Showing papers on "Grain boundary published in 1992"

In‐plane aligned YBa2Cu3O7−x thin films deposited on polycrystalline metallic substrates

Abstract: C‐axis oriented YBa2Cu3O7−x thin films are conventionally obtained on polycrystalline substrates, but a‐ and b‐axes are randomly distributed. Due to the weak links at the high‐angle grain boundaries in the a–b plane, the critical current density (Jc) are comparatively low, from 103 to 104 A/cm2 (77 K, 0 T), and the Jc decreases in magnetic field in a manner similar to bulk YBa2Cu3 O7−x samples. To reduce weak links at the high‐angle grain boundaries, biaxially oriented buffer layers of yttrium stabilized zirconia (YSZ) were formed on polycrystalline, Ni‐based alloy by ion‐beam assisted deposition (IBAD), and subsequently the a–b plane aligned YBa2Cu3 O7−x film was deposited by laser ablation. Jc of 2.5×105 A/cm2 (77 K, 0 T) and 2.2×104 A/cm2 (77 K, 8.0 T) were obtained. A new method to prevent intergranular weak links has been developed for potential applications using practical polycrystalline substrates.

Intrinsic stress in sputter-deposited thin films

Abstract: A review of the sputtered film stress literature shows that the intrinsic stress can be tensile or compressive depending on the energetics of the deposition process. Modeling studies of film growth and extensive experimental evidence show a direct link between the energetics of the deposition process and film microstructure, which in turn determines the nature and magnitude of the stress. The fundamental quantities are the particle flux and energy striking the condensing film, which are a function of many process parameters such as pressure (discharge voltage), target/sputtering gas mass ratio, cathode shape, bias voltage, and substrate orientation. Tensile stress is generally observed in zone 1-type, porous films and is explained in terms of the grain boundary relaxation model, whereas compressive stress, observed in zone T-type, dense films, is interpreted in terms of the atomic peening mechanism. Modeling of the atomic peening mechanism and experimental data indicate that the normalized moment...

Structural and thermodynamic properties of nanocrystalline fcc metals prepared by mechanical attrition

Abstract: Nanocrystalline fcc metals have been synthesized by mechanical attrition. The crystal refinement and the development of the microstructure have been investigated in detail by x-ray diffraction, differential scanning calorimetry, and transmission electron microscopy. The deformation process causes a decrease of the grain size of the fcc metals to 6–22 nm for the different elements. The final grain size scales with the melting point and the bulk modulus of the respective metal: the higher the melting point and the bulk modulus, the smaller the final grain size of the powder. Thus, the ultimate grain size achievable by this technique is determined by the competition between the heavy mechanical deformation introduced during milling and the recovery behavior of the metal. X-ray diffraction and thermal analysis of the nanocrystalline powders reveal that the crystal size refinement is accompanied by an increase in atomic-level strain and in the mechanically stored enthalpy in comparison to the undeformed state. The excess stored enthalpies of 10–40% of the heat of fusion exceed by far the values known for conventional deformation processes. The contributions of the atomic-level strain and the excess enthalpy of the grain boundaries to the stored enthalpies are critically assessed. The kinetics of grain growth in the nanocrystalline fcc metals are investigated by thermal analysis. The activation energy for grain boundary migration is derived from a modified Kissinger analysis, and estimates of the grain boundary enthalpy are given.

Separation of film thickness and grain boundary strengthening effects in Al thin films on Si

Abstract: We have measured stress variations with temperature as a function of film thickness and a given grain size in pure Al and Al–0.5% Cu films on Si substrates. The variation in thickness for a given grain size is brought about by using the same film and the repeated controlled growth and dissolution of a barrier anodic oxide which can be grown uniformly on the film. Stress measurements were made as a function of temperature by measuring wafer curvature after successively removing each 0.1 μm of Al film. The components of strengthening due to the film thickness and the presence of grain boundaries were separated by assuming that the flow stress of the film is simply the sum of these two components. It is found that strengthening due to film thickness varies inversely with the thickness, which is consistent with results obtained by us using laser-reflowed films in an earlier work. The Hall–Petch coefficients calculated from the strengthening due to the grain boundaries are slightly higher than those reported for bulk Al. However, it is also recognized that the variation of the flow stress as g−1, where g is the grain size, is more plausible than that predicted by the Hall–Petch relation (i.e., as g−1/2). The variations of these two components with temperature, and under tension and compression, are discussed.

Materials interfaces : atomic-level structure and properties

Abstract: Contributors. Introduction. Atomic-level geometry of crystalline interfaces D. Wolf. Experimental investigation of internal interfaces in solids D.N. Seidman. Bulk interfaces. Part I: Bulk interfaces. Correlation between the structure and energy of grain boundaries in metals D. Wolf, K.L. Merkle. Grain and interphase boundaries in ceramics and ceramic composites M.G. Norton, C.B. Carter. Special properties of E grain boundaries G. Palumbo, K.T. Aust. Grain boundary structure and migration D.A. Smith. Role of interfaces in melting and solid-state amorphization S.R. Phillpot, S. Yip, P.R. Okamoto, D. Wolf. Wetting of surfaces and grain boundaries D.R. Clarke, M. Gee. Part II: Semi-bulk and thin-film interfaces. Structural, electronic and magnetic properties of thin films and superlattices A. Continenza, C.Li, A.J. Freeman. Surfaces and interfaces as studied by scanning-tunneling microscopy R. Hamers. Epitaxy of semiconductor thin films J. Batstone. Phase behavior of monolayers S.G.J. Mochrie, D. Gibbs, D.M. Zehner. Elastic and structural properties of superlattices M. Grimsditch, I.K. Schuller. Computer simulation of the elastic behavior of interface materials D. Wolf, J. Jasczak. Interfaces within intercalation compounds M.S. Dresselhaus, G. Dresselhaus. Nanophase materials: structure-property correlations R.W. Siegel. Part III: Role of interface chemistry. Interfacial segregation, bonding and reactions C.L. Briant. Physics and chemistry of segregation at internal interfaces R. Kirchheim. Atomic resolution study of solute-atom segregation at grain boundaries: experiments and Monte Carlo simulations S.M. Foiles, D. Seidman. Amorphization by interfacial reaction W.L. Johnson. Relationship between structural and electronic properties of metal-semiconductor interfaces R. Tung. Electronic properties of semiconductor-semiconductor interfaces and their control using interface chemistry D.W. Niles, G. Margaritondo. Microscopic nature of metal-polymer interfaces P.S. Ho, B.D. Silverman, S-L. Chiu. Part IV: Fracture behavior. Tensile strength of interfaces A.S. Argon, V. Gupta. Microstructure and fracture resistance of metal/ceramic interfaces A.G. Evans, M. Ruhle. Role of interface dislocations and surface ledges in the work of adhesion D. Wolf, J. Jaszczak. Microstructural and segregation effects in the fracture of polycrystals D.J. Srolovitz, W. Yang.

Processing of bulk YBaCuO

Abstract: The author summarizes processing of bulk YBa2Cu3Ox superconductors with an emphasis placed on the relationship between microstructure and critical currents. Sintering is commonly used in ceramic processing but has been unsuccessful in producing high-Jc materials, primarily due to the weak links at grain boundaries. Melt processes have been found to be effective in increasing Jc values through a combination of grain alignment and the introduction of pinning centres. Some possible applications of melt processed superconductors in bulk form are also introduced in this review. Although many attempts have been made to fabricate long conductors, Jc values are still very small for long conductors, while the length is limited for high-Jc materials.

Large anisotropic thermal conductivity in synthetic diamond films

Abstract: AS high-power electronic devices are packed to progressively higher densities, synthetic diamond films are being considered as heat spreaders for the prevention of thermal damage (see ref. 1 for example). Although diamond single crystals are known to have the highest thermal conductivity for any material at room tem-perature (22 W cm−1 K−1 for diamond with natural isotopic abundance, compared with 4 W cm-1 K-1 for copper), the dependence of conductivity on the microstructure of polycrystalline diamond films is not understood. Using a newly developed laser technique2, we have measured thermal conductivity in the experimentally difficult direction perpendicular to the plane of the diamond film. Taken together with earlier in-plane measurements3, this gives a complete description of the local thermal conductivity, showing a significant gradient and anisotropy correlated with the inhomogeneous grain structure. Despite phonon scattering at lattice defects and grain boundaries, we find that the local conductivity near the top growth surface of a synthetic diamond film is, surprisingly, at least as high as that of gem-quality diamond single crystals.

Oxidation of MoSi2 and comparison with other silicide materials

Abstract: The oxidation behavior of MoSi2 in three fabrication conditions has been studied in oxygen and in air The cast material was studied over the temperature range 500–1400°C, the hot isostatically pressed (HIP) material was studied at 500 and 1000°C and the single crystals were studied at 500°C The cast material exhibited three regimes of behavior Above 1000°C a continuous protective silica scale formed Between 600 and 1000°C a silica scale formed, but formation of silica within grain boundaries, which are believed to be cracked, was observed At temperatures near 500°C accelerated linear oxidation, involving the formation of both Mo and Si oxides was observed and the specimen fragmented into powder (“pested”) The HIP material exhibited two regimes At 1000°C a protective silica film formed At temperatures around 500°C the HIP material underwent accelerated oxidation but did not fragment The oxidation of the single crystal was qualitatively the same as that for the HIP material at 500°C It was concluded, therefore, that accelerated oxidation is a necessary, but not sufficient, condition for pesting to occur The pesting of the cast material was concluded to occur by oxidation along pre-existing microcracks in the MoSi2 Preoxidation at 1000°C was found to be only partially successful in limiting accelerated oxidation during subsequent exposure at 500°C The oxidation of TaSi2 was observed to be qualitatively the same as that for MoSi2 However, the high thermodynamic stability and low volatility of Ta2O5 result in much higher temperatures being required for the occurrence of selective oxidation of Si

Stress and electromigration in Al-lines of integrated circuits

Abstract: The generation of tensile and compressive stress by the annihilation and production of vacancies which are subject to driving forces due to electromigration and due to the developing stress gradient is calculated. The rate of the stress changes is related to the deviation of the vacancy concentration from its equilibrium concentration. Depending on the magnitude of the rate constant different mechanisms of annihilation and production of vacancies (i.e. in the grain boundary itself, in adjacent grain boundaries or at dislocations) are covered. The resulting differential equations are solved numerically and analytically for some limiting cases. A quasi steady state concentration profile is established witin a short initial period of time, which is determined by one of the three processes diffusion, electromigration or rate of vacancy annihilation. During the quasi steady state the stresses increase linearly with time. When stresses are large enough to change the equilibrium vacancy concentration deviation from the linear increase occur and a rather long period follows where the true steady state is approached. If the time to failure, tf, of an Al-line is defined as the time necessary to reach a critical stress, tf is proportional to j−n where the current exponent changes from 1 at low critical stresses to 2 at higher stresses. The theoretical results are in excellent agreement with recent measurements of compressive stresses both with respect to the dependence on time and position.

Intercrystalline stable isotope diffusion: a fast grain boundary model

Abstract: We formulated a numerical model for stable isotope interdiffusion which predicts the temperatures recorded between two or more minerals, and the intragranular distribution of stable isotopes in each mineral, as functions of mineral grain sizes and shapes, diffusivities, modes, equilibrium isotopic fractionations, and the cooling rate of a rock. One of the principal assumptions of the model is that grain boundaries are regions of rapid transport of stable isotopes. This Fast Grain Boundary (FGB) model describes interdiffusion between any number of mineral grains, assuming that local equilibrium and mass balance restrictions apply on the grain boundaries throughout the volume modeled. The model can be used for a rock containing any number of minerals, and number of grain sizes of each mineral, several grain shapes, and any thermal history or domain size desired. Previous models describing stable isotope interdiffusion upon cooling have been based on Dodson's equation or an equivalent numerical analogue. The closure temperature of Dodson is the average, bulk temperature recorded between a mineral and an infinite reservoir. By using Dodson's equation, these models have treated the closure temperature as an innate characteristic of a given mineral, independent of the amounts and diffusion rates of other minerals. Such models do not accurately describe the mass balance of many stable isotope interdiffusion problems. Existing models for cation interdiffusion could be applied to stable isotopes with some modifications, but only describe exchange between two minerals under specific conditions. The results of FGB calculations differ considerably from the predictions of Dodson's equation in many rock types of interest. Actual calculations using the FGB model indicate that closure temperature and diffusion profiles are as strongly functions of modal abundance and relative differences in diffusion coefficient as they are functions of grain size and cooling rate. Closure temperatures recorded between two minerals which exchanged stable isotopes by diffusion are a function of modal abundance and differences in diffusion coefficient, and may differ from that predicted by Dodson's equation by hundreds of degrees C. Either or both of two minerals may preserve detectable zonation, which may in some instances be larger in the faster diffusing mineral. Rocks containing three or more minerals can record a large span of fractionations resulting from closed system processes alone. The results of FGB diffusion modeling indicate that the effects of diffusive exchange must be evaluated before interpreting mineral fractionations, concordant or discordant, recorded within any rock in which diffusion could have acted over observable scales. The predictions of this model are applicable to thermometry, evaluation of open or closed system retrogression, and determination of cooling rates or diffusion coefficients.

Gas Pressure and Temperature Dependences of Thermal Conductivity of Porous Ceramic Materials: Part 2, Refractories and Ceramics with Porosity Exceeding 30%

Abstract: A review of experimental results and theoretical models for thermal conductivities of ceramic materials with porosity less than 30% is given. It is shown that the abnormal non-monotonic pressure and temperature dependences of thermal conductivity arise from the effects of microcracks and porous grain boundaries, characterizing many industrial refractories, and from the competitive influences of classical and novel mechanisms of heat transfer in composite multiphase materials. The latter mechanisms include segregation and surface diffusion of impurities and defects in crystal structure, and the mechanism arising from chemical conversion and gas emission, occurring within pores of ceramic materials. A fractal model of porous materials' structure is proposed and used for analysis, explanation, and classification of thermophysical properties of ceramic materials measured in various thermodynamic conditions.

Modeling of agglomeration in polycrystalline thin films : application to TiSi2 on a silicon substrate

Abstract: An equilibrium model for agglomeration in polycrystalline thin films which considers the energy balance between the grain boundary energy and both surface and substrate interface energies is presented. It predicts that small grain size, low grain boundary energy, and high film surface and interface energies should promote resistance to agglomeration, and shows that the substrate‐film interface can play a significant role in the process. It also predicts a critical grain size limiting formation of a discontinuous island structure. This easily calculable value is significantly smaller than that found in previous modeling. The critical grain size, the importance of the substrate interface, and some of the assumptions are shown to be consistent with transmission microscope observations of TiSi2 thin films deposited on Si substrates.

Grain size effects in nanocrystalline materials

Abstract: Nanocrystalline materials have a grain size of only a few nanometers and are expected to possess very high hardness and strength values. Even though the hardness/strength is expected to increase with a decrease in grain size, recent observations have indicated that the hardness increases in some cases and decreases in other cases. A careful analysis of the available results on the basis of existing models suggests that there is a critical grain size below which the triple junction volume fraction increases considerably over the grain boundary volume fraction and this is suggested to be responsible for the observed softening at small grain sizes.

The influence of grain boundary inclination on the structure and energy of Σ=3 grain boundaries in copper

Abstract: In a combined theoretical and experimental study, the energies and structures of σ = 3, [011] tilt boundaries in Cu were investigated. Equilibrium atomistic structures and grain-boundary energies were calculated by static energy minimization using an embedded-atom potential. Cu bicrystals of the same boundary orientations were fabricated by welding of Cu single crystals. Grain-boundary energies were measured by the thermal grooving technique. The atomistic structure of the {211} twin boundary was investigated by high-resolution transmission electron microscopy (HRTEM). The calculated grain-boundary energies γb plotted against the inclination of the boundary plane show a minimum for the {111} twin boundary and a second minimum at an inclination of about 82° to the {111} boundary. The calculated dependence of γb on inclination is confirmed by the measured energies over the entire range. Common to all calculated boundary structures is a microfaceting into {111} and {211} twin facets. The structures ...

Preparation and deformation of synthetic aggregates of quartz

Abstract: Synthetic quartz aggregates of low porosity have been fabricated from natural quartz powder, impure silica gel, and high-purity silicic acid by hydrothermal isostatic pressing at 300 MPa. The resulting specimens have water contents comparable to those of natural quartzites and have been used for deformation tests. The flow stress at 1200–1300 K was found to vary in an inverse relationship to the water pressure estimated under the assumption that the measured water content homogeneously filled the measured porosity. It is questionable that the natural-quartz-origin specimens were fully equilibrated in respect to the activity of the water, but equilibration has probably been achieved in the amorphous-silica-origin specimens, which crystallized under the experimental conditions. In the deformation of the latter materials, the experimental activation energy at water pressures approaching the confining pressure was found to be about 150 kJ mol−1 for both. However, the impure gel-origin material gave a stress exponent of about 2.3, whereas the high-purity silicic-acid-origin material gave a stress exponent of about 4, in spite of the grain sizes being about 90μm and 20 μm, respectively. It is concluded that while the impure specimens have higher intragranular strengths, there is also a significant contribution of grain boundary processes to the strain in them which is absent in the high-purity specimens.

Hydrogen detrapping from grain boundaries and dislocations in high purity iron

Abstract: Hydrogen detrapping in high purity iron was studied by measuring evolution rates of quenched-in hydrogen from 80 to 800 K using a quadrupole mass spectrometer in an ultra high vacuum system. The peak of the evolution rate was observed at 395 K in single crystal specimens and 415 K in polycrystalline specimens with a heating rate of 1 K min−1. Effects of grain size and deformation on the evolution rate was also studied. It was shown that the results are consistent with the evolution rates calculated with the binding energy B = 0.51 ± 0.02 eV and the trap density term γCT = (4 ∼ 15) × 10−5 in polycrystalline iron, and B = 0.47 ± 0.02 eVand γCT = (2 ∼ 13) × 10−5 in single crystal iron. The dominant traps are considered to be grain boundaries in polycrystalline specimens and dislocations in single crystal specimens.

Effects of Crystalline Anisotropy on Fluid Distribution in Ultramafic Partial Melts

Abstract: The textures of experimentally produced ultramafic partial melts show consistent and significant deviations from the morphology predicted by isotropic equilibrium theory. Flat crystalline interfaces are pervasive in these systems and they coexist with smoothly curved boundaries which are predicted by the isotropic theory. Long-duration experimental runs on olivine-basalt mixtures held at pressures between 1.0 and 2.0 GPa and temperatures from 1350°C to 1400°C were evaluated. Scanning electron microscope images of samples with 2.5 or more volume percent melt showed at least 20% of observable grain boundaries to be wetted by the melt. In addition, approximately 60% of the melt tubules occurring along triple junctions in this sample were found to have at least one flat interface, an effect which increases the ratio of permeability to porosity. Both the experimental evidence and theoretical considerations indicate that the flat faces are stable equilibrium or steady state features in these partial melts, and that they are crystallographically controlled. The crystal-melt morphology is influenced by the crystalline equilibrium habit (obtained from minimization of surface energies of individual crystals) under the constraints of polycrystalline aggregates. A dependence of the style of melt distribution on melt fraction was observed in these runs. At very low melt fractions (less than 1 vol %) the texture is dominated by melt-filled triple junctions and mostly dry grain boundaries, whereas at higher melt fractions (but below 5 vol %) more melt pockets and melt films along grain boundaries appear. An interpretation of the observed texture is made, applying established crystal growth and interface theories to steady state partially molten systems. The extensive occurrence of flat or faceted crystallographic faces in partial melts requires major changes in the modeling of their permeabilities, as well as bulk elastic, anelastic, and electrical properties, from existing models of melt distribution. In regions of the upper mantle where olivine lattice preferred orientation is expected (e.g., in the vicinity of mid-ocean ridges) the presence of faceted faces and associated changes in melt distribution will produce anisotropic permeabilities and changes in seismic attenuation.

Theory of conduction and breakdown in perovskite thin films

Abstract: The mechanism of the dc electrical conduction and breakdown of perovskite-type titanates was investigated by impedance analysis. Based on an acceptor doped SrTiO3 model material, samples of different microstructures-ceramics, single crystals, and thin films-were employed. This approach allows us to distinguish conduction contributions of the bulk lattice, grain boundaries, and electrode interfaces. Based on defect chemistry studies, a predominant ionic contribution due to mobile oxygen vacancies and an additional p-type conduction were revealed for the bulk. At interfaces, space charge depletion layers of 100-500 nm width are formed in which the local conductivity is reduced by approx. four orders of magnitude compared to the bulk. Thin films show a similar depression of the conductivity. The combination of these facts may be indicative for considering thin films as distributed Schottky barriers. The field enhancement of the conductivity of thin films and of the interface depletion layers is compared and ...

Surface and grain boundary analysis of doped zirconia ceramics studied by AES and XPS

Abstract: The surface- and grain boundary composition of Y, Ce and Ti doped zirconia were studied by X-ray Photoelectron Spectroscopy and Auger Electron Spectroscopy/Scanning Auger Microscopy. The grain boundaries and free surfaces showed the same enrichment levels. After heat treatment ≥ 1000 ‡C all yttria doped samples showed yttrium enrichment. In the ZrO2-Y2O3 system the yttrium enrichment did not depend on the bulk concentration and amounted 30–34 mol% YO1.5 in all cases. As a consequence the segregation factor increases with decreasing solute concentration in the bulk. The thickness of the segregation layer was about 2–4 nm. In the ternary Y doped systems yttrium is the main segregant. In ceria-doped tetragonal zirconia polycrystals (Ce-TZP) systems significant segregation of cerium starts atT≥1300‡C and is mainly attributed to Ce3+. In Y,Ti-TZP systems also strong segregation of Ti4+ occurs. The absolute value of the increase of the surface concentration in fine grained material is smaller than in coarse grained material. This is mainly due to depletion of the bulk.

Crystallization of Fe73.5Cu1Nb3Si13.5B9 structure and kinetics examined by X-ray diffraction and Mossbauer effect spectroscopy

Abstract: Changes in the structure of the amorphous alloy Fe73.5Cu1Nb3Si13.5B9 were investigated after annealing for 1 h in a temperature range from 450-800 degrees C using X-ray diffraction scattering and Mossbauer effect spectroscopy. Between 520 and 550 degrees C nanocrystalline Fe80Si20 grains with the DO3 structure (diameter of about 10 nm) are embedded in an amorphous grain boundary phase. Above 650 degrees C the grain size increases and the amorphous grain boundary phase disappears. The Fe-B phases form and a new paramagnetic phase is observed. Furthermore the kinetics of the amorphous-to-nanocrystalline phase transition of this alloy was examined by X-ray diffraction observing the development of crystallization with time at a fixed annealing temperature of 520 degrees C. The beginning of crystallization appears at times less than 2 min, most grains developing in the first 10 to 20 min while after about 5 min the grain size remains constant with a diameter of about 10 nm.

The Role of grain boundary misorientation in intergranular cracking of Ni-16Cr-9Fe in 360 °C argon and high-Purity water

Abstract: The effect of grain boundary misorientation on the intergranular cracking behavior of pure Ni-16Cr-9Fe was assessed by determining if low-angle boundaries (LABs) or coincident site lattice boundaries (CSLBs) are more crack resistant than general high-angle boundaries (GHABs) in argon and high-purity water. Cracking susceptibility of boundary types was determined using constant extension rate tensile tests (CERTs) in 360 °C argon and in deaerated, high-purity water. Annealed samples contained 12 to 20 pct CSLBs, while CSLB-enhanced samples contained 27 to 44 pct CSLBs; GHAB proportions varied accordingly. Cracked boundary fractions for CSLB-enhanced samples tested in either environment ranged from 0.01 to 0.08, while those for annealed samples ranged from 0.07 to 0.10, indicating that samples with increased proportions of CSLBs are more crack resistant. No LABs cracked in either environment. In annealed samples, the proportion of CSLBs that cracked in water was 6.7 pct compared to 1.5 pct in argon; the proportion of GHABs that cracked in water was 9.3 pct compared to 6.6 pct for argon. Thus, CSLBs are more crack resistant than GHABs in either environment, and both are more crack resistant in argon than in water. The higher amounts of cracking and the higher CSLB cracking susceptibility in high-purity water indicate the presence of an environmental effect on cracking behavior. The beneficial effect of LABs and CSLBs is likely due to the ability of these boundaries to induce slip in neighboring grains by either transmitting or absorbing and re-emitting lattice dislocations, thereby reducing grain boundary stresses and the propensity for crack initiation. The results indicate that control of grain boundary proportions can improve the intergranular stress corrosion cracking susceptibility of pure Ni-16Cr-9Fe.

Synthesis of molybdenum disilicide by mechanical alloying

Abstract: Considerable interest and effort are being directed towards developing molybdenum disilicide (MoSi2) alloys with low oxygen content. During alloy synthesis, oxygen combines with Si to form glassy SiO2 precipitates at the MoSi2 grain boundaries, resulting in a degradation of its mechanical properties. We have used mechanical alloying, a high-energy ball-milling process, to synthesize alloy powders of MoSi2, MoSi2-27 mol.% MoSi3, MoSi2-50 mol.% Mo5Si3 and MoSi2-50 mol.% WSi2 starting from elemental powders. The processing of the powders, as well as the loading of the powders in graphite dies, was performed under high-purity argon inside a glovebox. The finer grain and particle size of the mechanically alloyed powders enabled us to hot-press them at 1500 °C, which is 300 °C lower than the temperature currently used for hot-pressing commercial powders. We have been successful in reducing the oxygen content in our alloys to about 310 ppm by weight, as measured by nuclear (d,p) reactions. We report the formation of metastable phases in the mechanically alloyed powders and their characterization by X-ray diffraction and differential thermal analysis. We also report the characterization of the hot-pressed alloys by optical and transmission electron microscopy, and the measurement of high-temperature mechanical properties.

The transition from mild to severe wear in alumina during sliding

Abstract: The occurrence of a wear transition in alumina during sliding has been investigated experimentally. The results show that a transition from initially mild wear to severe wear occurs abruptly, but only after a defined period of initial wear. The time required for this transition increases with decreasing grain size and decreasing applied load. Examination of wear samples revealed that, during the initial stage, surface material is removed by a plastic grooving process and is accompanied by the accumulation of subsurface dislocations arrays and twins. With continued sliding, internal stresses associated with this accumulating damage eventually results in grain boundary cracking and grain pull-out, which leads to the onset of fracture dominated, severe wear.