Showing papers on "Grain size published in 1990"

dc Electrical Degradation of Perovskite-Type Titanates: I, Ceramics

Abstract: The rate of the resistance degradation of doped SrTiO3 ceramics is investigated as a function of various external and material parameters. The effects of the mutually interrelated parameters dc voltage, dc electric field, and thickness of the dielectric are described by power laws. Electron microscopic potential contrast studies show a Maxwell-Wagner polarization leading to a concentration of the electric field at the grain boundaries during the degradation. Based on this finding, the voltage step per grain boundary, ΔΘgb, is introduced as a rate-determining parameter which allows an explanation of the influence of the grain size on the degradation rate as well as the difference in the power laws for ceramic and single-crystal samples.

Nanocrystalline metals prepared by high-energy ball milling

Abstract: This is a first systematic report on the synthesis of completely nanocrystalline metals by high-energy deformation processes. Pure metals with body-centered cubic (bcc) and hexagonal close-packed (hcp) structures are subjected to ball milling, resulting in a decrease of the average grain size to ≈9 nm for metals with bcc and to ≈13 nm for metals with hcp crystal structures. This new class of metastable materials exhibits an increase of the specific heat up to 15 pct at room temperature and a mechanically stored energy determined as up to 30 pct of the heat of fusion after 24 hours of high-energy ball milling. The grain boundary energy as determined by calorimetry is higher than the energy for fully equilibrated high-angle grain boundaries.

Micromechanics of pressure-induced grain crushing in porous rocks

Abstract: The hydrostatic compaction behavior of a suite of porous sandstones was investigated at confining pressures up to 600 MPa and constant pore pressures ranging up to 50 MPa. These five sandstones (Boise, Kayenta, St. Peter, Berea, and Weber) were selected because of their wide range of porosity (5–35%) and grain size (60–460 μm). We tested the law of effective stress for the porosity change as a function of pressure. Except for Weber sandstone (which has the lowest porosity and smallest grain size), the hydrostat of each sandstone shows an inflection point corresponding to a critical effective pressure beyond which an accelerated, irrecoverable compaction occurs. Our microstructural observations show that brittle grain crushing initiates at this critical pressure. We also observed distributed cleavage cracking in calcite and intensive kinking in mica. The critical pressures for grain crushing in our sandstones range from 75 to 380 MPa. In general, a sandstone with higher porosity and larger grain size has a critical pressure which is lower than that of a sandstone with lower porosity and smaller grain size. We formulate a Hertzian fracture model to analyze the micromechanics of grain crushing. Assuming that the solid grains have preexisting microcracks with dimensions which scale with grain size, we derive an expression for the critical pressure which depends on the porosity, grain size, and fracture toughness of the solid matrix. The theoretical prediction is in reasonable agreement with our experimental data as well as other data from soil and rock mechanics studies for which the critical pressures range over 3 orders of magnitude.

Development of Superplastic Structural Ceramics

Abstract: Superplastic structural ceramics (Y-TZP, A1203, Si3N4, and their composites) that can withstand biaxial stretching to large strains have been developed recently. Microstructural design of these ceramics first requires an ultrafine grain size that is stable against coarsening during sintering and deformation. A low sintering temperature is a necessary, but not a sufficient, condition for achieving the required microstructure. In many cases, the selection of an appropriate phase, such as tetragonal phase in zirconia or a phase in silicon nitride, which is resistant to grain growth, is crucial. The use of sintering aids and grain-growth inhibitors, particularly those that segregate to the grain boundaries, can be beneficial. Second-phase particles are especially effective in suppressing static and dynamic grain growth. Another major concern is to maintain an adequate grain-boundary cohesive strength, relative to the flow stress, to mitigate cavitation or grain-boundary cracking during large strain deformation. Existing evidence suggests that a lower grainboundary energy is instrumental in achieving this objective. The selection of an appropriate phase and the tailoring of the grain boundary or liquid-phase composition can sometimes drastically alter the cavitation resistance. Related observations on forming methods, forming characteristics, and sheet formability are also reviewed. The basic deformation characteristics are similar to diffusional creep and are dominated by R. E. Newnham-contributing editor

On the contribution of triple junctions to the structure and properties of nanocrystalline materials

Abstract: Nanocrystalline materials, having a crystal size less than ~10 rim, have been shown to possess unusual properties [1-3]. These properties are primarily the result of a substantial fraction of the atoms (20 50%) lying in intercrystalline regions [3]. On the basis of x-ray scattering [4], EXAFS [5], hydrogen solubility [6], small angle neutron scattering [7], and self diffusivity measurements [8] conducted on bulk nanocrystalline materials, it has been suggested [3] that the grain boundaries in these materials are more disordered than those in conventional polycrystals. Recent studies[9,10] involving direct observation of nanocrystalline interfaces by HREM, provide contradictory results. Wunderlich and co-workers[9] have shown that interfaces in nanocrystalline Pd show an 'extended' structure not typically observed in conventional systems. However, Thomas et al [10] observed that the interfacial structure of nanocrystalline Pd is consistent with that typically observed in coarse-grained materials.

The variability of critical shear stress, friction angle, and grain protrusion in water-worked sediments

Abstract: The erodibility of a grain on a rough bed is controlled by, among other factors, its relative projection above the mean bed, its exposure relative to upstream grains, and its friction angle. Here we report direct measurements of friction angles, grain projection and exposure, and small-scale topographic structure on a variety of water-worked mixed-grain sediment surfaces. Using a simple analytical model of the force balance on individual grains, we calculate the distribution of critical shear stress for idealized spherical grains on the measured bed topography. The friction angle, projection, and exposure of single grain sizes vary widely from point to point within a given bed surface; the variability within a single surface often exceeds the difference between the mean values of disparate surfaces. As a result, the critical shear stress for a given grain size on a sediment surface is characterized by a probability distribution, rather than a single value. On a given bed, the crtitical shear stress distributions of different grain sizes have similar lower bounds, but above their lower tails they diverge rapidly, with smaller grains having substantially higher median critical shear stresses. Large numbers of fines, trapp.ed within pockets on the bed or shielded by upstream grains, are effectively lost to the flow. Our calculations suggest that critical shear stress, as conventionally measured, is defined by the most erodible grains, entrained during transient shear stress excursions associated with the turbulent flow; this implies a physical basis for the indeterminacy of initial motion. These observations suggest that transport rate/shear stress relationships may be controlled, in part, by the increasing numbers of grains that become available for entrainment as mean shear stress increases. They also suggest that bed textures and grain size distributions may be controlled, within the constraints of an imposed shear stress and sediment supply regime, by the influence of each size fraction on the erodibility of other grain sizes present on the bed.

Mechanical properties of nanophase TiO 2 as determined by nanoindentation

Abstract: Nanoindenter techniques have been used to determine the hardness. Young’s modulus, and strain rate sensitivity of nanophase TiO2, which is currently available only in very small quantities and which cannot be tested by most conventional techniques. Hardness and Young’s modulus both increase linearly with sintering temperature over the range 25–900°C but come to within only 50–70% of the single crystal values. Strain rate sensitivity, on the other hand, is measurably greater for this material than for single crystal rutile, and the value of strain rate sensitivity increases as the grain size and the sintering temperature are decreased. In its as-compacted form, the strain rate sensitivity of nanophase TiO2 is approximately a quarter that of lead at room temperature, indicating a potential for significant ductility in these ceramic materials. Finally, a significant scatter in hardness values has been detected within individual nanophase samples. This is interpreted as arising from microstructural inhomogeneity in these materials.

Role of grain size in the strength and R-curve properties of alumina

Abstract: An investigation of the interrelationships between strength, crack-resistance (R-curve) characteristics, and grain size for alumina ceramics has been carried out. Results of identation-strength measurements on high-density aluminas with uniform grain structures in the size range 2 to 80 μm are presented. A theoretical fit to the data, obtained by adjusting parameters of a constitutive frictional-pullout relation in a grain-bridging model, allows determination of the critical microstructural parameters controlling the R-curve behavior of these aluminas. The primary role of grain size in the toughness characteristic is to determine the scale of grain pullout at the bridged interface. It is shown that the strength properties are a complex function of the bridged microstructure, governed at all but the finest grain sizes by the stabilizing effect of the R-curve. The analysis confirms the usual negative dependence of strength on grain size for natural flaws that are small relative to the grain size, but the dependence does not conform exactly to the −1/2 power predicted on the basis of classical “Griffith-Orowan” flaws. The analysis provides a self-consistent account of the well-documented transition from “Orowan” to “Petch” behavior.

Sintering characteristics of nanocrystalline TiO2

Abstract: The microstructural development of compacted nanocrystalline TiO2 powder was studied as a function of sintering temperature up to 1000°C. Grain growth was monitored using x-ray diffraction and scanning electron microscopy. The specific surface area and total porosity were determined quantitatively using the nitrogen adsorption BET. The density was measured by gravimetry using Archimedes principle. The green body density of compacted n-TiO2 with average grain size of 14 nm is as high as 75% of theoretical bulk density. Low temperature surface diffusion leads to the disappearance of small pores, while noticeable densification commences at 600°C and reaches near theoretical values at 900°C. Grain growth also begins at 600°C, accelerating rapidly by 1000°C. Hot isostatic pressing is observed to enhance densification while suppressing grain growth.

The influence of microstructure on the properties of ferroelectric ceramics

Abstract: The domain twinning in ferroelectric ceramics is dependent on grain size. In fine grained ceramic a simple lamellar structure allows two- dimensional stress relief, in coarse grained ceramic a banded lamellar structure takes away homogeneous stress in three dimensions. The different domain configurations and internal stresses lead to different dielectric properties and to different hysteresis curves. Inhomogeneous grains of BaTiO3 with some CdBi2Nb2O9 have a core with a normal domain pattern and a shell without domains at room temperature. Core and shell have different transition temperatures. The macroscopic dielectric constant therefore has very high values in a very broad temperature range. Ceramics which are properly prepared in order to have oriented grains exhibit properties which come near to the properties of single crystals.

Grain Size Control of Tetragonal Zirconia Polycrystals Using the Space Charge Concept

Abstract: Grain growth kinetics and grain-boundary segregation of 12Ce-TZP and 2Y-TZg containing divalent to pentavalent cationic dopants, were studied. In all cases, normal grain growth following the parabolic growth relation was observed at higher temperatures. The mobility of grain boundaries was suppressed by the addition of divalent and trivalent cations, unchanged or enhanced by the addition of tetravalent and pentavalent cations. Larger cations have a stronger effect in suppressing grain growth. From ESCA, AES, and STEM analysis of the near grain-boundary regions, it is further concluded that only divalent and trivalent cations segregate. These observations can be satisfactorily rationalized using the space charge concept and the model of impurity drag. [Key words: grain growth, tetragonal zircoda polycrystals, segregation, grain boundaries, dopants.]

Grain Growth in Sintered ZnO and ZnO-Bi2O3 Ceramics

Abstract: Grain growth in a high-purity ZnO and for the same ZnO with Bi2O3 additions from 0.5 to 4 wt% was studied for sintering from 900° to 1400°C in air. The results are discussed and compared with previous studies in terms of the phenomenological kinetic grain growth expression: Gn—Gn0=K0t exp(—Q/RT). For the pure ZnO, the grain growth exponent or n value was observed to be 3 while the apparent activation energy was 224 ± 16 kJ/mol. These parameters substantiate the Gupta and Coble conclusion of a Zn2+ lattice diffusion mechanism. Additions of Bi2O3 to promote liquidphase sintering increased the ZnO grain size and the grain growth exponent to about 5, but reduced the apparent activation energy to about 150 kJ/mol, independent of Bi2O3 content. The preexponential term K0 was also independent of Bi2O3 content. It is concluded that the grain growth of ZnO in liquid-phase-sintered ZnO-Bi2O3 ceramics is controlled by the phase boundary reaction of the solid ZnO grains and the Bi2O3-rich liquid phase.

The production of nanocrystalline powders by magnetron sputtering

Abstract: Magnetron sputtering has been used for the production of nanoscale particles of pure metals, binary alloys, intermetallics, and ceramics. Al, Mo, Cu91Mn9, Al52Ti48, and ZrO2 particles with diameters of 7–50 nm were synthesized. The structures and grain sizes of the particles were examined by means of x‐ray diffraction and both transmission and scanning electron microscopy. The particles were collected and compacted in situ in the sputtering chamber. The structure of these nanocrystalline materials is also described. This novel application of magnetron sputtering at high gas pressures is of interest for both the production of nanocrystalline materials and isolated clusters.

Cold deformation microstructures

Abstract: The evolution of the cold deformation microstructure is described for medium to high stacking fault energy, single phase fcc metals. Macroscopic strain accommodation for polycrystalline metals is considered, and it is suggested that grains subdivide during deformation on a smaller and smaller scale, and that each volume element is characterised by an individual combination of slip systems. A number of microstructural observations (especially of aluminium, nickel, and copper) are described, and dislocation arrangements are discussed on the basis of the general principle that they are low energy dislocation structures. It is shown that the microstructural evolution is quite similar in polycrystalline metals and in single crystals deforming by multislip, and ways in which metallurgical parameters such as stacking fault energy and grain size can affect the microstructure are examined. The general principle of grain subdivision during cold deformation is discussed with reference to the microstructural ...

The brittle compressive fracture of ice

Abstract: The brittle compressive fracture under uniaxial loading of fresh-water, granular ice Ih has been studied. Measurements are reported of the fracture stress at temperatures from −10 to −50°C at strain rates of 10 −3 and 10 −1 s −1 for grain sizes from approximately 1 to 10 mm. Also a summary is reported of measurements by Jones et al . (unpublished) of the kinetic coefficient of friction for ice on ice at temperatures from −10 to −40°C at sliding velocities from 5 × 10 −7 m s −1 to 5 × 10 −2 ms −1 . Observations via high speed photography of internal cracking during loading are included. The strength, albeit scattered, increases with decreasing grain size, with decreasing temperature and at −10°C with decreasing strain rate. Similarly, the coefficient of friction increases with decreasing temperature and at −10°C with decreasing sliding velocity. Wing cracks were observed on some inclined cracks nucleated during loading. The results are explained in terms of the frictional crack sliding-wing crack model [as developed by Ashby and Hallam, Acta metall. 34, 497 (1986)] of compressive fracture. Finally, a simple model is presented for the transition from ductile to brittle behavior. It is based upon the competition between the building up and the relaxation of internal stresses within the vicinity of the internal cracks, and it leads to a transition strain rate which can be expressed in terms of the fracture toughness, the creep rate, the kinetic coefficient of friction and the microstructural scale of the material.

The instability of polycrystalline thin films: Experiment and theory

Abstract: Dense polycrystalline thin films of ZrO2 (3 and 8 mol % Y2O3) were produced by the pyrolysis of zirconium acetate precursor films, which were deposited on single crystal Al2O3 substrates by spin-coating aqueous solutions of zirconium acetate and yttrium nitrate. Dense films were heat treated to encourage grain growth. With grain growth, these films broke into islands of ZrO2 grains. Identical areas were examined after each heat treatment to determine the mechanism that causes the polycrystalline film to uncover the substrate. Two mechanisms were detailed: (a) for a composition which inhibited grain growth and produced a polycrystalline film with very small grains, the smallest grains would disappear to uncover the substrate, and (b) for a composition which did not inhibit grain boundary motion, larger grains grew by enveloping a smaller grain and then developed more spherical surface morphologies, uncovering the substrate at three grain junctions. In both cases, the breakup phenomenon occurred when the average grain size was larger than the film thickness. Thermodynamic calculations show that this breakup lowers the free energy of the system when the grain-size-to-film-thickness ratio exceeds a critical value. These calculations also predict the conditions needed for polycrystalline thin film stability.

Effect of grain size on brittle and semibrittle strength: Implications for micromechanical modelling of failure in compression

Abstract: Triaxial experiments were performed at room temperature and confining pressures up to 450 MPa on four pure, dense calcite rocks whose average grain sizes range over four orders of magnitude. Volumetric strain was measured during some of the experiments and microstructural studies were conducted to identify the active deformation mechanisms. The brittle fracture strength and macroscopic initial “plastic” yield stress in the semibrittle field follow empirical Hall-Petch relations. The confining pressure at the brittle-ductile transition depends inversely on grain size, but the stress ratio σ3/σ1 at the transition is nearly the same for the different rocks. We assume that the initial flaw size scales with grain size and compare our experimental data to the fracture mechanics models of Ashby and Hallam (1986) for brittle fracture and Horii and Nemat-Nasser (1986) for the brittle-plastic transition in compression. The first model predicts that small confining pressures are sufficient to inhibit work softening behavior; however, our data indicate that localization occurs for significantly higher values of confining pressure than predicted. Furthermore, we find that localization is inhibited with increased confining pressure because of the increased activity of plastic flow mechanisms, rather than because of the increased difficulty of crack propagation alone. With certain assumptions, the model predicts the experimentally determined slope of the Hall-Petch relation in the brittle field, although it underestimates the compressive strength of the rocks. The second model predicts that the stress ratio σ3/σ1 at the brittle-plastic transition scales with the, square root of the grain size; however, the experimental data do not corroborate the model unless the square of the ratio of the mode I fracture toughness to the plastic yield stress in shear scales with the grain size. The stress ratio at the brittle-ductile transition is apparently a constant for many different rock types; we suggest that the physical basis for this relationship is that the ductility of most mineral aggregates falls within a small range.

R-curve behaviour of Al2O3 ceramics

Abstract: The fracture micromechanics and underlying physical processes of fracture in Al2O3-based ceramic specimens have been studied as a function of grain size by instrumented in situ dynamic scanning electron microscopy (SEM) using the double torsion technique The toughness is found to increase with grain size Crack bridging is found to extend over hundreds of grain diameters behind the crack tip, resulting in R-curve behaviour Evidence is amassed which points to frictional energy dissipation, rather than distrubuted microcracking or crack-closure due to elastic ligaments, as the dominant contribution to toughening The friction occurs at grains which bridge the crack faces and are pulled out as the faces separate Restraining stresses, which constrain the bridging grains in their sockets, are believed to be the result of both grain morphology and the thermal expansion anisotropy of the material Simple modelling indicates that only a few percent of the grains need be involved in the frictional process to account for the toughening The conclusion is supported by hysteresis measurements

Magnetite diagenesis in marine sediments from the Oregon continental margin

Abstract: The magnetochemistry of sediments from the Oregon continental margin is examined to determine the effects of iron-sulfur diagenesis on the paleomagnetic record Magnetic mineral dissolution and transformation into iron sulfides are a common feature in these suboxic to anoxic lutites These processes are evidenced in rapid decreases in natural remanent magnetization intensities and stabilities, systematic changes in other rock magnetic properties, and increases in solid phase sulfur concentrations with depth Hysteresis measurements are used to evaluate downcore changes in magnetite concentration and grain size Magnetite abundances decrease downcore from initial values of about 01%, and nominal grain diameters lie within a narrow pseudosingle domain range of 008 to 06 μm A first-order surface area reaction model, dA/dt = -kA, is proposed to explain the magnetite dissolution mechanism, where A is the total magnetite surface area and k is the rate constant The solution of this equation predicts that the surface area and concentration decrease exponentially, and the concentration, in addition, depends on grain size Application of this model in two cores where grain size varies with depth successfully explains the downcore profiles of both concentration and surface area Despite extensive magnetite destruction, magnetic directions in such sediments appear to reliably record long-wavelength trends of the geomagnetic field

Experimental study of grain-size sensitive flow of synthetic, hot-pressed calcite rocks

Abstract: Abstract Synthetic calcite rocks of controlled grain-size were prepared by crushing large, clear calcite crystals, centrifuged to separate restricted grain-size fractions, cold-pressed and then hot-pressed to produce low-porosity polycrystals of mean grain-size covering the range 2–40 µm. These were deformed dry over the range 400–700°C, mainly at 200 MPa confining pressure, to investigate the onset with decreasing grain-size of grain-size sensitive flow. The deformation of the fine-grained material can be described by a flow law of the form ė = A exp (−H/RT) σn dm in which at low stresses (<25 MPa differential stress) A = 104.9, H = 190 kJ mol−1, n = 1.7 and m = −1.9, when strain-rate, ė, is in s−1, stress, σ, is in MPa and grain-size, d is in µm. At higher stresses (25–250 MPa) A = 100, H = 190 kJ mol−1, n = 3.3 and m = −1.3. In the low stress regime, grains remain equidimensional during flow, a pre-existing preferred crystallographic orientation produced during cold-pressing tends to weaken and grain-boundary sliding is important. At higher stresses, a grain flattening fabric develops, the pre-existing preferred crystallographic orientation pattern is preserved, and recovery-accommodated intracrystalline plastic flow is inferred to be the dominant deformation mechanism. Grain-size sensitivity of the flow stress persists even into the intracrystalline plastic flow regime. Grain-size insensitive flow only develops at grain-sizes greater than 40 µm, as demonstrated by reference to mechanical data for the flow of Carrara and Taiwan marbles. The flow laws which describe most of the experimental data do not extrapolate well through the lowest strain-rate data at the coarser grain-sizes. This indicates that a complete description of the flow, which can be used reliably to extrapolate to geological conditions, requires one or more of the material parameters A, n and m to be a function of one or more of stress, strain-rate and grain-size. This implies variable contributions from different deformation mechanisms as the deformation conditions change.

A coherent model for the complex permeability in polycrystalline ferrites

Abstract: It is demonstrated that a grain size dependence exists for the rotational permeability of a series of MnZn polycrystalline ferrites, analogous to that predicted by the Globus model for wall permeability. To account for this behavior, a model has been developed which considers crystalline ferrite grains with intrinsic complex permeability, mu /sub i/, surrounded by thin, nonmagnetic grain boundaries. The effectively measured permeability of the polycrystal ( mu /sub e/) is related in the model to the intrinsic permeability, the grain size (D), and the grain boundary thickness ( delta ) according to the equation mu /sub e/= mu /sub i/D/ mu /sub i/ delta +D. The almost linear dependence of permeability with grain size for fine-grained polycrystals emerges if one considers the limit where D is so small that D > mu /sub i/ delta , it is found that the model predicts a constant rotational permeability equivalent to that in a single crystal of the same material. In the situation where the intrinsic permeability of the ferrite displays a relaxational behavior and follows the Snoeks relationship, it is found that both the low-frequency permeability and the resonance frequency of the polycrystal are modified, but in a manner whereby the Snoeks relationship remains valid. >

Grain-size effects in ferroelectric switching

Abstract: A model of ferroelectric switching that describes crystals composed of a large number of relatively small grains is presented. It is shown that, if the grain boundaries stop or significantly affect domain-wall growth, then the observed switching current transients will deviate from those predicted by the infinite-grain model of Ishibashi and Takagi. The model accounts for constant nucleation rate throughout the switching period and for constant domain-wall velocity. It is shown that at some time dependent only on the size of the grains and the domain-wall velocity, the switching-current transient changes from its small-time infinite-grain behavior to an exponential decay. The results are applied to two-dimensional Ising-model simulations.

Anisotropy of Grain Growth in Alumina

Abstract: Grain growth in theoretically dense undoped and MgO-doped polycrystalline Al2O3 was studied, and average grain-boundary migration rates were compared with those of a-plane and c-plane sapphire during migration into the same undoped and MgO-doped materials. The results are discussed in terms of a grain-size-dependent grain-boundary mobility-grain-boundary energy product, Mbγb. The grainsize dependencies of the Mbγb products for seed and matrix grains differ. Seed orientation appears to affect the nature of solute-boundary interactions. The importance of grain-boundary structure on migration characteristics is also indicated by a demonstration of twin-formation-enhanced grain growth.

Frequency dependence of AC susceptibility in high-temperature superconductors Flux creep and critical state at grain boundaries

Abstract: The loss peak of the AC susceptibility in polycrystalline high- T c superconductors shifts slightly to higher temperatures with increasing frequency of the applied AC magnetic field It is shown that a flux creep term, added to the current density term in the critical state equation, can account for the observed frequency dependence The magnitude of the peak shift is predicted to increase with decreasing average grain size and decreasing grain boundary junction current density The model predictions are compared with the experimental data of Nikolo et al Some of the parameters used in the calculation are determined by fitting data for χ′ and χ″ over the full temperature range using a recently developed model for granular superconductors In addition, the relation between the intergranular pinning potential and the activation energy, which is extracted from log-frequency versus inverse χ″-peak temperature data, is clarified

Grain refinement induced by a critical crystal growth velocity in undercooled melts

Abstract: Containerless undercooling by electromagnetic levitation has been used to measure both growth velocity and grain size as a function of undercooling in Cu70Ni30 and Cu69Ni30B1 alloys. At a critical undercooling ΔT* the temperature dependence of the growth velocity changes discontinuously. This is accompanied by a sudden drop of grain size in the solidified product by more than two orders of magnitude. The comparative investigations show that it is not a critical undercooling that initiates the grain refinement process, but rather a critical crystal growth velocity, which approximately equals the diffusive speed.

Mechanical properties and microstructural characterization of Al-0.5%Cu thin films

Abstract: Using a wafer curvature technique we have studied the variation of stress with tem-perature in Al-0.5%Cu thin films deposited on oxidized silicon wafers. Concurrently, the microstructural changes in the films induced by the thermal cycling inherent to this technique were studied with in-situ transmission electron microscopy heating experi-ments. On heating an as-sputtered film a stress drop occurs, corresponding to the onset of grain growth. The in-situ TEM experiments indicate that the extent of grain growth is significantly altered by the presence of compressive stresses in the film. During cool-ing, dislocation loops nucleate on {111} planes inclined to the film surface, although the grain size plays an important role in determining the extent to which this mechanism can account for the deformation. A native oxide can influence the stress levels in the film by pinning one end of the dislocation loops. Upon cooling below 200° C a rapid increase in stress occurs. Although this increase has been attributed to hardening due to the precipitation of excess copper, no evidence of precipitate-dislocation interactions were observed.

Growth mechanisms of oxide scales on ODS alloys in the temperature range 1000–1100°C

Abstract: After a short overview of the production, microstructure and mechanical properties of nickel- and iron-based Oxide Dispersion Strengthened (ODS) alloys, the oxidation properties of this class of materials is extensively discussed. The excellent oxidation resistance of ODS alloys is illustrated by comparing their behaviour with conventional chromia and alumina forming wrought alloys of the same base composition. ODS alloys exhibit improved scale adherence, decreased oxide growth rates, enhanced selective oxidation and decreased oxide grain size compared to corresponding non-ODS alloys. It is shown, that these experimental observations can be explained by a change in oxide growth mechanism. The presence of the oxide dispersion reduces cation diffusion in the scale, causing the oxides on the ODS alloys to grow mainly by oxygen grain boundary transport. As oxide grain size increases with time, the oxide growth kinetics obey a sub-parabolic time dependence especially in the case of the alumina forming iron-based ODS alloy. Wachstumsmechanismen von Oxidschichten auf ODS-Legierungen im Temperaturbereich 1000–1100°C Die vorliegende Arbeit gibt zunachst eine kurze Ubersicht uber Herstellung, Mikrogefuge und mechanische Eigenschaften von oxiddispersionsverfestigten (ODS-)Legierungen auf Nickel- und Eisenbasis. Anschliesend werden die Oxidationseigenschaften dieser Werkstoffgruppe ausfuhrlich diskutiert. Die hervorragende Oxidationsbestandigkeit von ODS-Legierungen wird aufgezeigt durch Vergleich derer Eigenschaften mit denen von konventionellen chrom- und aluminiumoxidbildenden Leigerungen gleicher Basiszusammensetzung. Oxidschichten auf ODS-Legierungen unterscheiden sich von denen auf konventionellen, dispersionsfreien Legierungen insbesondere durch bessere Haftung, reduziertes Wachstum, verbesserte selektive Oxidation und feineres Korn. Es wird gezeigt, das diese experimentellen Ergebnisse durch eine Anderung der Schichtwachstumsmechanismen gedeutet werden konnen. Die Oxiddispersionen bewirken eine Reduzierung des Kationentransports in der Deckschicht, so das die Oxidschichten auf den ODS-Legierungen nahezu ausschlieslich durch Sauerstofftransport uber Oxidkorngrenzen wachsen. Da die Oxidkorngrose mit zunehmender Oxidationszeit zunimmt, gehorcht die Schichtwachstumskinetik, insbesondere bei den aluminiumoxidbildenden Eisenbasislegierungen, einem subparabolischen Zeitgesetz.