Showing papers on "Grain growth published in 1973"

Irradiation creep due to the growth of interstitial loops

Abstract: A rate theory of the growth of irradiation-produced interstitial loops in uniaxially stressed materials which also contain voids, coherent and/or incoherent precipitates and network dislocations is derived and the resulting creep behaviour is determined. Allowance is made for the preferential nucleation of loops oriented perpendicular to the stress axis and for the subsequent enhanced growth of these same loops. It is shown that the first of these mechanisms dominates and gives a creep rate which can be correlated with the volume swelling rate. The magnitude of the effect is shown to be consistent with the limited amount of available experimental data appropriate to the void-swelling temperature regime.

Effect of Rare-Earth Ions on Electrical Conductivity of BaTiO3 Ceramics

Abstract: Lattice constants, grain size, electrical conductivity, and luminescence were measured for sintered BaTiO3 ceramics doped with 0 to 1.2 at.% rare-earth ions. BaTiO3 doped with low levels of rare-earth ions contains grains 10 μm in size and has lattice constants nearly equal to those of undoped ceramics. In this case, rare-earth ions occupy Ba2+ sites and yield donors. When grain growth is inhibited by high doping levels or by insufficient sintering, the lattice constants change, the rare-earth ions occupy both Ba2+ and Ti4+ sites, and, consequently, BaTiO3 becomes insulative because of charge compensation.

Influence of Additives on the Microstructure of Sintered Al2O3

Abstract: During an investigation to determine the effect of additives in inhibiting exaggerated grain growth, it was observed that several gave strikingly similar microstructures with pore clusters at grain centers surrounded by substantially pore-free annular regions. A few very large pores were found at the grain boundaries and occasionally in the small-pore-free annular regions. The development of this structure is explained as a sequence of exaggerated grain growth followed by normal grain growth. A discontinuous grain-boundary liquid phase facilitates the final grain-growth step and takes part in the coalescence of fine pores into coarse pores.

Electrostatic potential of interstellar grains

Abstract: The equilibrium electrostatic potential of interstellar grains, subjected to an ultraviolet radiation field in a dilute ambient plasma, is calculated for a variety of radiation fields and plasma parameters, and for two different grain materials. For an environment typical of an H I region, the results show that the various components of interstellar grains in the multicomponent model may charge to opposite potentials, thus favoring the formation of mixed composite grains due to Coulomb attraction. In H II regions, a three-shell model of the grain charge is presented, which includes a sphere of positively charged grains around the central star bounded by a transition region in which grains of low and of high photoyield are oppositely charged. Here grain growth and the formation of mixed composite grains may take place. In the outer shell all grains are charged negative. Estimated radii of these shells are given for exciting B0 and O5-type stars. (auth)

Grain growth and secondary recrystallization in iron

Abstract: The effect of impurities and of slight deformations as factors modifying the kinetics of normal grain growth after primary recrystallization in pure iron is investigated. Samples of pure iron and Armco iron, each with or without a content of precipitated oxides, were examined. Inclusions inhibit grain growth up to high temperatures, whilst the influence of strain depends on its amount: very low deformations (∼2%) generally block grain growth; for deformations around 5%, secondary recrystallization takes place; higher deformations (∼10%) accelerate the early stages of growth, leading to not very large final grain dimensions. 2.0% elongation has been found to be, in our conditions, the critical strain limit for secondary recrystallization.

Microstructural Changes During Heat Treatment of Sintered Al2O3

Abstract: Samples of both high-purity and Mg-doped Al2O3 sintered in H2, N2, O2, or vacuum were annealed at 1650° to 1850°C for times up to 64 h. In pore-free systems, grain growth is limited by the mobility of Mg-rich second-phase inclusions; in samples annealed in H2, grain growth is limited by pore dragging with a transition toward limitation by solid-inclusion dragging at high dopant levels; in samples annealed in N2 and O2, grain growth is characterized by a transition from an “anchoring effect” of the pores toward a combination of pore dragging by and unpinning from the grain boundaries. Time-dependence of grain growth is insufficient to determine the mechanisms and provide an adequate foundation for model-based calculations. Observations of microstructure and its change with time, together with the rate of grain growth as a function of composition, allow elimination of alternate hypotheses and determination of the process which controls the rate of grain growth and change in pore size.

Method of fabricating nickel base superalloys having improved stress rupture properties

Abstract: Improved superalloy powder materials consisting of a powder mixture of a substantially carbon free nickel base superalloy containing gamma prime phase and a metastable carbide such as vanadium carbide (VC), titanium carbide (TiC) or chromium carbide (Cr3C2) in an amount sufficient to provide a controlled carbon content in the resulting mixture. The mixture is then consolidated into a shape by heat and pressure, after which it may be mechanically worked at a temperature below the recrystallization temperature to thereby promote grain growth in the shape, followed by heat treating the shape at a temperature above the recrystallization temperature to solution the metastable carbide and permit grain growth. Finally, the shape is subjected to an aging treatment at a temperature below the recrystallization temperature to reprecipitate the gamma prime and carbide phases in the matrix and the new grain boundary areas.

The influence of grain size on the creep of uranium dioxide

Abstract: The creep of uranium dioxide has been investigated as a function of grain size. At high stresses, when creep is controlled by dislocation movement, grain boundaries exert a strengthening effect and this strengthening is correlated with the Hall-Petch equation. The degree of strengthening diminishes with increases in temperature. At lower stresses, when creep is controlled by mass transport, grain boundaries exert a weakening effect owing to the reduction in diffusion path length as grain size is reduced. In this range behaviour is correlated with the Nabarro-Herring equation with stress σ replaced by an effective stressσ E=σ−σ0 whereσ 0 is a threshold stress for diffusional creep associated with the limitation of the ability of boundaries to emit and absorb vacancies.σ 0 appears to decrease as grain size is increased.

The sintering and morphology of interconnected porosity in UO2 powder compacts

Abstract: Uranium dioxide powder compacts of ∼46% green density were sintered in flowing hydrogen at temperatures between 1500 and 1700° C. On annealing, the compacts readily formed an interconnected system of pores stabilized by grain boundaries. The volume of open porosity decreased with an activation energy of 4.6 J mol−1 at a rate controlled by grain growth. The grain-boundary migration removed the restraint on the porosity allowing shrinkage to commence. The compact surface area decreased with a higher activation energy of 6.0 J mol−1. The mechanism proposed for the diminishing area was the smoothing of the faceted powder grains. Nucleation of atomic layers on the facets was shown to account for the high activation energy. The equilibrium shapes that may be adopted by interconnected porosity were calculated using a model in which simpler geometry was substituted for the real anticlastic surface curvature. The model demonstrated the stabilizing effect of increasing grain-boundary energy and the formation of closed pores.

Breakdown of primary grain boundary inhibition in 3 pct si-fe sheet

Abstract: Grain boundary inhibition in 11-mil cold rolled decarburized 3 pct Si-Fe sheet was observed to break down by isothermal annealing at temperatures in excess of 1075°C. Small MnS particles (-r < ~260A) responsible for inhibiting primary grain growth in this material were observed to undergo complete dissolution within 3 min at temperatures of 1100°C and higher. Calculated dissolution rates were in good agreement with this observation. The dissolution of these particles produced appreciable primary grain growth during the early stages of (110)[00l] secondary recrystallization which was responsible for an overall reduction in the volume percent of (H0)[00l] grains produced on prolonged isothermal annealing at temperatures above 1075°C.

Isothermal Grain Growth of Pressure‐Sintered PLZT Ceramics

Abstract: PLZT specimens 99.99% of theoretical density containing 7 at.% La with a 65/35 Zr/Ti atom ratio were fabricated with average grain diameters of ∼1 μm by pressure sintering. The dense, single-phase specimens were isothermally heat-treated in a low PbO activity atmosphere. For grain diameters from 1 to 5 μm, grain growth obeyed the D3-D03=k't relation, with an apparent activation energy of 86 kcal/mol. The mechanism controlling the grain-growth rate is consistent with a solid-solution impurity drag caused by an La ion concentration gradient near the grain boundaries.

Structure and thermal stability of sputtered Au–Ta films

Abstract: The structure and thermal stability of bimetallic Au‐Ta films has been studied by x‐ray diffraction, electrical resistivity, and by microscopy. Annealing above 450 °C forms a TaAu compound. The growth of the TaAu layer at the Au–Ta interface follows a parabolic rate law. A low‐temperature depletion of gold at tantalum grain boundaries occurs below 350 °C with an activation energy of 0.41 eV. Grain growth occurs with an activation energy of 0.50–0.55 eV for gold and 0.40–0.50 eV for tantalum. Thermal stresses of the order of 1.0×109 dyn/cm2 exist in the sputtered films.

Superplasticity in a new dispersion strengthened zinc alloy

Abstract: A new dispersion strengthened zinc alloy has been developed which exhibits both high yield strengths at room temperature and superplasticity at elevated temperatures The alloy has a nominal composition of Zn-010 Ni-004 Mg A dispersion of submicron size precipitates was readily produced within this alloy by rapid solidification to form a supersaturated solid solution followed by thermal mechanical processing The precipitates suppressed superplastic deformation at room temperature and stabilized the grains at elevated temperatures, reducing the grain growth rate of zinc to 41 × 10−2 µ/h at 203°C The deformation characteristics of the alloy are investigated with special emphasis on the influence of the precipitates on the tensile properties It was found that the stress increased with a decrease in precipitate size and spacing for strain rates less than those at maximum rate sensitivity at 151°C and also for nonsuperplastic strain rates at 24°C The slope of the Hall-Petch plot was found to be sensitive to both the grain diameter and strain rate at 24°C The change of sign of the slope of the Hall-Petch plot at slow strain rates and small grain diameters was found to coincide with a rapid increase in the strain rate sensitivity An equation of the form σ =A d ψ exp(Q/RT) eηemhas been developed to describe the deformation characteristics of this alloy at elevated temperatures and presuperplastic strain rates

Formation of annealing twins, faceting, and grain boundary pinning in copper bicrystals

Abstract: In the course of a recent investigation of the kinetics of grain boundary migration in copper bicrystals, formation of annealing twins, faceting and grain boundary pinning have been observed. Stability and frequency of formation of annealing twins are related to boundary misorientation and temperature of anneal. Tendency for grainboundary faceting decreases with increasing temperature indicating that anisotropy of grain boundary energy is more pronounced at lower temperatures. In general, orientations of faceted boundaries corresponded to higher-order twin planes with respect to shrinking grains. At relatively high temperatures (950°C), specimen-thickness-dependent pinning effects also are observed. These observations are described and analyzed in terms of grain boundary structure, energy, and migration behavior.

Superplasticity and residual tensile properties of a microduplex copper-nickel-zinc alloy

Abstract: Structural superplasticity in two phase alloys of the copper-nickel-zinc system (nominal composition in wt. pct Cu-15Ni-38 Zn-0.2 Mn) occurs over a wide range of strain rates in the temperature range 850 to 1050∮F (454 to 565°). The upper temperature limit for super-plastic behavior in this system is determined by the reversion of the fine-grained two-phase structure to a single phase structure in which extensive grain growth is possible. Residual room temperature tensile properties and microstructure of the microduplex alloy after superplastic straining have been studied as a function of test temperature and total super-plastic strain. At test temperatures sufficiently removed from the phase transformation temperature, the high tensile properties and fine microstructure of the starting material are essentially retained after superplastic strains approaching 200 pct. In the immediate vicinity of the phase transformation temperature, rapid degradation of the microduplex structure occurs during superplastic deformation with a consequent severe degradation of the residual room temperature tensile properties.

Grain size and grain growth in an equiaxed alpha-beta titanium alloy

Abstract: Methods of revealing grain size in a two-phase α-β titanium alloy have been examined and observations on beta grain growth in the presence of alpha have been carried out. The technique proposed by Greenfield and Margolin1 for revealing β matrix grain sizes has been shown not to produce grain growth. However, for grain sizes of about 10 µm the G.M. technique does not reveal all the grains because of the similarity in orientation in neighboring grains. These clusters of similarly oriented grains are shown to persist as grain growth takes place but the misorientation between grains within a cluster decreases. Both the beta grain growth and alpha particle coarsening follow the same time dependency from which it is shown that a linear relationship exists between α particle size and β grain size. It is proposed that α particles must dissolve from theβ grain edges for β grain growth to occur. The linear dependency between beta grain size,Dβ, and alpha particle size,dα, can be rationalized either on the basis of geometrical or surface tension considerations.

Machining, surface work hardening, and strength of MgO

Abstract: Machining of MgO by sawing, grinding, sanding, and polishing introduces a surface layer of dense slip whose depth depends on material (e.g. yield stress and grain size) and machining parameters. Cracks, which were always more shallow than the depth of slip, extended from sawn and, to a lesser extent, from ground surfaces into single crystals and some large grains. Annealing removed the slip and decreased surface grain growth in fine-grained bodies but in general did not result in recrystallization. Surface work hardening increases hardness; it also increases bend strengths of some single crystal orientations and of larger-grained bodies, especially those with limited intragranular porosity. Fine-grained bodies were weakened by machining.

Creep in the hot pressing of titanium diboride powder

Abstract: 1. A study was made of the hot pressing of commercially pure and iron-containing titanium diboride powders, and the effects of temperature, time, and applied pressure upon the relative density of hot-pressed specimens were determined. 2. It is shown that the densification of the TiB2 powders investigated is described by an equation for the volume viscous flow of a porous solid, incorporating aϰ-creep equation. 3. The reduced creep rates of the solid phase during densification were established and found to be proportional to the cube of pressure for the commercially pure TiB2 powder and to the fourth power of pressure for the powder containing 1.5 wt.% Fe. 4. It is demonstrated that raising the iron content of titanium diboride powder markedly increases the rate of densification and grain growth, lowers the energy of activation for the process, and arrests densification at a late stage of hot pressing.

High permeability cube-on-edge oriented silicon steel and method of making it

Abstract: Cube-on-edge oriented silicon steel and method of making it wherein the silicon steel is characterized by high permeability values. To the standard melt chemistry for cube-on-edge oriented silicon steel from about 0.002 to about 0.012 percent boron and from about 0.003 to about 0.010 percent nitrogen are added. The melt material is cast into ingots or slabs, reheated, hot rolled to hot band, annealed, pickled, cold reduced to final gauge, subjected to a decarburizing step and given a final high temperature anneal to produce the desired final orientation. The temperature of the anneal following hot rolling bears an inverse relationship to the final gauge of the silicon steel. Prior to the final anneal the silicon steel is provided with an annealing separator. A grain growth inhibitor may be provided in the environment of the stock during the primary grain growth stage of the final anneal so as to inhibit primary grain growth and to favor the growth of cube-on-edge oriented nuclei during the secondary grain growth stage.

Controlled grain size metal oxide varistor and process for making

Abstract: Disclosed is a metal oxide varistor consisting primarily of metal oxide with a small percentage of preselected additives distributed substantially evenly therethrough. A grain growth inhibitor is combined with the metal oxide powder forming a final mix with peripheral regions having a high concentration of the inhibitor that surround and separate interior regions that have a low concentration of the inhibitor. Varistor bodies are formed from the final mix and the peripheral and interior regions retain their identities in the varistor bodies. The bodies can be pressed and sintered in the conventional manner. During sintering the grains within each interior region tend to combine and grow. However, growth is stopped at the peripheral regions due to the high concentration of inhibitor. Thus, the grain size in the processed pellet is dependent upon the size of the interior regions which can be controlled. If desired, one of the additives can be chosen to stimulate grain growth so as to promote the combination and growth within the interior regions. One preferred method of forming the interior regions is to coalesce the metal oxide powder by techniques such as spray drying and then coat the agglomerates thus formed with grain growth inhibitor.