Showing papers on "Grain growth published in 1980"

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.

Sintering and varistor characteristics of ZnO‐Bi2O3 ceramics

Abstract: The sintering behavior of binary ZnO ceramics (containing 0.01–1 mol% of added Bi2O3) showed that densification in the initial stage of sintering and grain growth of the ZnO matrix are enhanced in the presence of a liquid Bi2O3 phase. Exaggerated grain growth of ZnO was observed above 1100 °C and in mixtures containing as low as 0.05 mol % Bi2O3. Volatilization loss of Bi2O3 during sintering affected the microstructure of the sintered ceramics and resulted in the loss of varistor property and return to Ohmic behavior and high conductivity (∼1 Ω−1 cm−1). A simple microstructural model which predicts that the minimum content of Bi2O3 required to maintain varistor behavior is proportional to the thickness of intergranular layer and inversely proportional to the average grain size of the ZnO.

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...

Microstructure and normal grain growth in metals and ceramics. Part II. Experiment

Abstract: An extensive set of measurements on the grain growth and microstructure evolution of a series of high‐density Ni‐Zn ferrites annealed over periods ranging from 1/4 to 48 h is reported. A comparison of the data with predictions of the statistical multiple‐lognormal theory of normal grain growth presented in Part I is made. The time invariance of the standard deviations of both the grain size and population of individual topological classes is confirmed, demonstrating that the scaling and stability of normal grain growth are critically related to the dispersion of the grain sizes and that at least two parameters are needed to characterize a given microstructure. A parabolic growth law for the median diameter in both two and three dimensions is confirmed as well as the predicted power law dependence of the growth constants for each class on the topological index. A series of sectioning experiments verifies the near equivalence of the planar and spatial size parameters.

The kinetics of normal grain growth

Abstract: The relationship between the quasistationary distribution functions in normal grain growth and the corresponding grain-growth velocities is investigated. The restrictions imposed by volume conservation lead to a simple differential equation which describes quasistationary grain growth. This equation allows us to express the reduced growth velocity (dR/dt)/(dR*/dt), where R* is the scaling grain size, in terms of the corresponding distribution function and to express the distribution function by means of the reduced growth velocity. General conclusions about the shape of distribution functions can be drawn from these expressions.

Mechanical Properties of Fine‐Grained Magnesium Oxide at Large Compressive Strains

Abstract: Fine-grained polycrystalline MgO specimens were deformed in compression at constant strain rates of ∼6.7×10−6 s−1 to 6.7×10−5 s−1 at 1173 to 1423 K. Both mechanical data and microstructural observations are distinctive of a diffusion-accommodated flow, where the grain-size parameter is of prime importance in enhancing plasticity. Under suitable experimental conditions, superplastic strain was achieved in specimens; no change was detected in the equiaxed-grain shape configuration. Finally, the effect of grain growth during deformation was considered; specimens of initial grain size ∼0.1 μm showed an apparent hardening which was quasi-linear with time.

Compositional effects on the creep ductility of a low alloy steel

Abstract: The role which P plays in determining the creep ductility of 2.25Cr-1Mo steel is examined by notched bar creep rupture tests on high purity material selectively doped with combinations of Mn, Si and P. The impurity concentrations, hardness and grain size were carefully controlled. The ductility of as-tempered samples containing dopants was found to be higher than those without dopants; however the ductility of step cooled samples containing Mn and P was found to be lower than as-tempered samples. It is suggested that P, when segregated to the prior austenite grain boundaries, enhances the nucleation of grain boundary cavities while retarding their growth. Mechanisms for each process are proposed.

Effect of doping a Mn-Zn ferrite with GeO 2 and SnO 2

Abstract: The effects of various additions of GeO 2 or SnO 2 , in a Mn-Zn ferrite on the microstructure and magnetic properties have been studied. The behavior of GeO 2 additive has been found to be similar to that of SiO 2 . Up to a GeO 2 content of 0.64 mol%, the magnetic properties improve through the increase in the density. At a higher GeO 2 content of 1.28 mol%, the properties deteriorate due to the occurrence of discontinuous grain growth. Further addition of GeO 2 gives better grain structure but poor properties. The presented results suggest that Ge4+dissolves in the ferrite to a very limited extent and GeO 2 exists as a second phase. SnO 2 , on the other hand, has been found to be soluble in the ferrite and does not alter the microstructure even up to a content of 5.70 mol%. The essential property of this additive is found to be in its ability to modify the variation of μ i with temperature without deteriorating the μ i and core losses. The temperature variation of the bulk resistivity has also been studied.

Temperature dependence of Hall mobility in indium–tin oxide thin films

Abstract: Carrier concentrations and mobilities of vacuum‐prepared indium–tin oxide films have been measured to determine the effects of annealing. The apparent variation of electron mobility with temperature in these films is interpreted in terms of a grain‐boundary barrier model. No grain growth was observed in the films studied; all changes in electron density and mobility may be explained by oxygen diffusion in the grain boundaries and, from them, into the grains themselves.

Pore formation during oxidative annealing of Al2O3−∶Fe and slowing of grain growth by precipitates and pores

Abstract: High-density samples of polycrystalline Al2O3 doped with iron retain their high density when annealed at high temperature in a reducing atmosphere, but pores are formed, both inside grains and at grain boundaries, and the density decreases upon annealing in oxidizing atmospheres. An explanation for these effects is proposed. The presence of pores and second-phase particles of FeAl2O4 is found to affect the physical properties and to slow grain growth. Grain size is proportional to the number of second-phase particles to the power −1/3 indicating that the number of second-phase particles per grain remains constant during grain growth.

Manufacture of single crystal

Abstract: PURPOSE:To cheaply manufacture a single crystal through solid phase reaction at a relatively low temp. by contacting a single crystal very similar to a polycrystal body in crystal structure to the body showing discontinuous grain growth at a high temp. followed by heating to a specified temp. CONSTITUTION:A single crystal very similar to a polycrystal body in crystal structure is contacted to the body showing discontinuous grain growth at a high temp. At this time, the polished surfaces of the crystal and the body are contacted directly or with a soln. of an org. or inorg. salt, inbetween, contg. a metallic element forming the body or the crystal, or a soln. which dissolves part of the body or the crystal. They are then haated at a low temp. and joined by baking. This joined body is heated to a temp. below the temp. at which discontinuous grain growth of the polycrystal body occurs to induce solid phase reaction at the interface between fine grains of the polycrystal body and the single crystal. Thus, the fine grains are united with the crystal to grow a single crystal.

Theory of inclusion controlled grain growth

Abstract: Different inclusion/grain boundary interactions have been analysed with emphasis on the grain boundary structure and the geometry of the inclusion/boundary profiles. Crystalline inclusions can inhibit grain growth, provided one or more inclusions intersect each boundary. An equation for the limiting grain size, in terms of the volume fraction and the particle size distribution of the inclusions, is derived and is shown to explain the inclusion controlled limiting grain sizes in calcium fluoride and in alumina, as well as grain coarsening in steel. Conditions for the drag of amorphous particles or pores, and for pore isolation are discussed.

Grain Growth in Fe3O4

Abstract: Sintering and microstructural evolution were studied in Fe3O4 as a model system for spinel ferrites. Fe3O4 powder, purified by the salt-crystallization method, was sintered to ∼99.5% density in a CO-CO2 atmosphere. The pO2 Of the sintering atmosphere drastically affects the microstructure (grain size) of sintered Fe3O4 without significantly affecting density. The measured grain-boundary mobilities, M, of Fe3O4 fit the equation M=M0(T) pO2−1/2 with M0(T) = 2.5×105 exp[-(609kJ·mol-1/RT](m/s)(N/m2)−l. The grain-boundary migration process appeared to be pore-drag controlled, with lattice diffusion of oxygen as the most likely rate-limiting step.

A statistical investigation of normal and abnormal grain growth in iron

Abstract: The evolution of the statistical distribution of linear grain sizes during isothermal grain growth in unstrained or slightly strained pure iron specimens was investigated. A log normal distribution was confirmed to fit the data well for normal growth, and was shown to be applicable in the initial and final stages of abnormal growth. Analysis of statistical parameters of the size distributions proves that the coefficient of variation /gs¯d significantly distinguishes between abnormal and normal growth processes.

Electrical and structural properties of cadmium selenide thin film transistors

Abstract: Stable thin film transistors based on cadmium senenide and silicon dioxide have been prepared. The degree of stability implies a decay of only 10% in drain current in 100 years of continuous d.c. operation. The decay is solely due to tunnelling of electrons into insulator traps and has a logarithmic time dependence. The devices have field effect mobilities up to 140 cm 2 . Volt −1 sec −1 , switching ratios in the range 10 5 –10 6 , and good reproducibility. The CdSe films contain the hexagonal structure and grain growth occurs during anneal. Grain size and distribution are reproducible from run to run.

Resolution of the Gold Wire Grain Growth Failure Mechanism in Plastic Encapsulated Microelectronic Devices

Abstract: The relative statistical influence of bonding parameters and assembly materials on the failure frequency rate of microelectronic devices in plastic encapsulated packages due to grain growth, "wire creep" failure are resolved. Wire creep is a stressinduced break in a thermocompression gold wire ball bond which most frequently occurs within several wire diameters above the ball in plastic encapsulated microelectronic devices. Thermal Shock, Condition C, was used as the standard test by which to measure performance in this investigative program. A complete analysis was made of all materials and processing factors which would potentially contribute to the wire creep, grain growth failure mechanism. Over 75 test matrices were defined and executed with the demonstrated result that only four variables had apparent major influence on this failure mode: 1) the gold wire impurity constituents, 2) the type of wire bonding capillary, 3) the gold wire ball formation technique, and 4) the lead frame plating impurities. A full factorial experiment was conducted to delineate the relative influence of these four contributors. The results of this investigation show that: 1) lead frame plating impurities can significantly accelerate the wire creep, grain growth failure frequency; 2) hydrogen torch flame-off ball formation always results in wire creep, grain growth failure; 3) the type of capillary contributes to this failure to a lesser but still significant extent; and 4) the type of wire is the least significant contributor, but within this variable the level of grain modifying impurities is significant. Hypotheses, supported by additional experimental investigations, with foundations in metallurgy have been formulated. Corrective actions implemented as a result of this investigation have reduced the average cumulative failure frequency rate of 30 percent at 250 thermal shock cycles to zero cumulative failures at greater than 3000 thermal shock cycles.

Annealing behavior of evaporated Mo films—Direct observations by SEM

Abstract: Annealing behavior of molybdenum films 320–4900 A thick deposited by the e‐gun evaporation method on a silicon dioxide layer has been investigated directly by means of a high‐resolution scanning electron microscope (SEM). The grain growth of Mo films occurs drastically at 900 °C annealing or higher. The structures of Mo films annealed higher than 900 °C exhibit film thickness dependency. At 1000 °C annealing, 1200 A or thinner samples show columnarlike monolayer structure. In the case of the 4900‐A‐thick sample, it reveals a faceted multilayer structure, and its grain size is about 1700 A. Grain size annealing temperature dependency is well correlated to electrical resistivity dependency. In addition, it is also found that, at 900 °C annealing or lower, no interfacial reaction occurs.