Showing papers on "Grain boundary published in 1980"

Dopant segregation in polycrystalline silicon

Abstract: Dopant segregation at grain boundaries in polycrystalline silicon has been investigated. Arsenic, phosphorus, and boron were ion implanted into low‐pressure, chemically‐vapor‐deposited polycrystalline‐silicon films. All films were then annealed at 1000 °C for 1 h, and some were subsequently further annealed at 800, 850, or 900 °C for 64, 24, or 12 h, respectively. For phosphorus and arsenic the room‐temperature resistivity of the films was found to be higher after annealing at lower temperatures. By successively annealing the same sample at lower and higher temperatures, the resistivity would repeatedly increase and decrease, indicating reversible dopant segregation at the grain boundaries. Hall measurements were used to estimate the number of active dopant atoms within the grains and the number of atoms segregated at the grain boundaries as a function of annealing temperature. A theory of segregation in systems of small particles has been developed. Using this theory, the heat of segregation of arsenic and phosphorus in polycrystalline silicon was calculated. For boron no appreciable segregation was observed.

Plastic Deformation of Fine‐Grained Alumina (Al2O3): I, Interface‐Controlled Diffusional Creep

Abstract: Plastic deformation in fine-grained alumina polycrystals (grain size 1 to 15 μm) was studied. At least three distinct deformation mechanisms are important: diffusional creep, basal slip, and unaccommodated grain-boundary sliding. The first and most important of these processes is addressed in this paper. Analysis of the deformation dynamics suggests that both lattice and grain-boundary diffusion are important in the diffusional creep. Aluminum, rather than oxygen, lattice and grain-boundary diffusion are rate-controlling because oxygen diffusion is very slow in the lattice but very rapid in grain boundaries. Significantly, the diffusional creep can become interfacecontrolled at low stresses, causing the often-reported non-Newtonian creep behavior of fine-grained alumina.

Niobium carbonitride precipitation and austenite recrystallization in hot-rolled microalloyed steels

Abstract: The response of austenites to thermomechanical treatments is studied in a series of niobium (columbium) HSLA steels. Interactions between composition, plastic deformation, strain-induced precipitation, and austenite recrystallization are described and related to previous work in the field. Niobium in solution prior to deformation leads to significant retardation of subsequent austenite recrystallization if Nb(C,N) precipitation takes place prior to or during the early stages of recrystallization. Such straininduced precipitation proceeds in two stages: initially at austenitic grain boundaries and deformation bands, and later on substructural features in the unrecrystallized austenite. The latter precipitation is accelerated only if it occurs in the unrecrystallized austenite; if recrystallization precedes Nb(C,N) precipitation, then the precipitation reaction is much slower. Thus, the Nb(C,N) precipitation and austenite recrystallization reactions are coupled phenomena. The conditions necessary for such an interaction are analyzed, and it is proposed that the level of supersaturation of Nb(C,N) in the austenite at the deformation temperature is a critical factor in determining whether or not an effective interaction will operate at that temperature.

Theory of the electrical and photovoltaic properties of polycrystalline silicon

Abstract: Grain boundary states play a dominant role in determining the electrical and photovoltaic properties of polycrystalline silicon by acting as traps and recombination centers. The recombination loss at grain boundaries is the predominant loss mechanism in polycrystalline solar cells. Cell parameters are calculated based on a transformation of grain boundary recombination centers to a uniform distribution of such states throughout the grain. Effective carrier lifetime is expressed in terms of grain size, allowing calculation of short‐circuit current, open‐circuit voltage, and fill factor. Excellent agreement is observed between theory and experiment for almost all device parameters. It is indicated that one could fabricate 10% efficiency polycrystalline solar cells from 20‐μm‐thick material if the grain size exceeds 500 μm.

Microstructure and normal grain growth in metals and ceramics. Part I. Theory

Abstract: By extending the theory of Feltham and combining it with the work of Rhines and Craig, a detailed statistical theory of normal grain growth has been constructed. The theory exhibits all four attributes of normal grain growth: uniformity, scaling, stability, and lognormality. A prime new feature of the theory is the division of the grains into topological classes (14 planar, 34 spatial), each with a lognormal distribution of grain sizes. Growth is found to be controlled by the rate of loss of grains from the lowest topological class. Complete solutions are found for the grain growth kinetics of each class, as well as the transfer rates between classes. The latter result is used to explain how the median diameter of those classes in which grains are shrinking still manages to increase in the manner required to keep their number a constant fraction of the total population. A parabolic growth law is found for the median grain size of the whole population as well as the median grain size in each topological class. The growth constant for each class is found to increase approximately as the cube of the planar topological parameter or the square of the spatial topological parameter. The Rhines‐Craig structural gradient is shown to be independent of time and hence a basic constant of normal grain growth. Stability is due to a maximum in the grain boundary velocity with increasing grain size. The ratio of the maximum to median grain diameter is found to be e(=2.718). A comparison of the present theory is made with that of Hillert. Possible origins of the lognormality are discussed.

Grain boundary segregation

Abstract: Equilibrium grain boundary segregation is assessed in terms of its measurement, theory and industrial significance. Aspects of the phenomenon of this segregation, namely the localisation, chemical system dependence, response to environmental parameters, etc., are discussed in relation to the measurement techniques for grain boundary segregation. These techniques-the interfacial energy approach, AES, XPS, STEM with X-ray analysis and the atom-probe FIM-are analysed in terms of their unique contributions. The theories of segregation are presented in a unified framework which uses analogues of the theories for gas adsorption on free surfaces. The cases of limited and unlimited numbers of sites are treated. Thus, McLean's segregation theory is seen to be the counterpart of Langmuir adsorption and a self-interacting segregant is seen to follow the analogue of the Fowler theory. The Fowler theory is extended to ternary systems and accurate methods of estimating the value of the relevant fixed parameters are presented. The kinetics of segregation and desegregation are then outlined. A range of material aspects affected by grain boundary segregation are described.

Hydrogenation of transistors fabricated in polycrystalline-silicon films

Abstract: Transistors have been fabricated with their active channels in thin films of polycrystalline silicon. A subsequent hydrogen plasma treatment has been used to improve the transistor properties significantly by reducing the number of electrically active grain-boundary defects. Plasma conditions to maximize the hydrogenation effect have been briefly investigated.

Compressive creep of Si 3 N 4 /MgO alloys

Abstract: Highly localized strain fields are observed at grain boundaries in crept specimens of Si3N4/MgO alloys which were frozen under stress. These fields disappear upon annealing. Unresolved asperities between the grain pairs appear to give rise to the strain field during deformation. Viscoelastic effects responsible for primary creep and strain recovery are explained in terms of grain-boundary sliding on the glassy interphase which is accommodated by the elastic strain arising at the asperities. Each boundary containing an asperity can be modelled as a simple Kelvin element. The spectrum of these boundaries within the bulk gives rise to a spectrum of relaxation times that is observed for the strain recovery effect. The highly stressed region at the asperity also gives rise to the higher chemical potential required to drive diffusional creep. Although the source of the asperities was not observed, the possibility of opposing ledges of either single or multiple interplanar height is discussed.

Grain Boundary Segregation and Intergranular Fracture in Molybdenum

Abstract: The refractory group VIA metals generally exhibit intergranular brittleness when they are in the recrystallized condition. This causes severe problems in their fabrication and places major limitations on their practical application. The phenomenon, generally referred to as recrystallization embrittlement, results in large increases in the ductile-to-brittle transition temperature and a change in fracture mode in the lower shelf regime from cleavage to intergranular with a significant decrease in ductility. The embrittlement is widely considered to be associated with interstitial impurities but there have been few systematic studies to elucidate their effects. The present paper reports results from a systematic study of segregation and intergranular embrittlement in binary molybdenum-oxygen and ternary molybdenum-oxygen-carbon alloys. The experiments were carried out on ‘bamboo’ specimens containing a series of identical single grain boundaries traversing their cross-sections. Measurements have been made of the activation energy for oxygen segregation to grain boundaries in the binary molybdenum-oxygen alloys. The influence of carbon additions on the level of oxygen segregation has also been determined. In addition, the influence of oxygen segregation on the energy to fracture has been studied and this has involved quantitative measurements of the work of fracture and the contribution made by plastic deformation. Results from metallographic studies are also presented, showing the effects of segregation on fracture surface topography and dislocation structures immediately adjacent to the fracture surfaces. In discussing the results we consider the thermodynamics of oxygen segregation to grain boundaries and the role played by carbon in inhibiting segregation. It is proposed that carbon either increases the effective solubility of oxygen in molybdenum or acts as a trap for oxygen atoms. In either case the effect is to reduce the driving force for segregation. We also consider the influence of segregation on the work of fracture and show that the reduction in oxygen segregation resulting from the addition of carbon produces small increases in fracture energy. This increases the local stress to propagate a crack sufficiently to promote plastic deformation which blunts the crack tip. The consequent change in geometry reduces the stress concentration at the crack tip, thereby resulting in a large increase in the applied fracture stress and the work to fracture.

The effects on grain-boundary processes of the steps in the boundary plane associated with the cores of grain-boundary dislocations

Abstract: The contribution of the associated steps to the properties of grain-boundary dislocations are investigated. Two methods of step-height determination are given and it is shown that the step height, like the Burgers vector, must be conserved during the reactions of grain-boundary dislocations: this is not always automatic and additional coherent steps must take part in some reactions. The energies associated with steps have been estimated and compared with the elastic energies of grain-boundary dislocations; these comparisons suggest that (a) the reduction of elastic energy alone may not be a sufficient criterion for the progress of a reaction and (b) that arrays of dislocations may reduce their energy by containing dislocations embodying steps of varying height.

A quantitative metallographic assessment of structural degradation of type 316 stainless steel during creep‐fatigue

Abstract: — Optical and scanning electron microscopy have revealed the existence of grain boundary cavities in a series of specimens which had been cyclically deformed in the strain range ±0.25% with hold times ranging from 0 to 1000 min. The way in which these defects increase in size and number has been measured and found to correlate with the creep strain accumulated during the hold periods. A further observation is that a critical amount of deformation is required before either fatigue or creep type damage is nucleated. These observations lead to an alternative method to the currently accepted linear damage summation rule for estimating a lower bound of the creep-fatigue endurance.

Effects of grain boundaries in polycrystalline solar cells

Abstract: In polycrystalline solar cells, the short-circuit current is reduced (compared to single-crystal cells) due to recombination of minority carriers at the grain boundaries. The magnitude of this reduction is calculated for the case of a monochromatic beam of light, assuming that both the light beam and the grain boundaries are perpendicular to a p-n junction. The photoresponse of the junction is calculated in terms of the absorption coefficient of the light, the minority-carrier diffusion length, the surface recombination velocity at the grain boundary and the distance of the light beam from the grain boundary. The analysis gives an expression for an effective grain boundary width which depends on wavelength. This width determines the reduction in short-circuit current of solar cells made from material with columnar-type grains.

In situ fracture experiments in b.c.c. metals

Abstract: Crack propagation and the structure of the plastic zone formed ahead of a crack have been investigated during in situ electron microscope fracture experiments of the b.c.c. metals molybdenum and tungsten. Most frequently, the cracks in polycrystalline specimens propagated along the grain boundaries. In some cases, the cracks were preceded by thin twins. When the cracks propagated transgranularly, the plastic zone ahead of the cracks consisted of an inverse pile-up of dislocations. In these cases, the relationship between the plastic zone size and the crack tip displacement was compared with the predictions of the theory of Bilby, Cottrell and Swinden (BCS). It is concluded that the plastic zone size agrees with the predictions of the BCS theory if the theory is modified to include the surface effects in thin foils.

Morphological changes of carbides during creep and their effects on the creep properties of inconel 617 at 1000 °C

Abstract: Creep tests have been correlated with microstructural changes which occurred during creep of Inconel 617 at 1000 °C, 24.5 MPa. The following results were obtained: 1) Fine intragranular carbides which are precipitated during creep are effective in lowering the creep rate during the early stages of the creep regime (within 300 h). 2) Grain boundary carbides migrate from grain boundaries that are under compressive stress to grain boundaries that are under tensile stress. This is explained in terms of 1 the dissolution of relatively unstable carbides on the compressive boundaries, 2 the diffusion of the solute atoms to the tensile boundaries and 3 the reprecipitation of the carbides at the tensile boundaries. The rate of grain boundary carbide migration depends on grain size. 3) M23C6 type carbides, having high chromium content, and M6C type carbides, having high molybdenum content, co-exist on the grain boundaries. M23C6 type carbides, however, are quantitatively predominant. Furthermore, M6C occurs less frequently on the tensile boundaries than on the stress free grain boundaries. This is attributed to the difference of the diffusion coefficients of chromium and molybdenum. 4) The grain boundaries on which the carbides have dissolved start to migrate in the steady state creep region. The creep rate gradually increases with the occurrence of grain boundary migration. 5) The steady state creep rate depends not so much on the morphological changes of carbides as on the grain size of the matrix.

On the mechanisms of grain boundary migration

Abstract: Grain boundary dislocations and steps may have a major role in migration. The perfect grain boundary dislocations discussed in the present work can have steps at their cores and the step height depends on the dislocation and the boundary concerned. A model for migration is developed and it is correspondingly sensitive to the grain boundary crystallography. The problems of shear and long-range diffusion inherent in this model govern the details of the postulated dislocation processes and diffusion paths. There is direct evidence for migration by both dislocation and step mechanisms from in situ TEM heating experiments. Deformation associated with migrating boundaries in the absence of an external applied stress is interpreted in terms of the dislocation model. Movement of grain boundary dislocations, which are analogous to twinning dislocations, gives deformation and boundary migration. In the dislocation model, diffusion creep and boundary migration are related phenomena at a mechanistic level.

Direct Examinations of PTC Action of Single Grain Boundaries in Semiconducting BaTiO3 Ceramics

Abstract: R-T and I-V characteristics of single grains and grain boundaries in large-grained BaTiO3 PTC ceramics were studied with a two-probe technique using a micromanipulator and fine Al wire. The PTC originates in the grain boundary only and behaves differently in each boundary. Even below Tc, the ceramic resistance depends almost entirely on the boundary. I-V characteristics in the boundary follow Ohm's law and conduction by a space-charge-limited current with a trap, using different applied voltages. The PTC anomaly relates to activation of the trap in the boundary, not to barrier height. A band model in the intergranular layer, with dielectric BaTiO3 and the trap, is proposed.

The Dependence of Strength‐Controlling Fracture Energy on the Flaw‐Size to Grain‐Size Ratio

Abstract: The transition from single-crystal to polycrystalline fracture energies was studied as a function of the flaw-size to grain-size ratio by two methods. The primary method was calculating fracture energies from observed flaw sizes found at fracture origins in strength-test specimens. Some measurements were also made by varying the number of grains across the web in the applied-moment DCB test. Both methods agreed and generally snowed the transition to polycrystalline fracture energies being completed at flaw-size to grain-size ratios of ∼1 to ∼ 6 for the cubic materials studied. It is estimated that cracks less than ∼½ to ¼ of the grain size cannot be arrested at grain boundaries and that single-crystal fracture energies can be applied below this limit. The grain-size range over which this fracture-energy transition occurs was shown to be a function of extrinsic factors, such as texture, as well as intrinsic factors, such as the number and multiplicity of low-energy single-crystal fracture planes.

The noncontinuum crack tip deformation behavior of surface microcracks

Abstract: The crack tip opening displacement (CTOD) of small surface fatigue cracks (lengths of the grain size) in Al 2219-T851 depends upon the location of a crack relative to the grain boundaries. Both CTOD and crack tip closure stress are greatest when the crack tip is a large distance from the next grain boundary in the direction of crack propagation. Contrary to behavioral trends predicted by continuum fracture mechanics, crack length has no detectable effect on the contribution of plastic deformation to CTOD. It is apparent from these observations that the region of significant plastic deformation is confined by the grain boundaries, resulting in a plastic zone size that is insensitive to crack length and to external load.

The early stages of development of α-Al2O3 scales on Fe-Cr-Al and Fe-Cr-Al-Y Alloys at high temperature

Abstract: The development ofthe oxides on Fe-14%Cr-4%Al, Fe-27%Cr-4%Al, and similar alloys containing 0.008% Y, 0.023% Y, and 0.8% Y has been investigated during the early stages of oxidation in 1 atm oxygen at 1000 and 1200°C. In all cases, a steady-state α-Al2O3layer is established rapidly, after some initial formation of transient oxides rich in iron and chromium. For the yttrium-free alloys the steady-state situation is achieved more rapidly for the higher chromium-containing alloy and at the higher temperature. The amount of transient oxide formed is also determined by the specimen surface topography since the development of the α-Al2O3 layer is less rapid at the base of alloy asperities than at a flat alloy-oxide interface. Following establishment of the complete α-Al2O3layer, the oxide develops a convoluted oxide morphology at temperature, due to high compressive growth stresses in the oxide. These arise following reaction between oxygen ions diffusing inward down the oxide grain boundaries and aluminum ions diffusing outward through the bulk oxide. This results in lateral growth of the oxide and plastic deformation and movement of the alloy in a direction parallel to the alloy-oxide interface. The addition of yttrium to the alloys promotes the selective oxidation of aluminum. Also, the yttrium is incorporated into the growing oxide where it changes the mechanism of growth, reducing the production of the high compressive growth stresses and thus the development of the convoluted oxide morphology.

Deformation of fine-grained aluminium alloys

Abstract: The deformation of an ultrafine-grained aluminium alloy has been examined in tension and torsion. At grain sizes below about 3 μm the alloy exhibits inhomogeneous yielding but this is absent at larger grain sizes. If the lower yield strength values are plotted versus grain size the strengths are inversely dependent on grain size whereas the usual plot versus d −½ is non-linear and shows an enhanced strength at the finer grain sizes. However, if the inhomogeneous yielding region is avoided by extrapolating the work-hardening portion of the curve back to the elastic line, all the data can be well represented by the Hall–Petch relationship. Torsion tests, which allow the investigation of a large strain range, show that. the gredient of the grain size plots decreases slowly wIth strain but that the grain boundaries remain effective barriers to flow at strains up to at least 1.0. The deformation behaviour does not appear to agree with the current models for the influence of grain size on the flow stre...