Unstable Defects Research Articles

Stress relaxation and creep of 12 to 35 μm copper foil

Stress relaxation and creep of the electrodeposited and rolled copper foils, 12-35 μm thick, are investigated near yield stress and near room temperature. The stress relaxation does not obey a logarithmic time law; the creep appears to follow a power function. These deviations from the expected logarithmic behavior are thought to be caused by very small grain size, unstable non-equilibrium defect structure and extensive micropore population (vacancies and vacancy clusters) typical of the electrodeposit. Relaxation and creep are significantly lower for the rolled (than for the electrodeposited) foil. Decreasing the electrode-posit thickness has an effect of enhancing relaxation and creep, attributable to a limited nucleation on the cathode surface and consequent generation of microvoids between growth clusters in the vicinity of the substrate. The foil thickness effect on creep and stress relaxation is not observed for the rolled foil, which is prone to embrittlement and stiffening at about 323K.

Experimental investigations and modelling of relaxation behaviour of shot peening residual stresses at high temperature for nickel base superalloys

Residual stresses induced by two shot peening conditions in nickel base superalloy Inconel 718 and their relaxation after different exposure times (10 and 100 h) at 600 and 650°C were evaluated by an X-ray diffraction technique. The behaviour of stress relaxation (effects of exposure time and applied temperature) is particularly analysed and discussed. A significant decrease of the initial stress values was observed in the first period of exposure time, followed by slowing down and then stabilisation. Experimental results showed that the maximum relaxation always occurred at the surface. It was also observed that a higher applied temperature produced greater relaxation, but had no significant effect on the pattern of the depth profile of stress. The relaxation mechanism is essentially interpreted by thermal recovery characterised by a rapid annihilation of the unstable crystalline defects in highly deformed materials. Complex changes of various microscopic magnitudes under high temperature are described by macroscopic magnitudes related to the plastic strains and associated residual stresses. An existing model is developed and actualised by integrating the limiting state of recovery and the integrated width of X -ray diffraction peaks in order to predict relaxation of the residual stresses. The relaxation predicted by the model agrees satisfactorily with the experimental results in the studied cases.

Variables affecting pedicle screw plate fixation of an unstable L3-L4 defect.

Fresh frozen human cadaveric spinal specimens (T8-S1) were subjected to pure flexion extension bending moment and pure axial torque loadings while intervertebral rotations were recorded at the L3-L4, L2-L3, and Ll-L2 discs. A standardized unstable defect was created at the L3-L4 disc, and loading tests were repeated after application of bilateral Steffee plates in 2 configurations: a short plate with 2 pedicle screws (spanning the defect) and a longer plate with 3 pedicle screws (spanning the defect and 1 disc above). Each plating configuration was tested in the unlocked state (nuts compressing the plate down onto the spine) and locked state (nuts above and below the plate tightened against each other to clamp the plate to the screws). Locking the plates to the screws had no effect on any intervertebral rotation at any disc level. Use of a longer plate that also spanned the disc above the defect offered no advantage in controlling flexion extension rotations at the defect site. However, mean torsional rotation at the defect site with the 3-screw plate was approximately 50% of the mean for a 2-screw plate. Extension and torsional rotations at the L2-L3 disc (1 level above the defect site) were unaffected by application of a 2-screw plate; flexion rotation at this level increased slightly after plating. All motions at the L2-L3 disc were reduced (as would be expected) when the 3-screw plate spanned this uninjured disc. Plating the defect had no effect on disc rotations at the L1-L2 disc (2 levels above the fracture site).

Vibrational properties of an elastic continuum with dislocations and disclinations: a gauge approach

We study, in the framework of a gauge approach, the problem of small vibrations in an isotropic elastic continuum against the background of a static distortion due to a linear defect. In the approximation, which is linear in the dynamical displacements, the equations of motion for an isotropic elastic continuum with a topological defect are obtained. An analysis of the vibrational spectrum is presented for elastic materials with a screw dislocation, a wedge disclination and a disclination monopole. In the case of a screw dislocation the known results of classic defect theory are reproduced. For a wedge disclination, with a small non-integer Frank index, only minor changes of the spectrum are found to appear. It is shown that the situation is changed remarkably for topologically stable disclinations. In this case we deal with another topological sector, and the character of the vibrations differs essentially from that for topologically unstable defects.

Controlling spatiotemporal chaos.

A method for controlling spatiotemporal chaos in certain classes of spatially extended systems is proposed. In these systems, unstable defects emit convectively unstable waves which subsequently break and may nucleate new defects. Control is achieved via the stabilization of one such active wave source, which then sweeps all of the chaotic fluctuations to the system boundaries. This method is applied to the one- and two-dimensional complex Ginzburg-Landau equation, and to a recent model of spiral breakup in excitable media.

Cervical spondylolysis.

The term cervical spondylolysis describes a long-standing, perhaps congenital defect of the pars interarticularis of a cervical vertebra. We report 10 new cases of cervical spondylolysis and review the literature. All patients in this report were treated nonoperatively with subsequent symptomatic improvement. Cervical spondylolysis must be differentiated from its traumatic counterparts radiographically. Characteristic radiographic findings include well-corticated margins at the defect, a characteristic "bow tie" deformity, and ipsilateral dysplastic facets. Compensatory hypertrophic changes of the adjacent articular processes, spina bifida, and spondylolisthesis are frequently, but not always, seen in conjunction with cervical spondylolysis. The vast majority of patients with radiographically proven cervical spondylolysis can be treated confidently with conservative measures. Surgical intervention should be reserved for those who fail nonoperative management or who exhibit neurologic compromise referable to an unstable spondylolytic defect.

Characrerization of unstable point defects in crystals

Investigation of unstable defects generated by ultra-sound in ZnS and ZnSe crystals is reported. The experimental techniques use simultaneously both optical and acoustic excitation of crystal. Acoustophotoconductivity (APC) and acoustophotoluminescence (APL) were measured under the above-threshold intensity of ultrasound. By these measurements both deep and shallow energy levels of point defects and their complexes can be found.

Equilibrium Unstable Point Defects in Solids

ABSTRACTIt is shown that in addition to Frenkel’s and Schottky’s defects there is another type of equilibrium defects, such as an unstable pairs (UP) : vacancy and interstitial atom. UP has the life time of the order of 10-1010-12s, independent of temperature. The review of the unstable pairs physics is presented. Numerous experimental evidences of equilibrium UP in semiconductors, insulators, especially in superionic crystals, and metals are discussed.

Observation of a Configurationally Unstable Defect in Si

Observation of a Configurationally Unstable Defect in Si

Theoretical and experimental study of a quantized lattice configuration in a nearly unstable defect system.

Theoretical lattice dynamics analysis and new optical experiments on KI:${\mathrm{Ag}}^{+}$ reveal a nearly unstable low-temperature defect-host configuration, whose population decreases dramatically with temperature, but whose dynamics are consistent with a temperature-independent harmonic model involving collective motion. This behavior is in marked contrast to the temperature-dependent evolution of the dynamics of ``soft-mode'' systems.

Pulse radiolysis studies on the oxide glasses

A pulse radiolysis was applied to the alkali silicate, alkali borate and thallium borate glasses. The optical absorption spectra immediately after irradiation were found to differ largely from those by the X-ray or γ-ray steady state irradiation. This is due to the presence of the unstable defect centers in the short time period. In the alkali silicate glasses, the peak energies of the transient spectra increased with a decrease in the radius of the alkali ions. From the doping experiment with Cd(II) ion, it was concluded that the short lived defect centers in silicate glasses were electrons trapped on the alkali ions in the glass networks. In the binary alkali borate glasses the transient absorption due to the electrons trapped on the alkali ions could also been detected. The intensity of this absorption increased with an increase of the alkali content. On the other hand, in the X-ray irradiated thallium borate glasses, the absorption due to Tl∘ was observed in the visible range only when the thallium content is low. The absorption due to this center was already found in ps range and its intensity decreased gradually with time and becomes to the ones obtained by X-ray irradiation. When thallium oxide content exceeds 20 mol %, the the absorption due to Tl∘ could not be seen in the X-ray irradiated glasses, although in the early stage spectrum a broad component presumably due to Tl∘ was observed. This was understood by the the difference of the electron capturing strength of the thallous ion which depends on the glass composition.

Comparative Study of Gamma-Ray-Irradiation Effects on N-Type GaAs and InP

Radiation damage effects on electrical properties of n-type GaAs and InP bulk crystals have been investigated. Higher tolerance of InP to gamma-ray irradiation than GaAs was observed. The effects on GaAs strongly depend on the initial carrier concentration. Ihe critical value of concentration is taken to correspond to the border value of metal-nonmetal transition introduced by Mott. All InP samples with several different carrier concentrations showed almost completely unchanged properties after 1×109 R irradiation at 44°C. This extreme tolerance in InP to irradiation is believed to be caused by annihilation of unstable defects and also association of oppositely charged defects.

Investigation of the influence of unstable defects on generation of laser radiation in YAG:Nd3+

A study was made of the influence of unstable defects on the generation of laser radiation in YAG:Nd3+ crystals. It was found that F+ centers and iron impurity ions were the main unstable defects. The influence of these defects on lasing was manifested in different temperature ranges. At temperatures 287-312 K the F+ centers predominated, whereas above 312 K the effect was mainly due to iron impurity ions.

Radiation-induced transient acoustic loss in quartz crystals

When exposed to pulsed ionizing radiation, quartz oscillator crystals can exhibit transient acoustic losses (a decrease in Q) and transient frequency shifts. Although these changes are usually most pronounced in unswept quartz, they have also been seen in swept quartz. We have directly measured the transient acoustic loss in both synthetic and natural quartz. The synthetic quartz samples included hydrogen and lithium swept units. Irradiations were accomplished at REBA, a flash x-ray facility at Sandia Laboratories. Log decrement acoustic loss measurements were made over the 110–400 K temperature range and show a time dependence similar to a t−1/2 behavior characteristic of one-dimensional diffusion. The transient loss showed a broad temperature dependence ascribed to alkali ions moving along the Z-axis channels. Peaks at 360 and 380 K in the transient loss temperature spectra are thought to be due to an unstable defect which momentarily traps the alkali ion.

Study of structural changes for a two-phase titanium alloy with shock-wave loading

1. Original fcc interlayers are disrupted with high-speed shock-wave loading for lamellar two-phase alloy VT8. 2. Within the limits of a single colony, slip and twinning propagate unhindered across all of the α-platelets, and they are stopped at high-angle boundaries (colony boundaries). 3. With an increase in shock loading rate from 60 to 600 m/sec intensification of twinning is observed. 4. Formation of unstable packing defects, decorated mainly with hydrogen, along planes {1010}α and hydrides in β-phase with a shock loading rate of 600m/sec have been noted. 5. Electron-microscopic studies have not revealed formation of ω-phase with shock loading for the test alloy.

Development of flute-mark assemblages, 2. Evolution of trios of defects

Experiments using a bed of hardened Plaster of Paris dissolved beneath a turbulent water stream (practical analogue of natural mud-bed erosion) show that trios of defects evolve into compound flute marks which, at a critical age, become indistinguishable from structures evolved from single defects. The critical age is comparable with 0.6, defined in terms of the erosion velocity of the bed surrounding the mark, duration of erosion, and the overall size of the assemblage of defects. Marks formed from single defects (Allen, 1971) and from paired defects (Allen, 1973) also lose evidence of their parents broadly at this value of the age. Marks grown from trios increase in maximum depth on a similar law to the marks formed from single and paired defects. A natural assemblage of flute marks may be expected during an early stage of evolution to show a progressive reduction in the number-density of structures, as a consequence of the merging of marks produced at sufficiently closely spaced unstable defects.

On the behaviour of low temperature irradiated cadmium

Abstract The atomic displacement process in electron irradiated cadmium has been studied. The experimental data can be fitted using an energetic threshold for Frenkel pair formation Td , = 19 eV. Discussion of the nonlinearity of the resistivity increase per electron versus total electron flux shows that this behaviour may be explained by the existence of a very unstable Frenkel pair specie, the mutual annihilation of which may be initiated either thermally at ∼5.5 °K after the irradiation, or dynamically. In the latter case, the most important events for producing pair annihilation would be elastic interactions of secondary electrons with the atomic nuclei. With regard to the high c/a ratio in cadmium, clues exist showing that these unstable defects are likely to be Frenkel pairs situated in and interacting through densely packed atomic rows in the basal planes.

RESPONSE OF THERMALLY SOFTENED DEFECT STRUCTURES IN ALLOYS TO AN APPLIED STRESS

Alloy specimens, after predeformation at 4.2°K and intermittent holding at higher temperatures, respond to reapplication of stress by transient evolution of heat. These heat bursts have been measured and are interpreted as due to the breakdown of unstable defect configurations.

Effect of 60Co γ Rays on High-Resistivity p-Type Si

Defects created by γ rays in high-resistivity (1000 to 10 000 Ω·cm) boron-doped silicon have been studied by Hall effect and resistivity measurements. Only one energy level is observed at 0.268 eV above the valence band by irradiation at 300°K. Its statistical weight is equal to ½. The defect annealing between 100° and 200°C occurs by the agency of another defect. In some special cases, a more complex association with a third defect can be observed, which introduces another level at 0.21 eV above the valence band. All the results obtained are compatible with the divacancy assuption. Furthermore, we observe, by irradiation at 77°K, the formation of unstable defects, which disappear at 170°K.

Low-Temperature Irradiation of n-Type Germanium

The rate of change of conductivity of n-type single crystal plates of germanium at a temperature of ∼16°K during fast neutron irradiation is enhanced over that previously observed at higher temperatures. Annealing studies subsequent to irradiation reveal no evidence of thermally unstable defect states between ∼10 and ∼95°K. Annealing above ∼95°K indicates the presence of thermally unstable minority carrier traps of the type previously observed following irradiation at 120°K. The enhanced decrease in conductivity at 16°K compared to that observed at higher temperatures is believed to reflect a greater rate of introduction of defects, many of which anneal at temperatures above ∼95°K.