 0.48 to 1.9 GHz, at 298 K. A wide distribution of relaxation times about a mean value is indicated which can be analyzed between two limiting values. The relaxation parameters display concentration dependence and reflect the relaxation of segments containing the ⁺NH₃^?O₂C group (τ_O? 10 ps) and probably a molecular process as the dominant relaxation modes in the unassociated ion—pairs. Above the c.m.c. similar intramolecular processes occur together with relaxation of the micelles. Information has been obtained on the c.m.c. values, and the microwave absorption and relaxation parameters have been related to the formation of reversed micelle types of aggregates.     

Das kritische magnetische Verhalten von EuO

Mössbauer effect and magnetization measurements were employed in order to study the static and especially the dynamic magnetic properties of the nearly Heisenberg ferromagnet EuO near its Curie temperature,T _c=69.2 °K. The critical exponent β of the spontaneous magnetization was determined to be β=0.34±0.02. It was shown that critical slowing down of spin fluctuations takes place nearT _c with spin relaxation times between 7×10^?11 sec (T=1.01T _c) and 1.5×10^?1 sec (T=1.03T _c). The experimental values of the relaxation time were found to be in satisfactory agreement with theoretically computed ones. Just belowT _c the Mössbauer spectra exhibit relaxation effects, which are characteristic for the occurence of critical super-paramagnetism. Investigations of several samples indicated quantitatively, that critical superparamagnetism has its origin in the non ideal composition of the real crystal.     

Stress relaxation in cadmium-silver alloy single crystals

A series of cadmium — silver alloy single crystals containing up to 0·25 at % Ag were deformed in tension at a strain rate of 1×10^?4s^?1. The tensile tests were carried out at temperatures between 77 and 199 K. Stress relaxation experiments were performed to investigate the concentration dependence of the activation volume. Over the temperature range investigated, the activation volume at beginning of deformation decreases proportionally toc ^?2/3 wherec is the atomic concentration of silver as solute. The results are discussed on the basis of the interaction between dislocations and solute atoms.     

The decay of65Ge

Energies and relative intensities of γ-rays from the decay of⁶⁵Ge have been measured. The half-life was measured to be (31.2±0.7) sec. From the measured intensity of the annihilition radiation the intensity of the groundstate ß⁺ feeding was determined to be <10%. A total of 31 γ transitions was placed in a level scheme. The spin and parity of the groundstate of⁶⁵Ge was determined to be 3/2^? (5/2^?).     

Beta-delayed proton emission from 97Cd and 99Cd

Two new β-delayed proton precursors, ^97,99Cd, have been identified at the ISOLDE on-line isotope separator. The proton branching ratio for ⁹⁹Cd is (1.7^+1.1_?0.5) × 10^?3. The ⁹⁹Cd proton spectrum is discussed in terms of the statistical model with Porter-Thomas fluctuations. The half-life of ⁹⁹Cd was determined to be 16±3 sec. A search for ⁹⁸Cd was also performed and it was found to be a pure β-emitter with a probable half-life of ≈ 8 sec.     

Cavitation in water under tension near the freezing point

Experiments are reported on cavitation in water at an initial temperature of 0.7°C under the dynamic tension created when a compression wave interacts with a free liquid surface. It is found that the tensile strength of water increases from 20 to 50 MPa as the strain rate is varied from 1.8 × 10⁴ to 5.2 × 10⁴ s^?1. It is shown that the phase state of water obtained in experiments is in a double metastable region.