Showing papers on "Grain boundary published in 1983"

On the Structure of Tilt Grain Boundaries in Cubic Metals I. Symmetrical Tilt Boundaries

Abstract: The results of the study of symmetrical tilt boundaries, reported in the preceding part I, are generalized to asymmetrical tilt boundaries. A classification of tilt boundaries in cubic crystals is developed that reveals which boundaries to choose in order to study equilibrium faceting or intrinsic grain boundary dislocations (g.b.ds) accommodating a misorientation. Two series of atomistic studies of asymmetrical tilt boundary structures are presented based on this classification. The first is a study of long-period (27 ^ 97) [110] asymmetrical tilt boundaries in aluminium. The aims of this study are to investigate whether these boundaries are composed of fundamental structural elements, in the same way as was found in part I for symmetrical tilt boundaries, and to see if localized, distinct stress fields of edge g.b.ds exist throughout the misorientation range. With use of the results of this study, and the principle of continuity of boundary structure enunciated in part I, the boundary unit representation of a 27 — 1193 asymmetrical tilt boundary is derived as an example. It is generally found that the Burgers vectors of intrinsic secondary g.b.ds in tilt boundaries, based on favoured boundary reference structures, are non-primitive d.s.c. vectors. The reason for this is given and a simple formula is presented to derive the Burgers vectors of such dislocations for any favoured tilt boundary reference structure. It is pointed out that, in general, very low angle {0

The A‐C Conductivity of Polycrystalline LISICON, Li2 + 2x Zn1 − x GeO4, and a Model for Intergranular Constriction Resistances

Abstract: The a‐c conductivity of a polycrystalline, Li+ ion conducting solid electrolyte, LISICON, , has been measured over the frequency range 10−3–107 Hz and attention focused on the methods of data analysis. The data show intragranular (bulk) and grain boundary effects in which, for a given sample, both effects are characterized by a similar activation energy; a constriction resistance model for the latter is proposed based on air gaps existing in polycrystalline sinters of less than theoretical density. The data only crudely fit a Debye‐like equivalent circuit, composed of frequency independent resistors and capacitors, but fit well an equivalent circuit that contains Jonscher elements, i.e., frequency dependent admittances, in addition to normal and elements. Two such Jonscher elements are needed, one for the grain boundary and one for the intragranular contribution. With this circuit, departures from ideality in, e.g., the cpmplex impedance, admittance, and electric modulus planes are accounted for.

Theory of beam induced current characterization of grain boundaries in polycrystalline solar cells

Abstract: A theoretical analysis is given of the induced current profiles at grain boundaries in polycrystalline solar cells, as obtained by light or electron beam excitation. The area A and the variance σ2 of the contrast profile of a grain boundary are calculated for realistic generations as functions of the interface recombination velocity vs and the minority carrier diffusion length L. A graphical new procedure is proposed which allows the simultaneous determination of vs and L from the measured values of A and σ. The evaluation of an experimental electron beam induced current profile illustrates the applicability of the theory.

Density of gap states of silicon grain boundaries determined by optical absorption

Abstract: The results of optical absorption measurements on fine‐grain polycrystalline‐silicon thin films indicate that the singly occupied dangling silicon bond lies 0.65±0.15 eV below the conduction‐band minimum in the grain boundary. The grain boundary band gap is ∼1.0 eV and there is evidence for exponential tailing of the band edges. The optical absorption was determined by photothermal deflection spectroscopy. The dangling silicon bond density has been measured on polycrystalline‐silicon thin films as a function of hydrogen passivation of the grain boundaries and on silicon‐on‐saphhire films. The optical absorption exhibits a defect shoulder which varies as the dangling bond density.

Grain boundary sliding and stress concentration during creep

Abstract: Importance of grain boundary sliding to creep intergranular fracture is focussed. Previous metallographic and fractographic studies of creep intergranular fracture on metal bicrystals and polycrystals are briefly reviewed in order to show the close relationship between grain boundary sliding and fracture. Deformation ledge and migration irregularity are shown to be potential sites of stress concentration and crack nucleation on sliding grain boundaries without particles. The effect of grain boundary structure on creep intergranular fracture is discussed on the basis of the effect of grain boundary structure on sliding, the contribution of sliding to the overall creep deformation, and a sliding-fracture diagram. Recent observations of the effect of grain boundary structure on creep intergranular fracture on alpha iron-tin alloy polycrystals are shown.

Microstructure, tensile deformation, and fracture in aged ti 10V-2Fe-3Al

Abstract: In the /3-Ti alloy Ti-10V-2Fe-3Al a variety ofα-andω-aged microstructures with different yield stresses was established by combinations of forging and heat treatment. Tensile tests have shown that plastic deformation and fracture are strongly influenced by the morphology, size, and volume fraction of the different types of a-phase (primary a, secondaryα, grain boundaryα), as well as by the-phase. A detailed microscopical study revealed several deformation and fracture modes. It appears that at several sites stress and strain concentrations and subsequent void nucleation can occur and that the quantitative combinations of the differentα-types determine which sites are active. The dominant deformation mode for the (α +gb) solution treated andα-aged conditions was a strain localization in theα-aged matrix leading to voids at the interface between aged matrix and primary a-phase. In case of theβ-solution treated andα-aged microstructures the grain boundaryα leads to a strain localization in the softα-film and to void nucleation at grain boundary triple points at low macroscopic strains. Based on the above mechanisms it is discussed in detail how varying size, volume fraction, and morphology of theα-phase affect the ductility. The embrittling effect ofω-particles can be largely reduced by a grain refinement.

Effect of initial grain size on dynamic recrystallization of copper

Abstract: Oxygen free high conductivity (OFHC) and electrolytic tough pitch (ETP) copper specimens of different grain sizes (10–800 μm) were tested at various constant true strain rates in a modified Instron testing machine. The temperature range investigated was 725–1075 K. The effect of initial grain size on the transition from multiple to single peak flow was studied. The dependence of the stable dynamic grain size on the temperature corrected strain rate and the steady state stress was determined, as were the density of twins and of deformation bands. The density of twins decreased as the initial grain size was increased and when the recrystallization mechanism changed from static to dynamic. The density of deformation bands increased markedly with initial grain size. As the deformation bands are preferential nucleation sites for new dynamic grains, the results indicate that the initial effective grain size D o *

On the Structure of Tilt Grain Boundaries in Cubic Metals. III. Generalizations of the Structural Study and Implications for the Properties of Grain Boundaries

Abstract: The results of atomistic calculations of long-period tilt boundaries, which were reported in the preceding parts I and II, are generalized and represented concisely by using two-dimensional lattices, called decomposition lattices. The basis vectors of a decomposition lattice characterize the two fundamental structural elements composing all boundaries in a continuous series of boundary structures. Conversely, the governing condition on the basis vectors is that the boundary structure can change continuously throughout the misorientation range between the boundaries represented by the basis vectors. On assuming that no discontinuous changes in boundary structure occur at non-favoured boundary orientations, and that all boundaries considered are stable with respect to faceting, the governing condition may be used to deduce selection rules for adjacent favoured boundaries and the existence of other favoured boundaries in the misorientation range between two given favoured boundaries. The necessary condition for a discontinuous change in boundary structure to be possible at a non-favoured boundary orientation is formulated. Various aspects of intrinsic and extrinsic grain boundary dislocations (g.b.ds) are treated. It is first shown that the observation of intrinsic g.b.d. networks in the transmission electron microscope does not necessarily imply that the reference structure, preserved by those g.b.ds, is a favoured boundary. Secondly, it is argued that extrinsic g.b.ds provide imperfect steps with Burgers vector components parallel to the boundary that do not exist in equilibrium high-angle tilt boundaries. Finally, an explanation of the physical basis of plane matching dislocations is proposed. A general classification of grain boundary properties is introduced that is based on the results of this investigation of grain boundary structure. It is argued that only properties, such as grain boundary diffusion, that depend exclusively on the atomic structure of the boundary core may be used to detect favoured boundaries. Favoured boundaries exist at those misorientations where such a property is continuous but its first derivative, with respect to misorientation, is not. Grain boundary diffusion, the energy against misorientation relation and grain boundary sliding and migration are then discussed.

Space charge, elastic field, and dipole contributions to equilibrium solute segregation at interfaces

Abstract: The interaction potentials between solute ions and grain boundaries in ionic solids are identified as (1) the electrostatic interaction between the charged solutes and grain boundaries, (2) the elastic energy due to the size misfit of solutes in the matrix, and (3) the dipole interactions between the solute‐vacancy dipoles and the electric field in the grain‐boundary region. We include these interaction potentials to evaluate the minimum free energy and the equilibrium solute and defect distributions in the grain‐boundary region. Numerical calculations show that these interaction mechanisms, either acting individually or coupling with each other, lead to a nonuniform solute distribution near the grain boundary. Under certain conditions, both the elastic and dipole interactions can significantly modify the electrostatic potential near the boundary. Calculations also show that the grain‐boundary segregation of an aliovalent solute can be induced or altered by another aliovalent solute of different size misfit with the matrix.

Microstructural development of sub-eutectoid aged MgO-ZrO2 alloys

Abstract: The microstructural development in fully and partially stabilized MgO-ZrO2 alloys during sub-eutectoid heat treatment at 1100° C, has been studied by optical and electron microscopy and powder X-ray diffraction in order to correlate the observed microstructures with the mechanical properties. The materials used were 14 mol % MgO-ZrO2 (Mg-CSZ) of eutectoid composition and 9 mol % MgO-ZrO2 (Mg-PSZ). In Mg-CSZ the decomposition reaction proceeds almost to completion before the maximum thermal up-shock properties are attained. For the Mg-PSZ material the most significant event leading to the enhanced mechanical properties appears to be the nucleation and growth of a long-range ordered anion vacancy phase within the cubic zirconia matrix. This, in turn, leads to the destabilization of the tetragonal ZrO2 precipitates also present in Mg-PSZ, causing some of them to acquire monoclinic intergrowths whilst others transform to the monoclinic form on cooling. Beyond about 4 h ageing at 1100° C the decomposition product in the grain boundaries began to influence the mechanical properties.

Properties of microcrystalline silicon. IV. Electrical conductivity, electron spin resonance and the effect of gas adsorption

Abstract: For pt.III see ibid., vol.16, p.2005 (1983). Measured values of the electrical conductivity, sigma , and electron spin density (g=2.0057) of microcrystalline silicon can be essentially determined by the extent of the contamination of the samples by oxygen unless special precautions are taken as regards the sample preparation and/or handling. For samples deposited at a floating potential, two kinds of oxygen incorporation are identified: irreversible formation of Si-O bonds on the grain boundaries (and on the sample surface) and a reversible absorption which is probably associated with a nondissociative O2delta - (ads) state. The latter results in a decrease of sigma RT by up to five orders of magnitude, an increase of the activation energy, epsilon a, and of the preexponential factor, sigma 0, as well as in an increase of the electron spin density. A reversible desorption of oxygen leads to an increase of sigma RT up to not less than about 10-2 Omega -1 cm-1 and a decrease of the EPR signal below the detection limit of less than 1016 cm-3. In order to avoid such effects a negative bias has to be applied to the substrate during deposition. Samples of undoped mu c-Si deposited in this way show neither the incorporation of oxygen into the bulk nor significant changes in the dark conductivity even after long-term exposure to air.

Crack and cavity nucleation at interfaces during creep

Abstract: Athermal nucleation of microcracks and thermal nucleation of cavities during creep deformation are reviewed with an emphasis on effects of solute segregation to grain boundaries and cavity surfaces. The magnitude and the duration of stress concentration at a triple grain junction or at a grain boundary inclusion are estimated for transient Coble creep and steady state power-law creep conditions. Stable configurations of wedge-type microcracks are predicted by a Griffith-like crack model. The rate for thermal nucleation of cavities is obtained by the Fokker-Planck equation for vacancy clusters. Cracks and cavities are interdependent, and cavity nucleation occurs continuously throughout the three creep stages. The local stress concentration enhances microcrack and cavity nucleation. The cavity nucleation rate is generally increased as a result of solute segregation to the surfaces and interfaces and/or gas precipitation into cavity volume. This enhanced nucleation is more effective in a system with mobile solutes than with immobile solutes. Immobile solute or trace elements may affect the nucleation rate also by changing the grain boundary diffusivity. Experimental techniques for quantitative analyses of cavity nucleation processes are discussed.

Effect of reheating on microstructure and toughness of C–Mn weld metal

Abstract: Samples of C–Mn weld metal have been reheated to maximum temperatures of 900 and 1250°C, using an induction heating technique, and the resultant changes in microstructure and mechanical properties have been assessed. A model is proposed for brittle failure in this weld metal involving initial plasticity in grain boundary ferrite and crack initiation at nonmetallic inclusions. Variations between the brittle fracture susceptibilities of as-welded and reheated materials, and the scatter in brittle toughness results, can both be qualitatively understood in terms of the model.

Optical properties and grain boundary effects in CuInSe2

Abstract: The optical properties of CuInSe2 thin films and single crystals are reported. The wavelength dependence of the refractive index and extinction coefficient is measured using multiple angle‐of‐incidence ellipsometry. Absorption coefficients as high as 6×105/cm are reported—the highest for any semiconductor. CuInSe2 is confirmed to have a room temperature, direct‐bandgap transition near 0.96 eV for the single crystals, and 1.02 for the thin films. The difference is proposed to be due to nonuniformity in composition of the polycrystalline material. Additional absorption in the low absorption‐coefficient regime is likely due to transitions associated with phonon absorptions for the single crystals. The effects of heat treatments in Ar, N, and O are shown to improve the optical properties of the thin films due to improvement in film compositional uniformity. Heating in high vacuum causes Se desorption near the film surface, causing a related degradation in the absorption characteristics.

Effects of grain boundaries on the channel conductance of SOl MOSFET's

Abstract: A physical model that describes the effects of grain boundaries on the linear-region (strong-inversion) channel conductance of SOI (polysilicon on silicon-dioxide) MOSFET's is developed and supported experimentally. The model predicts an effective turn-on characteristic that occurs beyond the strong-inversion threshold, and henceforth defines the "carrier mobility threshold voltage" and the effective field-effect carrier mobility in the channel, which typically is higher than the actual (intragrain) mobility. These parameters, which are defined by the properties of the grain boundaries, can easily be misinterpreted experimentally as the threshold voltage and the actual carrier mobility.

Grain boundary microstructures in a liquid-phase sintered alumina (α-Al2O3)

Abstract: Grain boundary microstructures in a commercial 99.8% alumina ceramic have been analysed using transmission electron microscopy. Nearly all boundaries are wet by an amorphous film which should invalidate both the CSL and DSC lattice constructions. Facets of widely differing sizes are observed primarily on basal {0001}, rhombohedral {1012}, and prism {1120} planes. The facets are always multiatomic in height and range up to ∼ 1000 A; this probably results from substantial anisotropy in the Al2O3-glass interfacial energy. Basal annealing twins are observed without amorphous films; they lie predominantly on {1120} and show interfacial energies much lower than adjacent random boundaries.

A conduction model for semiconductor-grain-boundary-semiconductor barriers in polycrystalline-silicon films

Abstract: A quantitative trapping model is introduced to describe the electrical properties of a semiconductor-grain-boundary-semiconductor (SGBS) barrier in polysilicon films over a wide temperature range. The grain-boundary scattering effects on carrier transport are studied analytically by examining the behavior of the height and width of a rectangular grain-boundary potential barrier. The model also verifies the applicability of a single-crystal band diagram for the crystallite within which an impurity level exists. Carder transport includes not only thermionic field emission through the space-charge potential barrier resulting from trapping effects and through the grain-boundary scattering potential barrier but also thermionic emission over these barriers. Thermionic emission dominates at high temperatures; however, at low temperatures, thermionic field emission becomes more important and the grain-boundary scattering effects are an essential factor. By characterizing the experimental data of the I-V characteristics, resistivity, mobility, and carrier concentration, this model enhances the understanding of the current transport in polysilicon films with grain sizes from 100 A to 1 µm, doping levels from 1 × 1016to 8 × 1019cm-3, and measurement temperatures from -176 to 144°C. The limitations of the model are also discussed.

Hydrogen-induced cracking in 4340-type steel: Effects of composition, yield strength, and H2 pressure

Abstract: Measurements of the threshold stress intensity for hydrogen-induced crack extension,Kth at room temperature were made on bolt-loaded WOL specimens of a commercial 4340 steel and of laboratory heats in which the bulk concentrations of manganese, silicon, phosphorus, and sulfur were varied. The hydrogen pressure was varied from 200 to 1600 torr (~0.03 to 0.22 MPa), and the yield strengths were varied from ~170 to 270 ksi (~1200 to 1900 MPa). Measurements ofKIc in air were also made as a function of composition and yield strength. Significant differences betweenKIc in air andKth in H2 were found only in steels containing added Mn or Si; these elements are believed to promote segregation of phosphorus and sulfur to austenite grain boundaries. TheKth values were uniquely related to the percentage of intergranular fracture and also to a parameter containing the calculated maximum hydrogen concentration and the bulk concentrations of manganese, silicon, phosphorus, and sulfur. In a high purity steel free of manganese and silicon theKth was lower thanKIc only at yield strengths greater than 200 ksi (1400 MPa). The results are consistent with an additive reduction in cohesive strength by hydrogen and metalloid impurities. It is shown that theKth depends on hydrogen fugacity, yield strength, and grain boundary purity(i.e., cohesive strength).

Solute segregation and hydrogen-induced intergranular fracture in an alloy steel

Abstract: Hydrogen-induced intergranular fracture of laboratory heats of a 3.5 Ni-1.7 Cr steel doped with P, Sn, or Sb and having a yield strength of 840 MPa and a prior austenite grain size of 120 μm has been compared with that of an undoped steel at a hydrogen pressure of 0.17 MPa (1.68 atm). The intergranular concentrations of the impurities were controlled by varying the time of aging at 480 °C. Cracking of the undoped steel tested in hydrogen occurred along martensitic lath boundaries at high stresses. However, the susceptibility of the doped steels to hydrogen-induced intergranular cracking increased precipitously with impurity concentration. The susceptibility was measured in terms of the threshold stress intensity Kthfor the first detectable crack extension in precracked specimens and in terms of the threshold stress σth for microcrack formation in notched specimens. A comparison between the intergranular strength in hydrogen and in air revealed that absorption of hydrogen produced a profound intergranular weakening when the grain boundaries contained even a small amount of a segregated embrittling element. The relative embrittling potencies of P, Sn, and Sb in hydrogen gas were the same as in air. The combined effects of hydrogen and the impurities in reducing intergranular cohesion are discussed in terms of a newly proposed dynamic model which takes into account the accumulation of hydrogen ahead of a moving microcrack.

Helium bubble nucleation at grain boundaries

Abstract: Helium bubble nucleation has been studied at grain boundaries in a ternary austenitic steel after helium implantation in the temperature range 450–600°C. Transmission electron microscopy shows that all interfaces except the coherent twin are preferred nucleation sites for bubbles, and that the density of bubbles is greater on interfaces containing resolvable grain boundary dislocations (GBDs), A model has been developed which accounts for the bubble density as a function of GBD spacing. This model implies that there is a positive binding energy between He and GBDs until the dislocation spacing becomes very small. The diffusion coefficient of helium along GBDa is much lower than that arising from interstitial diffusion in the lattice. It appears that helium migrates in grain boundaries and along GBDs by a slow vacancy-type mechanism.

On the structure of twist grain boundaries in ionic oxides

Abstract: Recent calculations on coincidence twist grain boundaries in oxides with the rocksalt structure have shown that the interfaces are only weakly bound. This is in contrast with experiment in which twist grain boundaries have been observed in MgO. The static lattice relaxation calculation presented here predicts a restructured stable [001] twist boundary in NiO. The results are equally applicable to the other oxides with the same structure.