Showing papers on "Grain growth published in 1968"

Sintering of ZnO: I, Densification and Grain Growth

Abstract: The densification rate and grain growth rate of pure MgO powder compacts are measured between 1450 and 1650° C in air. Densification rate in a semilog plot appears to be linear up to about 94% of the theoretical, followed by marked nonlinearity. The time dependence of grain growth is 1/2 at the beginning and then decreases considerably with further sintering. Application of lattice diffusion model to the densification and grain growth data gives calculated diffusion coefficients in fair agreement with the directly measured diffusion coefficients for magnesium; they are also in fair agreement with the literature value of the diffusion coefficients calculated from various other kinetic processes in MgO.

Microstructural Development During the Liquid-Phase Sintering of Two-Phase Alloys, with Special Reference to the NbC/Co System

Abstract: An investigation was made of the grain growth and other microstructural changes occurring during the liquid-phase sintering of NbC alloys with ∼20 wt % cobalt. The effects of sintering time, sintering temperature, and small alloying additions were studied. It was found that the grain growth of NbC in liquid cobalt, at 1420° C, can be described by the equation: $$\bar d^3 - \bar d_0 ^3 = {\text{K}}t$$ where $$\bar d$$ is the mean linear intercept of the grains after time t, and $$\bar d_0$$ the initial mean intercept, K being a temperature-dependent constant with an “activation energy” of 95±15 kcal/mole. This equation suggests that grain growth occurs by a solution/ precipitation process controlled by diffusion in the liquid phase. Small alloying additions of WC, TiC or NbB2 inhibit the growth and/or alter the growth process, as well as affecting such properties as the shape and contiguity of the carbide grains. The relative significance of grain coalescence to grain growth in a liquid phase is discussed. By examining theoretically the effect of anisotropy of interface energy on the cube ⇌ sphere grain-shape change, it has been possible to explain the observed sensitivity of grain shape towards sintering conditions.

Grain Growth in UO2-Al,O3 in the Presence of a Liquid Phase

Abstract: The theory of grain growth in the presence of a liquid phase is examined using modifications of equations derived for coalescence of solid particles widely dispersed in a liquid. Although the grain diameter-time relation can still be represented by d3=kt, the absolute growth rates are increased as the amount of liquid is decreased. The grain growth kinetics in UO2 compacts containing 0.5 wtyo A12O3 were studied for temperatures between 1960° and 2200°C. The interrelation of grain size, temperature, and time is in agreement with that predicted by the theory.

Modifications to the Zener formula for limitation of grain size

Abstract: An experimental investigation of the ratio of grain boundary curvature to grain size is reported, from which it is deduced that the terminal grain size determined by inclusions should be about an order of magnitude less than that predicted by the Zener formula in its usual form. It is demonstrated that the Zener formula does not necessarily include the condition that each boundary is impeded by an inclusion, and a supplementary condition is derived to cover this possibility.

Thermal Recovery of Explosive Shock-loaded Stainless Steel

Abstract: The annealing response of shock-loaded type 304 stainless steel was studied in the pressure range of 120–1200 kbars, and compared with the response of stainless steel cold-rolled 5 and 45%. The annealing kinetics of shock-loaded stainless steel are characterized by a prominent recovery and grain growth stage, with little perceptible recrystallization in the classical sense. The results indicated a pronounoed structural difference between the shock-loaded and cold-rolled materials, as observed by transmission electron microscopy. The structure of stainless steel shock-loaded to 750 and 1200 kbars is observed to be strongly influenced by the transient shock heating effect, and a dislocation cell-type substructure is observed to occur predominantly in (100) grains. The hardness produced over the pressure range 120–1200 kbars by explosive shock-loading is shown to occur by a transition from dense dislocation arraya to twin-faults, and finally to what may be dense point defect concentrations at very h...

Internal strain energy and the strength of brittle materials

Abstract: The theory of Clarke for the formation of grain boundary cracks in anisotropic polycrystalline materials, is re-examined in the light of recent experimental data. The theory predicts correctly the conditions for the formation of grain boundary cracks of length similar to a grain dimension. However, the theory cannot be used to explain the experimentally observed strength/grain size and strength/irradiation dose relationships, for example for BeO. The theory supposes that the process controlling catastrophic fracture is the growth of a crack from a grain boundary pore with an energy absorption rate corresponding to the grain boundary surface energy of ∼103 erg/cm2. In practice, the process controlling catastrophic fracture is the subsequent growth of a crack from a grain dimension, with a higher energy absorption rate corresponding to an effective surface energy of ∼104 erg/cm2.

High‐Temperature Creep of Polycrystalline Magnesia: I, Effect of Simultaneous Grain Growth

Abstract: The creep of pure magnesia (99.9 +% MgO) was tested in transverse bending at temperatures from 1200° to 1500°C, strain rates near 10−2%/hr, and grain sizes of 4 to 50μ. In most cases, grain growth during the test affected the apparent creep behavior more than all the other variables combined. An analytical graphical method was used to separate the grain growth effect from other effects and to obtain more meaningful creep data. Creep occurred primarily by a viscous mechanism (Nabarro-Herring type, cation-lattice-diffusion controlling) with a minor amount of plastic creep (dislocation climb). The agreement with previous creep data was good.

High‐Temperature Creep of Polycrystalline Magnesia: II, Effects of Additives *

Abstract: The creep of magnesia doped with 0.035 to 2.26 cation % of nine other oxides and three binary mixtures thereof and of three seawater products (about 96, 98, and 99.5y0 MgO) was evaluated in transverse bending at 1200° to 1500°C, with strain rates of about 10−2%/hr, and average grain sizes of 5 to 50p. The results obtained were compared with those for pure magnesia. Most additives accentuated the plastic (diffusion-controlled) nature of the creep process presumably by pinning dislocations and/or slowing grain growth. In most cases the rate-determining diffusing species seemed to be the cation, Mg, but in two cases it was suspected that oxygen boundary diffusion was controlling. Porosities above ˜10% appear to increase the temperature dependence of creep, probably by introducing boundary sliding. The agreement of the creep data with those of other diffusion-controlled processes (electrical conductivity, sintering, and grain growth) is demonstrated.

Sintering and recrystallization of ZrC-ZrB2 compacts

Abstract: 1. By using finely-divided starting materials, it is possible substantially to improve the sinterability of ZrC, ZrB2, and cermets based on these compounds. The addition of ZrB2 to ZrC or of ZrC to ZrB2 has a very beneficial effect on sinterability. By sintering ZrC and ZrB2 powders with a specific surface area of 4–5 m2/g at 2300–2400°K it is possible to obtain specimens with a total porosity of not more than 2–4%. 2. To describe the process of collective recrystallization over a wide range of temperatures and holding times, Eq. (4) should be used instead of Eq. (1), which is normally employed for this purpose. 3. Grain growth is very intense in ZrC, slightly less intense in ZrB 2, and much less pronounced in ZrC-ZrB 2 cermets. Both the grain size and energy of activation of the cermets diminish with increasing amount of the second component.

Annealing of cylindrical Ni-Fe films

Abstract: The effect of annealing on the microstructure of cylindrical electrodeposited 81-percent Ni, 19-percent Fe films was studied with the electron microscope. The films were deposited onto 5-mil Be-Cu wires having either Au or Cu intermediate layers. The annealing was performed on both free-standing films and on films still attached to the substrate. The initial change in the free-standing films was a rapid recrystallization. After the whole film had recrystallized, normal grain growth was observed. There is evidence that the driving force for the annealing is provided by stresses present in the films. Films annealed while attached to a Au-Be-Cu substrate did not exhibit recrystallization or grain growth, and the electron microscope studies showed that the Ni-Fe grains were pinned by Be-Au which diffuses into the grain boundaries but does not alloy with the Ni-Fe. The films which were annealed while attached to a Cu-Be-Cu substrate behaved essentially like free-standing films. Electron beam microanalysis and electron diffraction studies revealed that Be-Cu diffuses into the Ni-Fe grain boundaries and then alloys with the Ni-Fe. Recrystallization of the alloy can then take place freely.

High density sintering of pure barium titanate

Abstract: The techniques of slow sintering, addition of a volatile flux, and grain growth inhibition have been used to produce high density sintering in pure barium titanate. The use of carbon as grain growth inhibitor gave a density of 99% theoretical at 0.7μm grain size. This technique may be used for other suitable ceramics, when the proposed application requires improved physical properties. An extension of the procedure gives a range of electrically conducting ceramics.

Inhibiting grain growth in metal composites

Abstract: A process for producing composite metal articles whereby a suitable grain size is obtained in the metal member having the lowest temperature of recrystallization suitable for mechanical deformation, and the article produced thereby.

Densification and Grain Growth in the Later Stages of Sintering of Alpha-Iron

Abstract: Only few studies have been made of the mechanism of sintering of body-centered cubic metals. On the author’s part, interest in the sintering of α-iron is enhanced by the observation [1, 2] of a dominant contribution of surface diffusion during the initial stage of sintering of wire spool models in the α range. Together with the empirical fact that fine iron powders do exhibit considerable shrinkage on sintering in the alpha range (which is incompatible with a surface diffusion mechanism) these wire spool experiments indicated that the mechanism of material transport might depend on the particle size of the sintering system. This stimulated a systematic study of particle size dependence which is still in progress. As a first step it was decided to study the sintering of carbonyl iron powder. This powder can be sintered to nearly full theoretical density below the transformation temperature, which makes it very suitable for a study of shrinkage mechanism.

Effect of Powder Particle Size on the Grain Size of the Sintered Material

Abstract: For many years powder metallurgy has been known as a method for fabrication of sintered materials with special physical properties not obtainable by any other more conventional metallurgical method. Reference is made, for example, to materials with controlled porosity, combinations of metals which do not form alloys, and sintered products with special electrical properties. Little attention, however, has been given heretofore to the grain structure of sintered materials, and hardly anything more is known with respect to the structure than the general fact that the grain size in sintered metals is usually smaller than that of the corresponding cast material heat-treated in a similar way.

Ultrasonic enhancement of grain growth in copper

Abstract: The influence of 20 kc ultrasonic energy on grain growth kinetics in O.F.H.C. copper was investigated. Half-wavelength specimens resulted in resonant standing waves with a maximum acoustic stress amplitude of 2·4 × 108 dynes/cm2. Acoustic energy caused an acceleration in grain growth rate. The grain growth exponent, n, in the equation D=βtn was increased from 0·11 to 0·31, where D is the grain diameter at growth time t and β is a constant.

Wear-resistant titanium carbide-steel materials

Abstract: 1. The infiltration of titanium carbide with high-speed steel for 30 min at 1500°C in a hydrogen atmosphere has practically no effect on the grain size of the starting TiC powder. By using fine starting titanium-carbide powders, alloys were obtained containing 52% of fine grains. 2. Additional heat treatment of the resulting alloys for 20 min at 1600°C was found to produce marked grain growth and to accelerate the process of formation of a ring-type structure; such a structure is apparently due to a sharp gradient of steel-component concentration across the section of the carbide grains. 3. Infiltrated alloys with a finer-grain structure exhibit increased hardness and wear resistance.

On the Grain Growth by Strain-Anneal in Cold-Rolled Silicon-Iron Sheet

Abstract: In the grain growth by strain-anneal of cold rolled silicon-iron sheet containing 3.30 wt% Sl, four kinds of grain growth were found up to 10% of reduction. It was presumed that these were Goss type secondary recrystallization, normal grain growth, cube type secondary recrystallization and grain boundary migration respectively all induced by strain. These grain growth by strain were also explained alternatively by an impurity effect with an additive effect on impurity.

The effects of specimen grain size and environment on the fatigue life of ofhc copper

Abstract: : The note presents the results of a study of the effects of grain size and environment on the fatigue life of OFHC copper specimens. Tests conducted in alternating torsion on specimens of two different grain size groups showed that an increase in specimen grain size results in a decrease in fatigue life at both high and low strain amplitudes. Similar tests under low and high humidity showed that humidity has a negligible effect on fatigue life for either high or low strain amplitude and for large or small grain size. The effects of elevated temperatures were found to be more complex. At high strain amplitudes the fatigue life decreased continuously as the temperature was increased. At low amplitudes the life dropped as the temperature was raised to 200C., but then remained approximately constant when the temperature was raised further to 350C. An examination of the microstructural changes caused by the fatigue tests revealed damage characteristics of Wood's H range for the tests at high amplitudes and room temperature, with slight variations for the tests at high temperature. Damage characteristics of Wood's S range were found for low amplitude tests at room temperature, with significant grain growth occurring at high temperature.