Showing papers in "Acta Materialia in 2000"

Stabilization of metallic supercooled liquid and bulk amorphous alloys

Abstract: Bulk metallic materials have ordinarily been produced by melting and solidification processes for the last several thousand years. However, metallic liquid is unstable at temperatures below the melting temperature and solidifies immediately into crystalline phases. Consequently, all bulk engineering alloys are composed of a crystalline structure. Recently, this common concept was exploded by the findings of the stabilization phenomenon of the supercooled liquid for a number of alloys in the Mg-, lanthanide-, Zr-, Ti-, Fe-, Co-, Pd-Cu- and Ni-based systems. The alloys with the stabilized supercooled liquid state have three features in their alloy components, i.e. multicomponent systems, significant atomic size ratios above 12%, and negative heats of mixing. The stabilization mechanism has also been investigated from experimental data of structure analyses and fundamental physical properties. The stabilization has enabled the pro- duction of bulk amorphous alloys in the thickness range of 1-100 mm by using various casting processes. Bulk amorphous Zr-based alloys exhibit high mechanical strength, high fracture toughness and good cor- rosion resistance and have been used for sporting goods materials. The stabilization also leads to the appearance of a large supercooled liquid region before crystallization and enables high-strain rate super- plasticity through Newtonian flow. The new Fe- and Co-based amorphous alloys exhibit a large super- cooled liquid region and good soft magnetic properties which are characterized by low coercive force and high permeability. Furthermore, homogeneous dispersion of nanoscale particles into Zr-based bulk amor- phous alloys was found to cause an improvement of tensile strength without detriment to good ductility. The discovery of the stabilization phenomenon, followed by the clarification of the stabilization criteria of the supercooled liquid, will promise the future definite development of bulk amorphous alloys as new basic science and engineering materials. # 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.

Nanostructured materials: basic concepts and microstructure☆

Abstract: Nanostructured Materials (NsM) are materials with a microstructure the characteristic length scale of which is on the order of a few (typically 1–10) nanometers. NsM may be in or far away from thermodynamic equilibrium. NsM synthesized by supramolecular chemistry are examples of NsM in thermodynamic equilibrium. NsM consisting of nanometer-sized crystallites (e.g. of Au or NaCl) with different crystallographic orientations and/or chemical compositions are far away from thermodynamic equilibrium. The properties of NsM deviate from those of single crystals (or coarse-grained polycrystals) and/or glasses with the same average chemical composition. This deviation results from the reduced size and/or dimensionality of the nanometer-sized crystallites as well as from the numerous interfaces between adjacent crystallites. An attempt is made to summarize the basic physical concepts and the microstructural features of equilibrium and non-equilibrium NsM.

Modelling of inoculation of metallic melts : Application to grain refinement of aluminium by Al-Ti-B

Abstract: A numerical model is presented for the prediction of grain size in inoculated castings and is tested against measured grain sizes obtained in standard grain-refiner tests on aluminium alloys. It is shown that for potent nucleants, such as commercial grain refiners for aluminium, the nucleation stage itself is not the controlling factor. The number of grains is determined by a free-growth condition in which a grain grows from a refiner particle at an undercooling inversely proportional to the particle diameter. With measured particle size distributions as input, the model makes quantitatively correct predictions for grain size and its variation with refiner addition level, cooling rate and melt composition. The model can assist in optimizing the use of existing refiners and in developing improved refiners.

High-temperature structural intermetallics

Abstract: In the last one and a half decades, a great deal of fundamental and developmental research has been made on high-temperature structural intermetallics aiming at the implementation of these intermetallics in aerospace, automotive and land-based applications. These intermetallics include aluminides formed with either titanium, nickel or iron and silicides formed with transition metals. Of these high-temperature intermetallics, TiAl-based alloys with great potential in both aerospace and automotive applications have been attracting particular attention. Recently TiAl turbocharger wheels have finally started being used for turbochargers for commercial passenger cars of a special type. The current status of the research and development of these high-temperature intermetallics is summarized and a perspective on what directions future research and development of high-temperature intermetallics should take is provided.

Microstructure and strength of nickel at large strains

Abstract: A quantitative microstructural analysis is presented for pure polycrystalline nickel (99.99%) cold rolled to reductions from 70 to 98% (evM 1.4–4.5). Applying transmission electron microscopy (TEM) techniques, key structural parameters, such as spacing between dislocation boundaries and high angle boundaries, as well as the misorientations across their boundaries, have been measured and analyzed. Application of scaling and similitude hypotheses to these microstructural parameters and their distributions revealed that the structures maintain a similar character with increasing strain. This similarity indicates that the measured parameters capture the important features of the structure. Scaling and similitude provide governing principles for structure formation. Based on this structural information and a detailed description of the morphology, structural parameters are identified, strength determining parameters chosen, and strength–structural relationships discussed. The suggestion is then made that two strengthening contributions should be considered: (i) dislocation strengthening due to the presence of low angle boundaries and (ii) grain boundary strengthening due to medium to high angle boundaries. The calculated individual strength contributions evolve differently with the strain and their addition leads to flow stress values and hardening rates in good agreement with those observed experimentally. No saturation of the calculated or experimental flow stress was observed.

Characterisation of strengthening precipitate phases in a Mg–Y–Nd alloy

Abstract: Strengthening precipitate phases in a Mg–Y–Nd based alloy (WE54), aged at 250°C, have been characterised using transmission electron microscopy. Precipitation at 250°C involves formation of three separate metastable phases, {11 2 0} α platelets of an as yet unidentified phase, and the phases designated β′ and β1, preceding formation of the equilibrium phase β. All three phases β′, β1 and β are present in significant fractions in peak-aged samples. The β′ phase has a base-centred orthorhombic structure, with a potential point group of mmm. The β1 phase has an f.c.c. structure (space group Fm 3 m , a=0.74±0.01 nm), which renders it isomorphous with a family of intermetallic compounds of the general form Mg3X, where X represents Nd, Ce, La, Pr, Dy and Sm. The equilibrium phase β has an f.c.c. structure (space group, F 4 3 m , a=2.2±0.1 nm), which makes it isomorphous with Mg5Gd. The formation of β1 phase is shown to generate significant shear strain energy, and a mechanism of shear strain energy accommodation is proposed, involving nucleation in association with β′ phase. With prolonged ageing at 250°C, the β1 phase transforms in situ to the equilibrium β phase.

Materials issues in microelectromechanical systems (MEMS)

Abstract: Microelectromechanical systems (MEMS) have recently become an important area of technology, building on the success of the microelectronics industry over the past 50 years. MEMS combine mechanical and electrical function in devices at very small scales. Examples include pressure sensors, accelerometers, gyroscopes and optical devices, as well as chemical, biomedical and fluidic applications. The status of MEMS technology is reviewed with particular emphasis on materials issues therein. The materials issues in MEMS are divided into three categories, the MEMS material set, microfabrication processes, and material characterization and design. Each of these areas is addressed, with particular emphasis on the potential impact of materials solutions. A discussion of the future of MEMS and the role of materials in that future is given.

Crystal structure of martensitic phases in Ni–Mn–Ga shape memory alloys

Abstract: The crystal structures of the different martensitic phases observed in a wide variety of Ni–Mn–Ga alloy compositions have been studied in detail. Similarly to the Ni–Al alloys, the non-modulated martensite can be well described by the L10 lattice, although it must be “doubled” in order to account for the L21 type of order of the parent phase. Concerning the well known five- and seven-layered martensites, two approaches taken from the literature are analysed and discussed. The first approach, widely accepted in Ni–Al alloys, describes the structure as long period stacking of {110}P close packed planes (10 M and 14 M structures), while the second one considers lattices modulated by shuffling. The two approaches are shown to be very similar and, in many cases, indistinguishable by the diffraction techniques using photographic recording. However, some physical arguments are given to interpret the five- and seven-layered structures with the second and first approaches, respectively. The structure of the “new” 10-layered martensite is studied, being described by a (5 5 ) stacking sequence of close-packed planes.

Failure mechanisms associated with the thermally grown oxide in plasma-sprayed thermal barrier coatings

Abstract: The microstructure and durability of a thermal barrier coating (TBC) produced by the thermal spray method have been characterized. Upon exposure, the bond coat chemistry and microstructure change by inter-diffusion with the substrate and upon thickening of the thermally grown oxide (TGO). A wedge impression test, in conjunction with observations by scanning electron microscopy, has been used to probe the failure mechanisms. At short exposure times, when the TGO thickness is less than about 5 μ m, the growth of the TGO does not affect the crack patterns in the TBC and delaminations induced by wedge impression propagate within the TBC about 30 μ m from the interface. An amorphous phase at the splat interfaces promotes this failure mode. As the thickness of TGO increases during exposure, cracks form in the TBC around imperfections at the interface. Moreover, induced delaminations develop a trajectory close to the interface, propagating not only through the TBC but also within the TGO and along the interfaces. A scaling result based on the misfit around imperfections caused by TGO growth has been used to rationalize the critical TGO thickness when the TBC fails.

Tensile testing of free-standing Cu, Ag and Al thin films and Ag/Cu multilayers

Abstract: Free standing polycrystalline thin films with a strong 〈111〉 texture were tested in uniaxial tension Studied were electron-beam deposited Ag, Cu and Al films, and Ag/Cu multilayers consisting of alternating Ag and Cu layers of equal thickness, between 15 nm and 15 μm (bilayer repeat length, λ , between 3 nm and 3 μm) The films had a total thickness of about 3 μm A thin polymeric two-dimensional diffraction grid was deposited on the film surface by microlithographic techniques Strains were measured in situ from the relative displacements of two laser spots diffracted from the grid The average values of the Young’s moduli, determined from hundreds of measurements, are 63 GPa for Ag, 102 GPa for Cu, 57 GPa for Al and 875 GPa for Ag/Cu multilayers In all cases, these values are about 20% lower than those calculated from the literature data and, for the Ag/Cu multilayers, are independent of λ No “supermodulus” effect was observed The 20% reduction in modulus is most likely the result of incomplete cohesion (“microcracking”) of the grain boundaries The ductility of the Ag/Cu multilayers decreases when λ is reduced For λ λ >80 nm, the yield stress increases with decreasing λ according to a Hall–Petch-type relation No softening with decreasing grain size was observed even at the lowest values of λ

Diffusion in the Ti–Al system

Abstract: Many properties of industrial Ti–Al alloys, such as high-temperature stability of the lamellar structure and creep resistance, are determined by diffusion rates in the phases and along the interfaces. The knowledge of diffusion characteristics and fundamental understanding of diffusion mechanisms are of great importance to the research and design of industrial Ti–Al alloys. This paper gives an overview of recent progress in experimental and theoretical studies of diffusion behavior in the phases of the Ti–Al system. The experimental methods used in modern diffusion measurements are briefly described, and recent experimental results for Ti and Al diffusion in α-Ti(Al), β-Ti(Al), and intermetallic phases α2-Ti3Al and γ-TiAl, are summarized. The results for interdiffusion and impurity diffusion in these phases are also discussed in detail. The second part of the paper provides an overview of current understanding of point defects and diffusion mechanisms in Ti3Al and TiAl. A statistical model of point-defect disorder in ordered compounds is presented and applied to Ti3Al and TiAl using input data generated with embedded-atom potentials. Possible atomic mechanisms of diffusion in these compounds are analyzed in detail, and methods of diffusion calculations under different mechanisms are reviewed. The relative importance of different mechanisms in Ti3Al and TiAl is evaluated by comparing their estimated activation energies. Prospective topics of further experimental and theoretical research in this area are outlined.

Solidification microstructures: recent developments, future directions

Abstract: The status of solidification science is critically evaluated and future directions of research in this technologically important area are proposed. The most important advances in solidification science and technology of the last decade are discussed: interface dynamics, phase selection, microstructure selection, peritectic growth, convection effects, multicomponent alloys, and numerical techniques. It is shown how the advent of new mathematical techniques (especially phase-field and cellular automata models) coupled with powerful computers now allows the following: modeling of complicated interface morphologies, taking into account not only steady state but also non-steady state phenomena; considering real alloys consisting of many elements through on-line use of large thermodynamic data banks; and taking into account natural and forced convection effects. A series of open questions and future prospects are also given. It is hoped that the reader is encouraged to explore this important and highly interesting field and to add her/his contributions to an ever better understanding and modeling of microstructure development.

Carbon nanotube-metal-oxide nanocomposites: microstructure, electrical conductivity and mechanical properties

Abstract: Carbon nanotube-metal-oxide composites (metal=Fe, Co or Fe/Co alloy; oxide=Al2O3, MgO or MgAl2O4) have been prepared by hot-pressing the corresponding composite powders, in which the carbon nanotubes, mostly single or double-walled, are very homogeneously dispersed between the metal-oxide grains. For the sake of comparison, ceramic and metal-oxide nanocomposites have also been prepared. The microstructure of the specimens has been studied and discussed in relation to the nature of the matrix, the electrical conductivity, the fracture strength and the fracture toughness. The carbon nanotube-metal-oxide composites are electrical conductors owing to the percolation of the carbon nanotubes.

Discrete and continuous deformation during nanoindentation of thin films

Abstract: This paper describes nanoindentation experiments on thin films of polycrystalline Al of known texture and different thicknesses, and of single crystal Al of different crystallographic orientations. Both single-crystalline and polycrystalline films, 400–1000 nm in thickness, are found to exhibit multiple bursts of indenter penetration displacement, h, at approximately constant indentation loads, P. Recent results from the nanoindentation studies of Suresh et al. (Suresh, S., Nieh T.-G. and Choi, B.W., Scripta mater., 1999, 41, 951) along with new microscopy observations of thin films of polycrystalline Cu on Si substrates are also examined in an attempt to extract some general trends on the discrete and continuous deformation processes. The onset of the first displacement burst, which is essentially independent of film thickness, appears to occur when the computed maximum shear stress at the indenter tip approaches the theoretical shear strength of the metal films for all the cases examined. It is reasoned that these displacement bursts are triggered by the nucleation of dislocations in the thin films. A simple model to estimate the size of the prismatic dislocation loops is presented along with observations of deformation using transmission electron microscopy and atomic force microscopy. It is demonstrated that the response of the nanoindented film is composed of purely elastic behavior with intermittent microplasticity. The overall plastic response of the metal films, as determined from nanoindentation, is shown to scale with film thickness, in qualitative agreement with the trends seen in wafer curvature or X-ray diffraction measurements.

Interfaces and stresses in thin films

Abstract: A review of the current understanding of the effect of interfaces on the intrinsic stresses in polycrystalline thin films is given. Special attention is paid to the measurement, modeling and application of surface and interface stresses. Mechanisms for generating the compressive and tensile components of the intrinsic stress are assessed. Prospects for future research are presented.

TEM study of continuous precipitation in Mg-9 wt%Al-1 wt%Zn alloy

Abstract: The development of continuous precipitate morphology in heat-treated Mg–9 wt%Al–1 wt%Zn alloy (AZ91) for a range of ageing temperatures is investigated in detail using TEM. The matrix/precipitate orientation relationships (ORs), sizes, shapes and the number of precipitates per unit volume (NV) are described for ageing at temperatures from 70 to 300°C. Most of the continuous precipitates have a Burgers OR and are plate-shaped with the primary habit plane parallel to the basal plane of the matrix. These precipitates are initially lozenge-shaped plates that elongate with time at temperature to become long laths. Two other smaller populations of precipitates that have ORs different from the Burgers OR are also present. These precipitates are rod-shaped with their long direction either perpendicular to or inclined to the basal plane. The relationship between the continuous precipitate morphology and the hardness response is discussed and comparisons are made with high-strength aluminium alloys.

Surface rumpling of a (Ni, Pt)Al bond coat induced by cyclic oxidation

Abstract: The surface of an initially flat, platinum-modified nickel aluminide bond coat formed on a single crystal superalloy is shown to progressively roughen (“rumple”) with thermal cycling in air. Far less surface roughening occurs after isothermal oxidation or after the same number of thermal cycles but with a shorter high-temperature exposure in each cycle. Mechanisms of the observed rumpling and the implications of the bond coat surface evolution leading to the failure of thermal barrier coatings are discussed. It is concluded that local volume changes in the bond coat, caused by aluminum depletion and subsequent decomposition of the β -(Ni, Pt)Al phase, are responsible for the observed rumpling.

Work hardening in heterogeneous alloys - A microstructural approach based on three internal state variables

Abstract: A new work-hardening model for homogeneous and heterogeneous cell-forming alloys is introduced. It distinguishes three internal state variables in terms of three categories of dislocations: mobile dislocations, immobile dislocations in the cell interiors and immobile dislocations in the cell walls. For each dislocation population an evolution law is derived taking into account dislocation generation, annihilation and storage by dipole and lock formation. In particular, these rate equations take into account the number of active glide systems and, thus, introduce texture in the model in addition to the Taylor factor. Microstructure is represented by the dislocation cell structure as well as second-phase particles, which may undergo changes by precipitation and Ostwald ripening. Interaction of mobile dislocations with the microstructure is taken into account through an effective slip length of the mobile dislocations. For the same set of parameters, the predictions are in excellent agreement with measured stress–strain curves of both a precipitation-hardened aluminium alloy (Al–4.16 wt% Cu–1.37 wt% Mg, AlCuMg2) and a precipitation-free model alloy (Al–0.35 wt% Cu–0.25 wt% Mg), the composition of which corresponds to the matrix of the two-phase alloy.

Nano-scale materials development for future magnetic applications

Abstract: Developments in the field of magnetic materials which show promise for future applications are reviewed. In particular recent work in nanocrystalline materials is reviewed, with either soft or hard behavior as well as advances in the magnetic materials used for magnetic recording. The role of microstructure on the extrinsic magnetic properties of the materials is stressed and it is emphasized how careful control of the microstructure has played an important role in their improvement. Important microstructural features such as grain size, grain shape and crystallographic texture all are major contributors to the properties of the materials. In addition, the critical role that new instrumentation has played in the better understanding of the nano-phase magnetic materials is demonstrated.

The effect of strain path on the development of deformation structures in severely deformed aluminium alloys processed by ECAE

Abstract: Equal channel angular extrusion (ECAE) has been used to investigate the formation of submicron grain structures in Al-alloys deformed to ultra-high plastic strains by different strain paths. The different strain paths were obtained by rotating billets through 0, 90, and 180° between each extrusion cycle. High resolution EBSD analysis has been employed to measure the boundary misorientations within the deformation structures. This has highlighted great differences in the evolution of the deformed state, as a function of the strain path, even after effective strains as high as 16. It has been demonstrated that the most effective method of forming a submicron grain structure by severe plastic deformation is to maintain a constant strain path. Processing routes involving a 180° rotation reverse the shear strain every second pass and this prevents the build up of significant numbers of new high angle boundaries. When a sample is processed with an alternate clockwise and anticlockwise 90° rotation, between each extrusion cycle the billet is deformed on two shear planes, each of which experiences half the total strain, compared to the single shear plane when there is no rotation. This reduces the rate of formation of high angle boundaries. With a constant clockwise 90° rotation the sample is also deformed on two alternate shear planes, but the total strain becomes redundant every fourth extrusion cycle. However, in this case each shear is reversed out of sequence after first deforming the billet on the alternate shear plane. This appears to be a much more effective means of forming new high angle boundaries than 180° rotation, where the shear strain is immediately reversed each alternate cycle, but is still less efficient than deformation with a constant strain path.

Multi-objective optimization in material design and selection

Abstract: The development or selection of a material to meet given design requirements generally requires that a compromise be struck between several, usually conflicting, objectives. The ways in which multi-objective optimization methods can be adapted to address this problem are explored. It is found that trade-off surfaces give a way of visualizing the alternative compromises, and that value functions (or “utility” functions) identify the part of the surface on which optimal solutions lie. The method is illustrated with examples.

Quasi-static and dynamic mechanical response of Haliotis rufescens (abalone) shells

Abstract: Quasi-static and dynamic compression and three-point bending tests have been carried out on Haliotis rufescens (abalone) shells. The mechanical response of the abalone shell is correlated with its microstructure and damage mechanisms. The mechanical response is found to vary significantly from specimen to specimen and requires the application of Weibull statistics in order to be quantitatively evaluated. The abalone shell exhibited orientation dependence of strength, as well as significant strain-rate sensitivity; the failure strength at loading rates between 10×103 and 25×103 GPa/s was approx. 50% higher than the quasi-static strength. The compressive strength when loaded perpendicular to the shell surface was approx. 50% higher than parallel to the shell surface. The compressive strength of abalone is 1.5–3 times the tensile strength (as determined from flexural tests), in contrast with monolithic ceramics, for which the compressive strength is typically an order-of-magnitude greater than the tensile strength. Quasi-static compressive failure occurred gradually, in a mode sometimes described as “graceful failure”. The shear strength of the organic/ceramic interfaces was determined to be approx. 30 MPa by means of a shear test. Considerable inelastic deformation of the organic layers (up to a shear strain of 0.4) preceded failure. Crack deflection, delocalization of damage, plastic microbuckling (kinking), and viscoplastic deformation of the organic layers are the most important mechanisms contributing to the unique mechanical properties of this shell. The plastic microbuckling is analysed in terms of the equations proposed by Argon (Treatise of Materials Science and Technology. Academic Press, New York, 1972, p. 79) and Budiansky (Comput. Struct. 1983, 16, 3).

Deformation of single crystal hadfield steel by twinning and slip

Abstract: The stress–strain behavior of Hadfield steel (Fe, 12.34 Mn, 1.03 C, in wt%) single crystals was studied for selected crystallographic orientations ( [ 1 11] , [001] and [ 1 23] ) under tension and compression. The overall stress–strain response was strongly dependent on the crystallographic orientation and applied stress direction. Transmission electron microscopy and in situ optical microscopy demonstrated that twinning is the dominant deformation mechanism in [ 1 11] crystals subjected to tension, and [001] crystals subjected to compression at the onset of inelastic deformation. In the orientations that experience twinning, the activation of multiple twinning systems produces a higher strain-hardening coefficient than observed in typical f.c.c. alloys. Based on these experimental observations, a model is presented that predicts the orientation and stress direction effects on the critical stress for initiating twinning. The model incorporates the role of local pile-up stresses, stacking fault energy, the influence of the applied stress on the separation of partial dislocations, and the increase in the friction stress due to a high solute concentration. On the other hand, multiple slip was determined to be the dominant deformation mechanism in [ 1 11] crystals subjected to compression, and [001] crystals deformed under tension. Furthermore, the [ 1 23] crystals experience single slip in both tension and compression with planar type dislocations. Using electron back-scattered diffraction patterns, macroscopic shear bands (MSBs) were identified with a misorientation of 9° in the compressed [ 1 11] single crystals at strains as low as 1%.

A novel mechanism for generating auxetic behaviour in reticulated foams: missing rib foam model

Abstract: Foams have previously been fabricated with a negative Poisson's ratio (termed auxetic foams). A novel model is proposed to explain this and to describe the strain-dependent Poisson's function behaviour of honeycomb and foam materials. The model is two-dimensional and is based upon the observation of broken cell ribs in foams processed via the compression and heating technique usually employed to convert conventional foams to auxetic behaviour. The model has two forms: the “intact” form is a network of ribs with biaxial symmetry, and the “auxetic” form is a similar network but with a proportion of cell ribs removed. The model output is compared with that of an existing two-dimensional model and experimental data, and is found to be superior in predicting the Poisson's function and marginally better at predicting the stress–strain behaviour of the experimental data than the existing model, using realistic values for geometric parameters.

A crystallographic mechanism for fatigue crack propagation through grain boundaries

Abstract: A crystallographic model is proposed which takes into account both crack-plane twist and tilt effects on crack retardation at grain boundaries. The twist and tilt angles of the crack-plane deflection at a grain boundary are the key factors that control the path and growth rate of a short crack. Because of crack-plane twist, the area between the traces on the grain-boundary plane of the crack planes across the boundary has to be fractured in order for the crack to propagate through the boundary. This presents significant resistance to crack growth. As the area to be fractured increases with the extent of crack growth beneath the surface of observation, the grain boundary could still resist crack growth after the crack tip has passed the grain boundary on the surface, until the crack propagates through the whole boundary below the surface. A grain boundary with a large twist component could cause a short crack to arrest or branch. Studies of short fatigue crack growth in an Al-Li 8090 alloy plate provide evidence that supports the model.

Martensitic transformations and shape-memory materials ☆

Abstract: The authors review theoretical research on martensitic phase transformations in shape-memory materials, with emphasis on recently derived theory and predictions of interest for alloy development. Research on special lattice parameters corresponding to certain microstructures, complex crystal structures and 6M martensite, the relation of micro-scale to macro-scale deformations, ferromagnetic and ferroelectric martensites, and martensite at small scales is covered.

Microstructural evolution in thermally sprayed WC-Co coatings: Comparison between nanocomposite and conventional starting powders

Abstract: WC–Co coatings have been deposited by high velocity oxy-fuel thermal spraying of conventional and nanocomposite powders which contain WC grains in the size range 2–5 μm and 70–250 nm, respectively. The coatings differed not only in microstructural scale, but also in the nature and proportion of the phases present and in the overall degree of decarburization. A model describing the evolution of microstructure has been developed. As a WC–Co particle is heated in the hot gas jet, the cobalt phase melts and the WC grains begin to dissolve in it. The periphery of the semi-molten particle becomes decarburized by oxidation, promoting further WC dissolution in this region. Particle quenching on impact with the substrate results in precipitation from the melt of W2C and possibly W depending on the local melt composition. The larger surface to volume ratio of the WC in the nanocomposite material promotes more rapid dissolution and thus decarburization. Consequently, W2C is observed in both coatings, whereas W is found only in the nanocomposite deposit.

Plastic deformation behaviour of fine-grained materials

Abstract: A phase mixture model in which a polycrystalline material is regarded as a mixture of a crystalline phase and a grain-boundary phase is presented. The model aims to describe the plastic deformation behaviour of fine-grained materials. The mechanical properties of the crystalline phase are modelled using unified viscoplastic constitutive relations, which take dislocation density evolution and diffusion creep into account. The total strain rate of a crystallite is calculated by summation of the contributions of dislocation, boundary diffusion and lattice diffusion mechanisms. The deformation mechanism for the grain-boundary phase is modelled as a diffusional flow of matter through the grain boundary. Using a simple rule of mixtures, the grain size dependence of the overall plastic deformation behaviour of the material is analysed. Rate effects are also investigated. The results of the calculations are compared with previously published experimental data.

Gadolinia-doped ceria and yttria stabilized zirconia interfaces: regarding their application for SOFC technology ☆

Abstract: For solid oxide fuel cells (SOFCs) operating at intermediate temperatures the adjacency of the state-of-the-art yttria-stabilized zirconia (YSZ) electrolyte with ceria-based materials to both anodic and cathodic sides is regarded as crucial for the effectiveness of the cell. Solid-state reaction, however, and interdiffusion phenomena between YSZ and ceria-based materials can cause degradation of the electrolyte. When a gadolinia-doped-ceria (GDC) layer is used to protect YSZ against interaction with Co-containing cathodes, an unfavorable solid state reaction at the YSZ–GDC interface can be efficiently suppressed when a thin (≤1 μm thick) interlayer with nominal composition of Ce 0.43 Zr 0.43 Gd 0.10 Y 0.04 O 1.93 is incorporated at the interface. When ceria is to be employed at the electrolyte–anode interface to reduce polarization losses, use of a ceria–40% vol Ni cermet is recommended, since suppression of the reactivity between YSZ and ceria can also be achieved in the presence of Ni.

Mechanics of materials: top-down approaches to fracture

Abstract: The utility and robustness of the mechanics of materials is illustrated through a review of several recent applications to fracture phenomena, including adhesive failures, the role of plasticity in enhancing toughness in films and multilayers, and crack growth resistance in ductile structural alloys. The commonalty among the approaches rests in a reliance on experiments to provide calibration of the failure process at the smallest scale.