Density Of Lattice Defects Research Articles

低サイクル疲労中の格子欠陥の挙動に及ぼす温度変化の影響

Torsional fatigue tests were carried out for commercial aluminium wires by changing temperature from 77K to R.T. or vice versa, and resistivity was measured as a function of number of cycles at 77K.After the temperature change from R.T. to 77K, the resistivity increased with number of cycles, and saturated at an advanced stage of fatigue. This phenomenon was explained from the change in lattice defect density given byρ12=α/β{1-(1-P)e-(β/2)γ}C=Cs[1-1-P/K2-β/2{K2e-(β/2)γ-(β/2-K2P/1-P)e-K2γ}]Cs=Kfα2/K2β, P=ρi1/2β/αwhere ρi is the dislocation density prior to temperature change, ρ, C and γ is the dislocation density, the concentration of point defects and the cumulative strain after temperature change, respectively, and Kf, K2, α and β are constants.The decrease in resistivity induced during fatigue after the temperature change from 77K to R.T. was thought to occur mainly due to dynamic annealing of point defects.

Structural and electrical properties of intercalated and ion-implanted highly ordered graphite fibers

Because of their high degree of structural order, high mechanical strength, and small diameter (\ensuremath{\sim}1 \ensuremath{\mu}m), benzene-derived graphite fibers provide an excellent host material for a structural analysis of both the intercalation and ion-implantation processes in graphite. Using the $c$-axis lattice imaging technique, with a high-resolution electron microscope, we observe well-staged intercalated regions in this fiber matrix. With regard to ion implantation, we find a distribution in the density of lattice defects with depth from the fiber surface. Ion implantation is also found to increase the graphite interlayer spacing. This information is correlated with electrical properties of these fibers.

Solid state reactions for the epitaxial growth of spinel films

Solid state reactions are shown to provide a method for the growth of completely single crystal, stoichiometric spinel films, exhibiting a perfect orientation on the substrate and a very low density of lattice defects. The atomic-scale processes involved in the film growth and in the formation and subsequent annihilation of lattice defects have been investigated by different electron-optical and ionometric methods. These investigations outline the feasibility and certain limitations of this method of epitaxial film growth.

An atom probe study of the aging of iron-nickel-carbon martensite

The atom probe field ion microscope has been used to study the redistribution of carbon in iron-nickel-carbon martensites aged in the temperature range 22 to 250 °C. The Ms temperature of the alloy used was −50 °C. During all stages of aging a considerable amount of redistribution of carbon was detected in the form of carbon segregation to twin boundaries and other lattice defects and in the formation of small clusters and carbides. Small carbon-rich areas were detected at 40 °C, and disc-shaped carbide particles were found after 24 hours at 100 °C. Thin cementite lamellae were observed after one hour at 250 °C. It is evident that a close similarity exists between the sequence of reactions occurring during the tempering process and the sequence of precipitation reactions observed in age-hardening alloys. The main additional complications that arise in the case of martensites are due to the high densities of lattice defects that are present and to the strong interaction between carbon atoms and such defects.

Effect of cooling rate in liquid nitrogen on the transformation rate of retained austenite in steel R6M5

1. Increasing the cooling rate in liquid nitrogen from 3–5 to 50–70 deg/min changes the transformation of retained austenite in quenched steel R6M5 from martensitic to bainitic-martensitic. 2. The bainitic transformation during rapid cooling shifts the martensitic transformation proper to lower temperatures (−100 to −150°) and thus changes the morphology of martensite crystals. 3. After partial stabilization at +20° for 24 h the austenite is transformed by the bainitic-martensitic mechanism during slow cooling and by the bainitic mechanism with rapid cooling to the temperature of liquid nitrogen, which reduces the amount of austenite transformed. 4. Rapid cooling in liquid nitrogen has a stronger shrinkage effect during subsequent heating in the temperature range of −60 to +120°, i.e., increases the density of martensite lattice defects. 5. The characteristics of the transformation of retained austenite (not stabilized or partially stabilized) must be taken into consideration in selecting the cold treatment for tools made of high-speed teel.

Predictions of the random potential energy models of domain wall motion: An experimental investigation on high-purity iron

In several phenomenological models of magnetic domain wall motion, the wall-defect interactions are pictured by a random potential energy V( x). These models predict that the following quantities have to remain practically constant: aH c, bH 2 c, bH c/ a, bM s/ a 2, M 0 ( H c: coercive field, a and b: parameters of the Rayleigh law, M s: saturated magnetization, M 0: magnetization at the upper limit of the Rayleigh region). These predictions were tested on polycrystalline specimens of high-purity iron in which the densities of lattice defects (grain boundaries and dislocations) could be varied. The experimental data showed that the above-mentioned quantities, instead of remaining constant, can change by a factor of 3 or more. In view of these results, the statistical assumptions which underly the models of random potential energy are discussed.

On the Study of Crystals Containing High Density of Lattice Defects

On the Study of Crystals Containing High Density of Lattice Defects

Epitaxial AlGaAsSbGaSb (AlGaSb) heterostructures for injection lasers

AbstractA possibility is shown of obtaining nearly perfect heterojunctions in the GaAlAsSb—GaAlSb system which do not contain any remarkable density of lattice mismatch defects at the heteroboundaries. Application to low threshold DH injection lasers for 1.4–1.8 μm spectral region for optical communications, moist‐o‐graphs etc.

Dislocation Motion in Metals Investigated by Means of Pulsed Nuclear Magnetic Resonance

ABSTRACTWe report the first use of nuclear magnetic resonance to investigate dislocation motion in metals. The spin-lattice relaxation rate in the rotating frame T 1ρ−1, of 27Al in polycrystalline, ultrapure Aluminium foils has been measured as a function of plastic-deformation rate έ for two different temperatures (77K and 300K). For έ = 0, the relaxation rate is determined by conduction electrons. For a finite deformation rate έ, an additional contribution to the relaxation rate arising from fluctuations in the nuclear quadrupole interaction due to dislocation motion is observed. From the motion-induced part of the relaxation rate the mean jump distance of a mobile dislocation is calculated which is determined by the density of lattice defects acting as obstacles for moving dislocations.

Grain boundaries as dislocation sources in the premacroyield strain region

The early stages of yielding in high purity copper (plastic strains of 1–10 × 10 −4) have been studied by both quantitative transmission electron microscopy (TEM) and tensile data analysis. The TEM observations, in the form of comparative lattice defect densities, and the tensile data, in the form of stress vs square root of plastic strain plots, both indicate a two stage deformation process separated by a transition region. Each stage is characterized by a dominant deformation mode. In the first stage, this mode is the generation of lattice dislocations at grain boundaries, with the dislocations remaining at or near the boundaries. In the second stage, these dislocations are emitted into the grain interiors, where the grownin dislocations are moving and multiplying as well. These two stages are separated by a transition region where both modes are competitive.

Electron diffraction in crystalline specimens containing a high density of lattice defects. II. Numerical evaluation for the case of narrow dislocation dipoles–comparison with experimetnal data

The equation established in a preceding paper for the description of the diffraction of fast electrons in crystals containing a high density of lattice defects is worked out for the case of narrow dislocation dipoles. The diffraction pattern of a systematic 10-beam case is calculated as a function of the density of the dislocation dipoles, the width of the dipoles, the acceleration voltage, and the direction of incidence of the electrons. The results are applied to the diffraction patterns observed on a cyclically deformed copper crystal containing dense arrays of dislocation dipoles. The analysis yields ϱ = 3.4 × 1011/cm2 (ϱ = dislocation density) which compares well with other, indirect, estimates. Die in einer vorangegangenen Arbeit aufgestellte Gleichung zur Beschreibung der Beugung schneller Elektronen in Kristallen mit hoher Dichte an Gitterfehlern wird auf den Spezialfall enger Versetzungsdipole angewandt. Es wird das Beugungsdiagramm fur einen systematischen 10-Strahlfall als Funktion der Dichte der Versetzungsdipole, der Dipolweite, der Beschleunigungsspannung der Elektronen und deren Einstrahlrichtung berechnet. Die Resultate werden auf Beugungsdiagramme eines wechselverformten Kupfereinkristalls mit hoher Versetzungsdichte ϱ angewandt. Es ergibt sich ein Wert ϱ = 3,4 × 1011/cm2, der mit anderen, mehr indirekten Abschatzungen gut ubereinstimmt.

Electron diffraction in crystalline specimens containing a high density of lattice defects. I. Development of a statistical theory

A statistical theory is developed of characteristic features exhibited by the electron diffraction pattern of a crystalline specimen containing a high density of lattice defects: with increasing defect density the intensities of the (originally strong) low-order diffraction spots decrease, whereas the intensities of the (originally weak) higher-order diffraction spots increase. Thus the diffraction pattern tends towards an equidistribution of the diffracted intensity over a large number of diffraction spots. The theory leads to a fairly simple equation. Its evaluation for a particular kind of lattice defects requires essentially only the solution of the differential equation of the diffraction contrast in the Bloch wave notation for a representative example of the lattice defect considered. Es wird eine statistische Theorie entwickelt zur Interpretation charakteristischer Besonderheiten des Elektronenbeugungsdiagramms kristalliner Präparate mit Gitterfehlern in hoher Dichte: mit steigender Dichte der Gitterfehler nimmt die Intensität der (ursprünglich starken) Reflexe niedriger Ordnung ab, während die Intensität der (ursprünglich schwachen) Reflexe höherer Ordnung zunimmt. Die Theorie mündet in eine relativ einfache Gleichung, deren Auswertung für eine bestimmte Art von Gitterfehlern im wesentlichen nur die Lösung der Differentialgleichungen des elektronen-mikroskopischen Beugungskontrastes (in der Blochwellendarstellung) für einen repräsentativen Vertreter der betrachteten Gitterfehler erfordert.

Positron Lifetimes in Deformed Copper

Positron lifetime measurements were performed for Cu samples with different densities of lattice defects. The lifetime spectra were successfully resolved into two components with the help of the well established analysis program. Obtained results were quite consistent with those expected from the trapping model. The positron trapping mechanism from free to trapped states and the initial condition of the model were especially checked. Deduced values obtained for τ c (lifetime of free positrons) and τ t (lifetime of trapped positrons) were 122±5 psec and 176±5 psec, respectively.

Nearly perfect crystal growth of III–V compounds by the temperature difference method under controlled vapour pressure

Nearly perfect crystals of III–V compounds are grown by a new LPE method which can produce homogeneous and high-quality crystals with controlled stoichiometry. Homogeneous epitaxial growth is performed at fixed temperature with a temperature difference in a melt, under controlled vapour pressure of the group V elements. In the experiment, the fluctuation of growing temperature is ascertained to produce a number of new defects and the effect of controlled vapour pressure is investigated. It has been ascertained that an optimum vapour pressure, which is the function of growing temperature, can suppress the generation of “rooty faults” to a minimum density. There is close agreement between the optimum vapour pressure in the heat treatment of crystals to reduce the generation density of lattice defects and the optimum vapour pressure to obtain defect-free crystals in the process of crystal growth. The epitaxial layers prepared under optimum vapour pressure have the highest mobility and the lowest additional carrier concentration. The lattice constant of the epitaxially grown crystal under controlled vapour pressure also shows a minimum value at optimum pressure. This also shows that the controlled vapour pressure is important in lattice fitting of epitaxial layers. The epitaxial layers grown by the temperature difference method (TDM) under optimum control of the vapour pressure (CVP) are almost as perfect crystallographically as measured by X-ray topography. The experiments are performed for GaP and GaAs.

Chemical Effects of Osmium Neutron Capture Recoils. III. Cation Effect in Crystalline Mixtures of K4Os(CN)6–K4Fe(CN)6–Cs4Fe(CN)6

Osmium recoil effects were studied in crystalline mixtures of the type K/ sub 4/Os(CN)/sub 6/-K/sub 4/Fe(CN)/sub 6/-Cs/sub 4/Fe(CN)/sub 6/ in order to investigate the role of the cation in diluted systems. Neutron irradiations were carried out at a thermal neutron flux of 1.8 x 10/sup 12/ n/cm/sup 2/s fast neutron flux of 1.8 x 10/sup 12/ n/cm/sup 2/s, and gamma ray dose rate of about 1.4 x 10/sup 7/ rad/hr The potassium-cesium salt mixtures were each irradiated for 6.0 hr and then analyzed by paper electrophoresis. Tabulated results of cyanoosmate(II) parent retention determined as a function of Cs/sub 4/Fe(CN)/sub 6/ mole fraction in the cesium-potassium crystalline mixtures indicate that replacement of cesium for potassium causes a large decrease in parent retention. In general, the probability of parent reformation is- not only determined by the density of lattice defects and radiation damage in the crystal, but also by the energy of activation for release from traps. It is expected that the more ionic cesium salt with a larger band gap would increase the trap depth and activation energy for reactions giving rise to initial retention. (JGB)

X-ray topography as a tool in crystal growth studies

X-ray topography is a non-destructive method revealing extended imperfections such as dislocations, stacking faults, growth horizons, twins, inclusions and so on; in non too imperfect crystals, it has rapidly rpoved a very useful tool in studying problems related to crystal growth. The configuration, nature and density of lattice defects usually vary considerably depending on the method and conditions of growth as well as on the chemical nature of the crystal. One very frequent observes very different degrees of perfection for the same crystal grown by different methods, but similar configurations of defects for different crystals grown by the same method.The dislocation configuration arising in crystals grown from the solution will be studied in more detail. Dislocations in this case tend to be linear and their orientation closely related to that of the growth face. They usually constitute bundles originating at inclusions and along the interface between the seed and the new crystal, which is where the majority of dislocations are created. The importance of the surface of the seed and of the initial stages of the growth is therefore primordial for the quality of the crystal to be grown. Any fluctuation of the conditions of growth will usually produce a growth horizon along which dislocations are sometimes formed.

Thermal Recovery Processes in Explosion-Shocked Nickel

Electron microscopy has been used extensivley to investigate annealing processes in metals, but most of the work refers to cold-worked materials. Only a limited amount of information has been published concerning the microstructural changes occurring during annealing of explosively shockloaded metals. It is known, however, that the small extent of permanent deformation and the very high densities of lattice defects induced by shock-loading of metals, particularly at high explosive pressures, results in thermal recovery processes which are quite different from those occurring in conventionally deformed metals. The present study was concerned with the microstructure of pure polycrystalline nickel after explosive shock treatment at 70, 320 and 1000 kb and subsequent heat treatments between 600 and 780°C.

Effect of grain size on the properties of low-carbon steel after deformation + aging

1. The change in the mechanical characteristics, particularly the yield strength, after aging is greater in coarse-grained than fine-grained steel. 2. Impact bending tests showed that the rise of the cold brittleness threshold after aging is greater for the coarse-grained steel. 3. The somewhat smaller broadening of (310) after aging of the coarse-grained steel, with greater distortion and density of lattice defects after deformation, is evidently due to more intense relaxation processes occurring during aging as compared with the fine-grained steel.

Increase of the Anisotropy of the Energy Gap in Superconducting Thallium under Pressure

A study has been made of the dependence of the superconducting transition temperature of thallium on the density of lattice defects due to plastic deformation, as a function of hydrostatic pressures up to 4 kbar. The change in critical temperature with resistivity ratio is found to be pressure-dependent. It can be accounted for by a theory of gap anisotropy given by Markowitz and Kadanoff and yields a strongly pressure-dependent anisotropy parameter $\ensuremath{\lambda}〈{a}^{2}〉$ which varies from 0.0008 at zero pressure to 0.007 at $p=4$ kbar. Only a little change in the "valence" effect with pressure is observed. The well-known anomalous enhancement of the transition temperature of thallium at pressures up to 2 kbar can be explained quantitatively by this increase of the gap anisotropy under pressure.

Contribution à l'étude de la transformation allotropique du cobalt

AbstractThe allotropic transformation of cobalt has been followed with a direct reading X‐ray spectrometer. The α→β transition occurs stepwise between 400° and 500° C, each increase in temperature in the given interval giving rise to the practically instantaneous formation of a new amount of β. The reverse transition occurs similarly but over a much wider temperature range and is not complete even far below 0° C.The proportion of β remaining at room temperature is dependent on the density of lattice defects as measured by the broadening of definite α lines. This proportion can be increased to 90% by severe deformation followed by heat treatment, below 500°. Deformation after heat treatment suppresses totally the β phase. Heat treatment of short duration in the 500° ‐ 1480° C region tends to restore the lattice order, decreasing at the same time the proportion of β, retained in the cold, to some 35%.