Mechanisms of high-speed deformation of polycrystalline copper

The mechanisms of high-speed deformation of ultrafine-grained copper produced during severe plastic deformation by equal-channel angular pressing were analyzed using numerical modeling in comparison with those in the case of coarse-crystalline copper. The activity of annihilation processes during nonconservative motion and double cross slip of dislocations was estimated. Their effect on the macroscopic behavior of samples is shown.     

Change of electrical resistivity of polycrystalline copper during tensile deformation

The change of electrical resistivity of polycrystalline copper of various purities was measured during tensile deformation. The purity of material did not substantially influence the results. With the use of intermediate annealing, the contributions of point defects and dislocations to the total resistivity increment were separated. The linear relationship between the stress and square root of the dislocation density was found. The Saada's relation was checked and found to be valid only in the case of point defects produced in originally annealed copper.ikova 22, Brno, Czechoslovakia.     

Specific features of helium penetration into single-crystal and nanocrystalline copper under deformation in liquid helium

This paper reports on experimental data on the penetration of helium atoms into single-crystal and nanocrystalline copper samples subjected to tensile and compressive strains at T=4.2 K, respectively. The dependences of the helium concentration N in the samples on the strain ? and the curves of helium extraction in the temperature range 300–1000 K at different strains ? are determined. It is found that the dependences N(?) and σ(?) correlate qualitatively with each other for single-crystal copper and do not correlate for nanocrystalline copper. This is associated with the different mechanisms of deformation in these samples. The deformation proceeds through the dislocation mechanism in single-crystal copper and through the jumpwise (twinning, rotational) mechanisms in nanocrystalline copper during local heating in regions of plastic shears. These factors are also responsible for the considerable difference between the curves of helium extraction from samples of both types. The curves of helium extraction exhibit two maxima for single-crystal copper and five maxima for nanocrystalline copper samples. The results obtained are discussed in terms of both the dynamic dislocation pipe diffusion and grain-boundary mechanisms of particle penetration from the surrounding medium into copper through different-type moving defects under applied stresses and due to the gradient of the chemical potential at the metal-surrounding medium interface.     

Spreading resistance microscopy of polycrystalline and single-crystal ferroelectric films

Thin films based on lead zirconate titanate with stoichiometric composition near the morphotropic boundary have been studied using atomic-force microscopy methods. The dependence of the local conductivity on the local polarization direction has been observed for all samples, independently of substrate type, deposition method, and film thickness. It has been shown that the current response to the applied voltage exhibits a long current relaxation, about several tens of seconds, which is two to three orders of magnitude greater than the current relaxation time in an external circuit, associated with the ferroelectric domain switching. The conductivity features have been explained by recharging of traps localized at ferroelectric grain boundaries near electrodes and involved in polarization charge screening.     

Kinetics and mechanism of copper fracture during deformation in surface-active baths

Relations have been derived here between the macroscopic characteristics of liquid metal embrittlement (durability _c under creep and strain _c prior to rupture under tension) and the parameters which characterize the micromechanism of fracture (surface energy at the crystal bath interface, energy of grain boundaries, temperature, structure of the crystal-bath interface, etc.), on the basis of test data indicating that the subcritical stage of microcrack development governs the fracture process, and on the assumption that transition to supercritical fracture occurs when the crack angle at the tip opens to its critical width _c. It is also shown here that, as the rate of subcritical crack development changes by three orders of magnitude, the magnitude of the critical angle _c changes only by a factor of 3.0 and may, to the first approximation, be regarded as independent of the bath composition. The values of _c and _c calculated according to this approximation agree closely enough with values based on tests.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 22–29, July, 1976.     

Shear modulus of shock-compressed LY12 aluminium up to melting point

Asymmetric plate impact experiments are conducted on LY12 aluminium alloy in a pressure range of 85-131 GPa. The longitudinal sound speeds axe obtained from the time-resolved particle speed profiles of the specimen measured with Velocity Interferometer System for Any Reflector （VISAR） technique, and they are shown to be good agreement with our previously reported data of this alloy in a pressure range of 20-70 GPa, and also with those of 2024 aluminium reported by McQueen. Using all of the longitudinal speeds and the corresponding bulk speeds calculated from the Gruneisen equation of state （EOS）, shear moduli of LY12 aluminium alloy are obtained. A comparison of the shear moduli in the solid phase region with those estimated from the Steinberg model demonstrate that the latter are systematically lower than the measurements. By re-analysing the pressure effect on the shear modulus, a modified equation is proposed, in which the pressure term of P/η^1/3 in the Steinberg model is replaced by a linear term. Good agreement between experiments and the modified equation is obtained, which implies that the shear modulus of LY12 aluminium varies linearly both with pressure and with temperature throughout the whole solid phase region. On the other hand, shear modulus of aluminium in a solid-liquid mixed phrase region decreases gradually and smoothly, a feature that is very different from the drastic dropping at the melting point under static conditions.     

Shear modulus of shock-compressed LY12 aluminium up to melting point

Asymmetric plate impact experiments are conducted on LY12 aluminium alloy in a pressure range of 85--131\,GPa. The longitudinal sound speeds are obtained from the time-resolved particle speed profiles of the specimen measured with Velocity Interferometer System for Any Reflector (VISAR) technique, and they are shown to be good agreement with our previously reported data of this alloy in a pressure range of 20--70\,GPa, and also with those of 2024 aluminium reported by McQueen. Using all of the longitudinal speeds and the corresponding bulk speeds calculated from the Gruneisen equation of state (EOS), shear moduli of LY12 aluminium alloy are obtained. A comparison of the shear moduli in the solid phase region with those estimated from the Steinberg model demonstrate that the latter are systematically lower than the measurements. By re-analysing the pressure effect on the shear modulus, a modified equation is proposed, in which the pressure term of $P/\eta^{1/3}$ in the Steinberg model is replaced by a linear term. Good agreement between experiments and the modified equation is obtained, which implies that the shear modulus of LY12 aluminium varies linearly both with pressure and with temperature throughout the whole solid phase region. On the other hand, shear modulus of aluminium in a solid-liquid mixed phrase region decreases gradually and smoothly, a feature that is very different from the drastic dropping at the melting point under static conditions.     

Kinetics of high-temperature deformation of polycrystalline OFHC copper and the role of dislocation core diffusion

The mechanisms of the high-temperature deformation of oxygen-free high-conductivity (OFHC) copper have been evaluated over a wide temperature (300–950°C) and strain rate (0.001–100?s^?1) regime. The stress–strain behaviour in hot compression is typical of the occurrence of dynamic recrystallization with an initial peak in the flow stress followed by a steady state, preceded by oscillations at lower strain rates and higher temperatures. The results are analysed using the kinetic rate equation involving a hyperbolic sine relation of the steady-state flow stress with the strain rate. In the temperature and strain rate range covering 500–950°C and 0.001–10?s^?1, a stress exponent of 5 and an apparent activation energy of 145?kJ/mol were evaluated from this analysis. The power law relationship also yielded similar values (5.18 and 152?kJ/mol, respectively). On the basis of these parameters, the rate-controlling mechanism is suggested to be dislocation core diffusion. The flow stress for the OFHC copper data reported by earlier investigators for different oxygen contents is consistent with the above analysis and revealed that an oxygen content of less than about 40?ppm does not have any significant effect on the core diffusion since it is too low to ‘clog’ the dislocation pipes. At strain rates greater than 10?s^?1 and in the temperature range 750–950°C, the stress exponent is about 3.5 and the apparent activation energy is 78?kJ/mol, which suggests that the plastic flow is controlled by grain boundary diffusion.     

Kinetics and mechanism of copper fracture due to deformation in surface-active baths

The authors propose a molecular mechanism of subcritical crack development under conditions of liquid-metal embrittlement. It is suggested that cracks develop as a result of diffusive mass transfer from the tip of a crack and that the role of the liquid is to provide a wide and “fast” diffusion channel. The specific action of surface-active baths here is connected with the atomic roughness of the crystal-bath interface, at rather low temperatures already, which leads to a higher concentration of dissolution centers at the crack front and produces conditions at the interface favorable to diffusion currents in the bath. It is demonstrated that this model of fracture correctly depicts the basic trends of subcritical crack development in the copperbath (Bi-Pb) system and may be used for analyzing the fracture kinetics in other systems too.     

Kinetics and mechanism of copper fracture due to deformation in surface-active paths

The general kinetic characteristics of copper fracture in the presence of surface-active bismuth-lead baths during creep and elongation under tension are explained. It is shown that the subcritical stage of crack development controls the process, whereupon the effects of stresses , temperature, strain rate , surface energy at the copper-bath interface _SL, and surface energy at the grain boundaries _b on the rate of crack development l/ are analyzed. The basic conclusions are that: a)l/=(–) ( and being constants here); b) the crack development activating energy ) the reduction of energy _b, achieved by intergranular internal adsorption of 0.5% antimony, lowers the value of about 50 times; d) a 30% increase in surface energy _SL reduces the cracking rate 30 times, according to the relation (where A=6 · 10^–15 cm²); and e) .Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 7–15, May, 1976.     

Mechanisms of fracture of the free surface of shock-compressed metals

The mechanisms of the ejection of aluminum and copper microparticles from the free surfaces of these metals have been studied under conditions of the escape of a moderate-intensity shock wave from a sample. The free surfaces of samples contained 0.7–0.9 mm deep artificial wells and protrusions simulating (on a greater scale of 10: 1) the natural surface roughness retained upon mechanical processing. The pressure in a shock-wave pulse at the base of a protrusion was controlled within P = 5–20 GPa (i.e., below the melting region), and the variable duration of pressure pulses was 0.02, 0.2, and 1 μs. Analysis of the free surfaces of postloaded samples showed that, for certain loading and roughness parameters, the ejection of metal from vertices of protruding ridges or pyramids (as a result of the longitudinal fracture) was about ten times greater than the amount of metal ejected in the form of cumulative jets from wells. The amount of ejected metal and the size distribution of metal microparticles were quantitatively characterized using “soft collecting targets” and by measuring mass losses of samples upon fracture.     

Mechanical properties of self-irradiated single-crystal copper

Molecular dynamics simulations are performed to investigate the influence of irradiation damage on the mechanical properties of copper. In the simulation, the energy of primary knocked-on atoms （PKAs） ranges from 1 to 10 keV, and the results indicate that the number of point defects （vacancies and interstitials） increases linearly with the PKA energy. We choose three kinds of simulation samples： un-irradiated and irradiated samples, and comparison samples. The un-irradiated samples are defect-free, while irradiation induces vacancies and interstitials in the irradiated samples. It is found that due to the presence of the irradiation-induced defects, the compressive Young modulus of the single-crystal Cu increases, while the tensile Young modulus decreases, and that both the tensile and compressive yield stresses experience a dramatic decrease. To analyze the effects of vacancies and interstitials independently, the mechanical properties of the comparison samples, which only contain randomly distributed vacancies, are investigated. The results indicate that the vacancies are responsible for the change of Young modulus, while the interstitials determine the yield strain.     

Dynamic characteristics of single-crystal and polycrystalline memory cells with toothlike projections

The dynamic characteristics of polycrystalline and single-crystal cells and of polycrystalline cells with artificial magnetic texture in toothlike projections are obtained and analyzed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, Vol. 12, No. 5, pp. 11–17, May, 1969.     

Cyclic deformation of polycrystalline Cu films

Fatigue impairs the reliability of macroscopic metallic components utilized in a variety of technological applications. However, the fatigue behaviour of thin metal films and small-scale components used in microelectronics and mechanical microdevices has yet to be explored in detail. The fatigue behaviour in submicrometre thin films is likely to differ from that in bulk material, since the volume necessary for the formation of dislocation structures typical of cyclic deformation in bulk material is larger than that available in thin films. The thin-film dimensions and microstructure, therefore, affect the microscopic processes responsible for fatigue. The fatigue behaviour of Cu films 0.4, 0.8 and 3.0 µm thick on polyimide substrates was investigated. The specimens were fatigued at a total strain amplitude of 0.5% using an electromechanical tensile-testing machine. This work focuses on the characterization of fatigue mechanisms and the resulting fatigue damage of thin Cu films. Extrusions similar to those observed in bulk material were found at the film surfaces after cyclic loading. Voids observed beneath the extrusions, close to the film-substrate interface, contributed significantly to thin-film failure. Thinner films were more fatigue resistant and contained fewer and smaller extrusions than thicker films did. A small thickness appears to inhibit void nucleation. This observation is explained in terms of vacancy diffusion and annihilation at free surfaces or grain boundaries. Transmission electron microscopy investigations confirmed that no long-range dislocation structures have developed during fatigue loading of the films investigated.     

Polyol-mediated synthesis of single-crystal tellurium nanowires directly from polycrystalline powder

Single-crystal trigonal tellurium (t-Te) nanowires were successfully synthesized by the direct transformation of polycrystalline powder via a convenient polyol-mediated route. These nanowires have an average diameter of 80 nm and lengths up to tens of micrometers. The high-resolution transmission electron microscopy image and the selected-area electron-diffraction pattern reveal that the as-obtained tellurium nanowires are single crystalline with a growth direction of [001]. The mechanism of this transformation perhaps follows the dissolution–recrystallization mechanism. PACS 61.10.Nz; 61.82.Rx; 68.37.Lp; 73.61.Cw     

Strength and plastic deformation of polycrystalline diamond composites

ABSTRACT

The use of nanopolycrystalline diamond has allowed a systematic study on deformation of polycrystalline diamond composites (PCDCs). Bulk PCDCs samples containing either Co or SiC as a binding agent were deformed under high pressure and temperature to strains up to 18% at strain rates ～10^?5?s^?1. All samples exhibit strong work hardening. The strength of PCDCs depends on the amount and type of binding agents and is consistently weaker than that of diamond single crystals. The weakening may be due to the binder materials, which play an important role in affecting grain boundary structures. In SiC-based PCDC, significant grain fragmentation occurs. Nearly all grain boundaries are wetted by SiC after large deformation, resulting in lower strength. In Co-based PCDC, the microstructure is dominated by dislocations, deformation twins, and separated grain boundaries. The density of deformation twins increases significantly with strain, with the twin domain width reaching as low as 10–20?nm at 14% strain.     

Effect of stress state on deformation and fracture of nanocrystalline copper:Molecular dynamics simulation

Deformation in a microcomponent is often constrained by surrounding joined material making the component under mixed loading and multiple stress states. In this study, molecular dynamics(MD) simulation are conducted to probe the effect of stress states on the deformation and fracture of nanocrystalline Cu. Tensile strain is applied on a Cu single crystal,bicrystal and polycrystal respectively, under two different tension boundary conditions. Simulations are first conducted on the bicrystal and polycrystal models without lattice imperfection. The results reveal that, compared with the performance of simulation models under free boundary condition, the transverse stress caused by the constrained boundary condition leads to a much higher tensile stress and can severely limit the plastic deformation, which in return promotes cleavage fracture in the model. Simulations are then performed on Cu single crystal and polycrystal with an initial crack. Under constrained boundary condition, the crack tip propagates rapidly in the single crystal in a cleavage manner while the crack becomes blunting and extends along the grain boundaries in the polycrystal. Under free boundary condition, massive dislocation activities dominate the deformation mechanisms and the crack plays a little role in both single crystals and polycrystals.     

Conductivity of single-crystal and polycrystalline bilayer metal films under the conditions of interdiffusion

The electrical conductivity of a bilayer film with a single-crystal or polycrystalline structure is theoretically investigated under the conditions of metal interdiffusion at an arbitrary ratio between the thickness of layers and the mean free path of electrons in the layers. It is shown that analysis of the changes in the electrical conductivity of the bilayer film due to diffusion annealing allows one to elucidate the nature of the processes of bulk and grain-boundary diffusion, to determine the effective depth of penetration of impurity atoms into the bulk and grain boundaries of the sample, and to obtain information on the bulk and grain-boundary diffusion coefficient.