Showing papers on "Grain boundary published in 1971"

On grain boundary sliding and diffusional creep

Abstract: The problem of sliding at a nonplanar grain boundary is considered in detail. The stress field, and sliding displacement and velocity can be calculated at a boundary with a shape which is periodic in the sliding direction (a wavy or stepped grain boundary): a) when deformation within the crystals which meet at the boundary is purely elastic, b) when diffusional flow of matter from point to point on the boundary is permitted. The results give solutions to the following problems. 1) How much sliding occurs in a polycrystal when neither diffusive flow nor dislocation motion is possible? 2) What is the sliding rate at a wavy or stepped grain boundary when diffusional flow of matter occurs? 3) What is the rate of diffusional creep in a polycrystal in which grain boundaries slide? 4) How is this creep rate affected by grain shape, and grain boundary migration? 5) How does an array of discrete particles influence the sliding rate at a grain boundary and the diffusional creep rate of a polycrystal? The results are compared with published solutions to some of these problems.

Nonohmic Properties of Zinc Oxide Ceramics

Abstract: Nonohmic properties of ZnO ceramics with five additives of Bi2O3, CoO, MnO, Cr2O3, and Sb2O3 are studied in relation to sintering temperature, additive content, and temperature dependence. The observation of electron photomicrographs and X-ray microanalysis proves a ceramic microstructure such that ZnO and these five oxides form, at the grain boundaries, segregation layers which are responsible for the nonohmic properties. The electrical resistivity and dielectric constant of segregation layers are estimated to be 1013 ohm-cm, and 170, respectively by using a simple model. The electric field strength corresponding to the steep rise in the current is also estimated to be 104 V/cm by taking account of the concentration of applied voltage at the segregation layer. In view of these data and simple model, a possible explanation for nonohmic properties is discussed.

Hall Mobility in Chemically Deposited Polycrystalline Silicon

Abstract: Hall‐mobility measurements have been performed on polycrystalline silicon films deposited on a silicon oxide surface by the thermal decomposition of silane. Samples with doping impurities added during deposition or by diffusion from a doped vapor‐deposited oxide showed similar behavior. For both n‐type and p‐type samples approximately 5 μ thick, the mobility reached a maximum value of about 40 cm2/V sec at a free carrier concentration of about 1018 cm−3 and decreased for both higher and lower carrier concentrations. The observed Hall mobility was generally higher in p‐type samples than in n‐type samples. The decrease in observed mobility with decreasing carrier concentration is attributed to the effects of high resistivity space‐charge regions surrounding grain boundaries in the polycrystalline material. The mobility was seen to increase as the film thickness increased for samples with similar doping, indicating a more ordered structure in thicker films.

Point Defects and Sintering of Lead Zirconate-Titanate

Abstract: The crystal chemistry of point defects in lead zirconate-titanate is discussed. The results are used to interpret sintering and grain-growth behavior. Lattice vacancies are created thermally, by substitutional impurities with incorrect valences, and by changes in stoichiometry. Charged O vacancies are introduced when Al3+ replaces Ti4+, and charged Pb vacancies occur when Nb5+ replaces Ti4+. These vacancies are believed to be associated with the impurity ions and cause them to be adsorbed at grain boundaries. This behavior retards grain growth and thereby expedites densification. Aluminum ions (deficient valence) compensate for niobium ions (excess valence). These “paired” defects are not associated with vacancies and are not adsorbed; thus, they do not impede grain growth. Sintering follows Coble's model of bulk diffusion of vacancies from pores to grain boundaries. Oxygen vacancies are believed to be the slowest-moving species. Aluminum: niobium compensation is confirmed by ferroelectric measurements. Doping with Al decreases the mobility of ferroelectric domain boundaries, whereas Nb increases it. Doping with both ions produces ferroelectric properties similar to those of the undoped material.

Void Formation and Growth During Electromigration in Thin Films

Abstract: One of the modes of electromigration damage is shown to be related to irregularities in the grain‐boundary structure of films. Based on the restriction of electromigration to grain boundaries, calculations are performed to determine the magnitude and distribution of excess vacancy concentrations at sites where electromigration flux divergencies may be present. The attainment of an equilibrium vacancy supersaturation (Δc/c0) is found to be practically instantaneous. The maximum supersaturations are less than 1 under all conditions used. This is found to be inadequate for void formation by vacancy condensation unless heterogeneous nucleating sites are present (e. g., oxide particles, boundary‐surface oxide intersection). The rate of observable hole formation is accounted for by either void growth at a grain‐boundary triple‐point junction or by grain‐boundary grooving, each being related to the electromigration flux. Grooving seems most likely in pure films with all boundaries having similar character (e. g....

Intergranular Corrosion of Austenitic Stainless Steel

Abstract: The well‐known susceptibility of austenitic stainless steels to intergranular corrosion after heat‐treatment in the temperature range of 550°–800°C (i.e., "sensitization") has long been attributed to depletion of Cr from regions of the alloy matrix adjacent to grain boundaries in which had precipitated. Those regions of the steel in which the local Cr composition falls below about 12% have a diminished ability to form a passive film and hence corrode preferentially.We have made thermodynamic calculations of the equilibria in and near grain boundaries in order to determine the chromium content in the vicinity of carbide particles. The calculations show that the equilibrium chromium content is a strong function of the temperature and of the carbon and alloy content of the steel. It is inferred from the analysis that variations in the susceptibility to intergranular attack are determined primarily by changes in the equilibrium chromium content near the carbides and not by changes in the number and distribution of particles. For instance, above 800°C the chromium content near the carbides is high enough to produce passivity, and the alloy is immune not because of the absence of carbides but because chromium depletion is not severe. Experimental studies of rates of grain boundary attack as functions of temperature and composition have confirmed the predictions of the analysis.A further factor of less importance is the distribution of carbides in the grain boundary. Calculations were made to determine the Cr concentration gradient within the boundary between carbide particles as a function of temperature and composition as well as the Cr gradient normal to the boundary. These calculations predict a strong interdependence between susceptibility to intergranular corrosion, carbide particle spacing, and sensitizing temperature. Experimental results on thinned samples of sensitized material corroded in a Strauss solution and examined by electron microscopy are in good agreement with predicted effects.

Electromigration and failure in electronics: An introduction

Abstract: Some fundamental aspects of electromigration phenomena as they have been studied in "bulk" metallic conductors are reviewed. In an electric field atoms are subjected to a force due to the field, and to a force which results from the motion of electrical carriers, electrons, or holes. In bulk samples, and at high temperatures, these forces cause the displacement of atoms by a lattice mechanism which is also responsible for the diffusion of atoms in a concentration gradient. In thin films, electromigration has been found to occur at lower temperatures (and higher current densities) by a grain boundary diffusion mechanism. Electromigration may cause failures at material discontinuities, such as found at terminals, at temperature gradients, or at structural inhomogeneities. The process of crack formation, as observed in aluminum thin films, is described. Failure times are a function of the activation energy for diffusion and of exponents of the current density which vary for different failure modes. The effects of film purity, orientation, grain size, glass overcoat, and solute additions on lifetime are reviewed. Practical guidelines for the design of thin-film interconnections, and for the interpretation of accelerated test data are given.

Computer Calculations of the Structure and Energy of High‐Angle Grain Boundaries

Abstract: A method that has been developed to obtain the geometry of a grain boundary associated with a minimum internal energy is described in detail. A Morse potential is employed to represent the interatomic forces. The procedure was applied to a series of boundaries of the coincidence orientations. The resulting structures are consistent with experimental observations.

Grain-boundary sliding and diffusion creep in polycrystalline solids

Abstract: The diffusion creep of polycrystalline materials must be accompanied by grain-boundary sliding in order that material continuity be preserved. Since both grain-boundary sliding and the addition of atoms to a boundary between two grains result in a relative translation of the grains, it is possible to regard the material as deforming by the motion of its constituent crystals. A detailed analysis of the grain-boundary sliding occurring during diffusion creep in simple tension has been made, based on this point of view. It is concluded that the relative velocities of displacement by grain-boundary sliding and by diffusion at a particular point on a grain boundary are fixed by the geometry of the grains. The relationship between these relative displacement velocities is found and it is concluded that sliding is occurring at most grain boundaries in a normal polyerystalline material undergoing diffusion creep. It is also shown that sliding on most boundaries makes a positive contribution to specimen s...

Quench rate effects in Al-Zn-Mg-Cu alloys

Abstract: The effect of quench rate on alloys with a bcomposition of 6.75 pct Zn, 2.5 pct Mg, 1.2 pct Cu and various levels and combinations of the minor addition elements (MAE’s) Cr, Mn, and Zr is reported. The level of chromium and to a lesser extent the level of zirconium determine quench sensitivity. Slow quenching of chromium- or zirconium-bearing alloys led to the nucleation of (MgZn) plates or laths at MAE-bearing particles which had certain orientation relationships with the matrix. The density of the hardening precipitation (100 to 200A in diam) was found to be high in slowly quenched, sensitive alloys. Even when the yield strength had fallen to 50 pct of that of the fast-quenched material, a large fraction of the fine precipitate was still formed. A fracture mechanism is proposed to account for the magnitude of quench sensitivity in such materials. An embrittlement, at low quench rates, was found in alloys containing manganese or very low levels of MAE. This embrittlement was thought to be due to precipitation in the grain boundaries, and not to the width of the precipitate-free zone.

Temper embrittlement in a Ni-Cr steel containing phosphorus as impurity

Abstract: Phosphorus segregates to prior austenite grain boundaries in ferrite during embrittling treatments (below 1000°F) and contributes to temper embrittlement of a Ni-Cr-C-P steel. The phosphorus is confined to a width of 5 to 10A, at a very high concentration of approximately 5 wt pct (0.1 monolayer) in the vicinity of the grain boundaries; segregation of nickel is also observed to a lesser degree and spread over a distance somewhat larger than 50a. Nickel segregation to the grain boundaries appears to be of minor consequence either in terms of its own effect or in terms of its interaction with phosphorus. Fracture appearance transition temperatures of the various samples are directly correlatable to the phosporous content at grain boundaries through a linear plot. Deembrittlement of the steel is found to consist of reversing the boundary segregation and driving the phosphorus and nickel away from the prior austenite boundaries. The amount of phosphorus segregated at grain boundaries of the embrittled Ni-Cr-C-P steel is found to be compatible with an equilibrium segregation model.

A study of grain-boundary grooving at the platinum/alumina interface

Abstract: Grain-boundary grooving at the interface between solid platinum and solid alumina has been studied and its relevance to spheroidisation and particle coarsening noted. Analysis of the equilibrium shape of the grooves in conjunction with zero creep studies on platinum shows that the platinum/alumina interfacial energy is 1050 ergs/cm2 at 1400° C. The rate of growth of the grooves shows that volume diffusion is the operative mass transport mechanism at all temperatures considered (1240 to 1540° C); however, there are indications that diffusion through the metal is rate-determining at lower temperatures (< 1410° C) while diffusion through the ceramic phase predominates at higher temperatures.

Influence of Grain Growth on Dielectric Properties of Nb‐Doped BaTiO3

Abstract: Effects of grain size and grain growth in Nb-doped BaTiO3 on temperature and frequency dependencies of the dielectric constant were investigated. When 0.65 μm powder is sintered to an average grain size of 1 μm, two dielectric constant peaks indicate the presence of Nb-free BaTiO3 and of Nb-containing material. Single peaks are observed above room temperature after additional grain growth or when 0.07 μm powder is sintered to an average grain size of 1 μm. The Curie point of pure BaTiO3 with 1 μm grains is 4 to 6°C lower than that of material with grains >10 μm. Thermodynamically, this behavior is accounted for by a phase inversion stress ∼ the room-temperature stress.

Room-Temperature Recrystallization in Thick Bias-Sputtered Copper Deposits

Abstract: Deposits of copper up to 1 mm thick were formed at room temperature in a triode sputtering apparatus using a krypton discharge under various conditions of sputtering rate, gas purity, and substrate bias. Room‐temperature recrystallization and grain growth displaying no twins were observed within 9 h after removal from the sputtering apparatus. This resulted in a dominant orientation of {100} planes parallel to the substrate‐deposit interface. Recrystallization rate increased as a function of increasing deposition rate, gas purity, and substrate bias. It was hypothesized that high intrinsic stress levels in the deposit provided both vacancies and the driving force for their diffusion to grain boundaries. Grain boundaries composed of {100} planes then adsorbed vacancies and moved by a combination of climb and glide. This movement was made possible by the elimination of barrier impurities, primarily oxygen, with the negative bias applied to the substrate during sputtering.

Extraneous grain boundary dislocations in low and high angle (001) twist boundaries in gold

Abstract: A study of extraneous grain boundary dislocation (GBD) structures in low and high angle (001) twist boundaries in gold was carried out by transmission electron microscopy using bicrystal specimens prepared by welding evaporated single crystal thin films together face-to-face GBD structures were observed which could be explained by the introduction of dislocations with lattice Burgers vectors ±a/2[101], ±a/2[101], ±a/2[011], or ±a/2[011] into the boundaries and the subsequent interactions of these dislocations with the networks of misfit screw GBD's which are associated with the intrinsic equilibrium structure of the twist boundaries (Schober and Balluffi 1970 b) In many cases the interactions in the boundary essentially eliminated the components of the Burgers vectors of the extraneous GBD's in the plane of the boundary and produced effective Burgers vectors almost (or exactly) perpendicular to the boundary In all cases the components normal to the boundary of the effective Burgers vectors wer

The interrelationship of fracture toughness and microstructure in a Ti-5.25Al-5.5V-0.9Fe-0.5Cu alloy

Abstract: Fracture toughness of anα-Β titanium alloy heat treated to a constant yield strength has been found to depend upon the morphology of α produced or remaining after the initial solution treatment. In equiaxed α structures, fracture toughness depends linearly upon the grain boundary area per unit volume,Sv, and is independent of equiaxed α particle size or spacing. In a grain boundary α structure fracture toughness depends both onSv and, within limits, linearly on the thickness of the α. Explanations are offered for the observed propagation of cracks at α-@#@ Β interfaces and for the observation that high fracture toughness can accompany low tensile ductility.

Effects of Grain Growth on the Distribution of Nb in BaTiO3 Ceramics

Abstract: Measurements of Nb diffusion into large- and small-grained BaTiO3 disks show a high ratio of grain boundary to bulk diffusivity. Well defined X-ray diffraction lines are found in Nb-doped BaTiO3 only when significant grain growth occurs during sintering. When grain growth of a 0.65 μm grain size powder is limited at ∼1 μm, excess line broadening results. This is attributed to the simultaneous presence of Nb-free and Nb-rich regions. Because of its low bulk diffusivity little Nb penetrates into the original BaTiO3 grain cores, and a solid solution forms only in the regions of recrystallization. When grain growth is limited, the “sintering reaction” results in a non-homogeneous system; when appreciable grain growth occurs, most of the original grain cores are eliminated and an essentially uniform system is obtained.

The four stages of grain growth

Abstract: A study of grain growth in dilute cadmium and lead base alloys has revealed that there are four stages of grain growth as a function of annealing temperature and solute concentration. One of these stages, which can result in a substantial change in grain size, involves irregular grain growth or secondary recrystallization.

A study of stress-corrosion in Al-Zn-Mg alloys

Abstract: This study of Al-Zn-Mg alloys was undertaken to establish the specific combination of mechanical and thermal conditions which is necessary for stress-corrosion susceptibility to occur in these alloys. The work included a study by electron transmission microscopy of the microstructure of this alloy system. A new mechanism is proposed to account for the observations made. It suggests that the high corrosion potential at grain boundaries in stress-corrosion susceptibile Al-Zn-Mg alloys is due to segregation of zinc and magnesium. By taking precautions to reduce this segregation, the stress-corrosion life can be increased.

The relation between grain-boundary orientation and intergranular cracking.

Abstract: The role of grain and grain-boundary orientation on the susceptibility of a boundary to intergranular cracking was studied in purified iron. Cracks were introduced by cathodic charging with hydrogen at room temperature, in the absence of an externally applied stress. In order to analyze the data, a new graphical method was devised which successfully represented the degrees of freedom necessary to describe the boundary orientation. The results showed that boundaries with angular misorientation of up to about 20 deg never exhibited cracking, independent of the boundary type or the common rotation axis. At higher angles, large regions of cracked, uncracked, and mixed behavior were observed. The results were rationalized on the basis that boundary porosity,i.e. the degree of atomic misfit, controls the cracking susceptibility of a grain boundary. The effects of interstitial solute content and distribution on the frequency of grain-boundary cracking were also studied.

Dislocation structure and contrast in high angle grain boundaries

Abstract: The dislocation aspects of the earlier model of Bishop and Chalmers for the structure of high angle symmetric tilt boundaries in metals is extended to twist and more general boundaries having both tilt and twist components. The expected strain contrast in high angle grain boundaries is discussed. It is concluded that strain contrast effects will be observed whenever there is not a long-range cancellation of the strain field of the boundary because of perturbations in the primary dislocation array, provided that the perturbations are sufficiently strong and widely spaced. Two types of contrast are discussed. One will occur at near-coincidence orientations due to perturbations in the periodicity of the primary dislocation array. This can be described in terms of secondary dislocations. The second is the result of a small tilt or twist about a second axis superimposed on an otherwise simple tilt boundary.

Electrical Properties of Single Crystals, Bicrystals, and Polycrystals of MgO

Abstract: The high oxygen pressure conductivity of high-purity single-crystal magnesium oxide at high temperatures varied as the 1/4 power of the oxygen partial pressure with a 3 eV activation energy; the low pressure conductivity varied as the -1/6 power of the pressure with an activation energy of 4 eV. The predominant defects proposed are (1) holes and singly ionized Mg vacancies at high pressure and (2) electrons and doubly ionized oxygen vacancies at low pressures. The effect of impurities was noted. Changes in the grain boundary conductance relative to the crystal conductance of MgO were observed as a function of temperature and pressure, but not as a function of grain boundary orientation, transport direction, or total impurity content. Increased high pressure ionic conductivity in polycrystalline material offered possible evidence for increased grain boundary diffusion.