Showing papers on "Grain boundary published in 1985"

High-temperature ordered intermetallic alloys

Abstract: This book contains over 50 selections. Some of the titles are: Order-disorder behavior of grain boundaries in a two-dimensional model ordered alloy; Dislocation reactions at grain boundaries in Ll/sub 2/ ordered alloys; Creep cavitation in a nickel aluminide; Effects of elastic anisotropy on the anomalious yield behavior of cubic ordered alloys; and Processing technology for nickel aluminides.

Grain boundaries in semiconductors

Abstract: This review presents the available experimental and theoretical understanding on the structure and electronic properties of grain boundaries in semiconducting materials. High-resolution electron microscope images of interfaces are interpreted within the framework of the structural unit model of grain boundaries, and the electronic properties of the grain boundaries discussed with relation to the popular symmetric Schottky barrier model for charge trapping and potential barrier formation. It is shown that these models give some limited understanding of the physical processes that occur at grain boundaries in elemental semiconductors, but that in compound semiconducting materials the effects of non-stoichiometry at the boundary regions must also be considered. Segregation of impurity and dopant species to the boundaries can have significant influence on their electrical properties, and the question of what structural or chemical features are responsible for the observed properties is posed. Diffusion at semiconductor grain boundaries is also discussed, and finally the electrical properties of zinc oxide varistor material are presented in the light of the models of carrier interactions with grain boundaries.

Defects and defect processes in nonmetallic solids

Abstract: An up-to-date discussion of defects in nonmetals emphasizing point defects and point-defect processes is presented The treatment encompasses electronic, vibrational, and optical properties of defective solids and some discussion of extended defects such as dislocations and grain boundaries Nonstoichiometry, fast-ion conduction, nonradiative transitions, and their consequences are considered, and radiation damage and enhanced diffusion, radiolysis, and the photographic effect are addressed A chapter on surfaces discusses surface reconstruction and surface electronic states and associated properties such as catalysis and corrosion Other topics examined include crystalline and amorphous materials and the significance of defects in metal-insulator transitions, intercalates, and polymers 463 references

Secondary grain growth in thin films of semiconductors: Theoretical aspects

Abstract: Secondary grain growth in thin films can lead to grain sizes much greater than the film thickness. Surface energy anisotropy often provides an important fraction of the driving force for secondary grain growth, especially in the early stages of growth. Surface‐energy‐driven secondary grain growth leads to the development of large grains with restricted crystallographic textures. A model is presented for growth of secondary grains into a uniform matrix of columnar normal grains. The model indicates that secondary grain growth rates should increase with grain boundary energy, surface energy anisotropy, grain boundary mobility, and temperature. While final secondary grain sizes will decrease with film thickness, their growth rates will increase. The final secondary grain sizes and orientations will be strongly affected by grain sizes and orientations in the initial film. The models presented here provide analytical tools for experimental study of secondary grain growth in thin films. They will be used in for...

The inhomogeneity of plastic deformation

Abstract: Considerable local strain differences occur in single crystals as a consequence of slip bands. A significant contribution to this phenomenon is the effect of the deformation rate on the inhomogeneity of its distribution. In polycrystalline materials the plastic deformation is affected by grain boundaries and their vicinity as well as by various orientations of single grains in onephase metals and alloys, and by various component properties in multiphase materials. In some cases it has been possible to describe these phenomena by means of micromechanics. Valuable information has also been acquired by means of three-dimensional stereology.

Polycrystalline strengthening

Abstract: The strength of polycrystalline specimens can be related to interaction phenomena taking place during elastic and plastic deformation. Such phenomena are reviewed in term of macroscopic and microscopic strain accommodation processes required to maintain strain continuity across the grain boundaries. The strength-grain size relationships can be described in a number of empirical equations relating the yield stress and the flow stress in tension to various structural parameters. A number of such equations are reviewed and their predictive capability is discussed. Structural information of importance for the understanding of polycrystalline strengthening is obtained mainly from surface relief patterns and from bulk structures observed by transmission electron microscopy of thin foils. The results obtained by these methods are discussed and correlations are proposed. A number of features characterizing the deformed structure are summarized and the behavior of a number of metals and alloys is reviewed with emphasis on the structural changes in the interior of the grains and in the vicinity of the grain boundaries. The models for strain accommodation during deformation are discussed on the basis of the microstructures found, and this structural information is correlated with a number of strength structural equations. Finally, the flow stress of fcc and bcc polycrystalline specimens is related to the occurrence of microstructures formed by macroscopic and microscopic strain accommodation processes during plastic straining, and it is concluded that macroscopic processes may be strength determining at larger strains whereas microscopic effects may be of importance at small strains.

Thermodynamics of the tetragonal to monoclinic phase transformation in constrained zirconia microcrystals

Abstract: End-point thermodynamic analyses were made of the tetragonal to monoclinic transformation (t→m) occurring in ZrO2 precipitates in a Ca-PSZ alloy and particles in Al2O3-ZrO2 composites. Calculated plots of the reciprocal critical size for transformation temperature were in excellent agreement with experimental data for both systems. Contributions to the total free energy change included bulk chemical, dilatational and residual shear strain energies and also interfacial energies. The latter term consisted of contributions from the change in the chemical surface free energy, the presence of twin boundaries in the precipitate (particle)-matrix interfacial energy. The major impediment to the transformation was the shear strain energy which could not be reduced sufficiently by twinning alone. The t → m reaction proceeded spontaneously when the energy barrier was reduced by the response of the particle-matrix interface. The response comprised loss of coherency and grain boundary microcracking for the Ca-PSZ and Al2O3-ZrO2 alloys, respectively. These results are in accord with recent suggestions that either a stress-free strain or a free surface is a necessary conditions for the initiation of a martensitic transformation.

Adiabatic shear localization in titanium and Ti-6 pct Al-4 pct V alloy

Abstract: Ballistic impact experiments were conducted on 12.5 mm thick commercial purity titanium and Ti-6 pct Al-4 pct V alloy plates using steel “stepped” projectiles with 10.5 mm diameter. The impact velocities varied between 578 m per second and 846 m per second, and a flash X-ray technique was used to determine projectile velocity and to assure the normality of impact. The microstructural damage mechanisms associated with impact (shear band formation, shock wave propagation, and dynamic fracture) were analyzed by optical, and scanning and transmission electron microscopy. Elliptical and spherical cavities were observed along the bands. Microindentation hardness differences between the bands and adjacent regions were slight for the targets; for the projectiles, the hardness in the band was significantly lower than that of surrounding regions. Observation of the fractured regions along the bands showed unique features indicating possible melting. Transmission-electron microscopy of a shear band in titanium revealed microcrystalline features (∼0.3 µm diameter) with poorly defined grain boundaries.

Swelling of Hot-Pressed A12O3

Abstract: Fully dense aluminas, prepared by hot-pressing, were found to swell during annealing at 1600°C in air, but not during annealing in a reducing atmosphere (po2= 10-7 Pa). The reaction followed the relation p - po = -K log t, where po and p are the initial and final densities, respectively, t is the time, and AT is a constant. The rate of swelling was enhanced by MgO solute. The reduction in density resulted from the nucleation and growth of grain-boundary pores. Pore formation was attributed to the reaction of carbon and sulfur impurities at the boundaries with oxygen, which had diffused down the grain boundaries from the ambient, to form CO/CO2 and SO2 gas at high pressures. Preliminary results indicate that this reaction can be avoided by preannealing powders in flowing oxygen prior to hot-pressing. The consequences of internal gas-forming reactions to other processes such as high-temperature creep and sintering are also discussed.

On the formation of porosity and microchannels in growing scales

Abstract: It is well known that oxide scales develop porosity and microchannels that permit inward transport of molecular species from the ambient gas even under conditions when there is no evidence of cracking of the scales. It is proposed that such porosity and microchannels develop as a result of grain growth and of plastic deformation (grain-boundary sliding, diffusion creep, etc.) under compressive stresses in the scales. The presence of small amounts of impurities enriched at grain boundaries in the scales may greatly affect deformation and mechanical and transport properties in scales.

Microstructural aspects of superplasticity

Abstract: The microstructural aspects of the superplastic phenomenon are reviewed. The experimental results of a very large number of investigations are critically analysed in the context of: grain shape and size; grain growth; grain boundary sliding and migration, grain rotation and rearrangement; diffusion and dislocation activity. It is shown, that in spite of often conflicting evidence in the literature, a common pattern of microstructural behaviour emerges for all the materials and conditions investigated to date.

Grain Boundaries in Polycrystalline Silicon

Abstract: Significant progress in the understanding of the structural and electrical properties of semiconductor grain boundaries has been made in the last ten years. Much of this work was stimulated by promising technological applications of poly crystalline semiconductors, such as cheap solar cells ( 1 ), large area device arrays for display use (2), integrated circuit gates and interconnections (3), and voltage-controlled resistors (varistors) (4). In addition to industrial research support for these studies, government funding in this area has been readily available. A large part of this interest has focused on polycrystalline silicon, not only because of its attractiveness for many of the above-mentioned uses, but also because it is the best characterized, purest semiconductor available, which allows the researcher to concentrate on grain boundary rather than bulk properties. While this review mainly focuses on this material, it will become clear that many of the concepts discussed can be (and have been) carried over quite successfully to model grain boundary properties in other semiconducting materials. The organization of the text is straightforward; it logically follows the concepts needed to understand the rather complex properties of a given piece of polycrystalline silicon. However, as will become apparent to the reader, our present, somewhat incomplete level of understanding in this area was not acquired in the same simple order that is presented here. While grain boundaries affect the mechanical and chemical properties of silicon in important ways, their major effect-and the one that most influences the operation of the devices mentioned above-is on the

1/f noise and grain-boundary diffusion in aluminum and aluminum alloys.

Abstract: We have measured the $\frac{1}{f}$ noise in polycrystalline films of Al, Al-Si(1%), and Al-Cu(4%) in the temperature range of 300 to 600 K. The temperature dependence indicated activation energies of 0.69, 0.80, and 0.89 eV, respectively. These energies are similar to the activation energies found for Al diffusion along grain boundaries for films of the same size and composition measured in the same temperature range. Measurements of samples with identical compositions but differing widths and thicknesses revealed significant departures from the usual inverse volume dependence of the $\frac{1}{f}$ noise.

Precipitation and aging in high-conductivity Cu–Cr alloys with additions of zirconium and magnesium

Abstract: The precipitation reactions responsible for age hardening in a high-conductivity Cu–Cr–Zr–Mg alloy have been investigated by analytical transmission electron microscopy and compared briefly with the processes that occur in simpler Cu–Cr and Cu–Cr–Mg alloys. Aging at low temperatures (400°C) results in the formation of Guinier–Preston zones. Peak hardness, obtained by aging for 24 h at 450°C, is found to be a result of the fine scale precipitation of an ordered compound, possibly of the Heusler type, with the suggested composition CrCu2(Zr, Mg). Overaging results in the formation of coarse precipitates of Cr and CU4Zr. The intergranular precipitate which forms in the Cu–Cr–Zr–Mg alloy is Cu4Zr. This phase precipitates both as discrete particles on the grain boundaries and as thin ( ~ 5 nm) continuous intergranular films.MST/89