Showing papers in "Journal of Materials Science in 1994"

The physics and mechanics of fibre-reinforced brittle matrix composites

Abstract: This review compiles knowledge about the mechanical and structural performance of brittle matrix composites. The overall philosophy recognizes the need for models that allow efficient interpolation between experimental results, as the constituents and the fibre architecture are varied. This approach is necessary because empirical methods are prohibitively expensive. Moreover, the field is not yet mature, though evolving rapidly. Consequently, an attempt is made to provide a framework into which models could be inserted, and then validated by means of an efficient experimental matrix. The most comprehensive available models and the status of experimental assessments are reviewed. The phenomena given emphasis include: the stress/strain behaviour in tension and shear, the ultimate tensile strength and notch sensitivity, fatigue, stress corrosion and creep.

Structure and plastic deformation of polyethylene

Abstract: A review is given of the structure of semi-crystalline polymers and the mechanisms of plastic deformation in them. High-density polyethylene (HDPE) is taken as the specific example because of the large number of detailed studies performed on this material. The early findings are also compared and contrasted with very recent detailed large-strain deformation studies and computer simulations of deformation-induced texture development in HDPE.

Cast magnesium alloys for elevated temperature applications

Abstract: The alloy development, microstructure, properties and uses of cast magnesium alloys for elevated temperature applications are reviewed. The alloying principles and strengthening mechanisms of magnesium are discussed to identify the potential alloy systems for elevated temperature applications in automotive and aerospace industries. It is concluded that the Mg-Zr family of sand cast alloys exhibit adequate mechanical properties at both ambient and elevated temperatures for aerospace applications, and Ca-modified sand cast AS41 alloy might provide a cost-effective alternative for the Zr-containing alloys. For diecasting applications, no current alloy systems meets the requirements of good high temperature properties, acceptable castability and low cost for critical automotive components, future development is especially needed in this area. Development of dispersion strengthened magnesium alloys and improvement of current Mg-Al-RE and Mg-Al-Si systems are the potential routes to expand diecast magnesium alloys to elevated temperature applications.

Solid-state synthesis and characterization of the ternary phase Ti3SiC2

Abstract: Ti3SiC2 is the only true ternary compound in the Ti-Si-C system. It seems to exhibit promising thermal and mechanical behaviour. With the exception of its layered crystal structure, most of its properties are unknown, owing to the great difficulty of synthesis. A new procedure of solid-state synthesis with several steps is proposed, which results in Ti3SiC2 with less than 5 at % of TiC. Ti3SiC2 is stable at least up to 1300 °C. Beyond this temperature, it can decompose with formation of non-stoichiometric titanium carbide and gaseous silicon, with kinetics highly dependent on the nature of the surroundings. As an example, graphite can initiate this process by reacting with silicon, while alumina does not favour the decomposition which remains very slow. The oxidation of Ti3SiC2 under flowing oxygen starts at 400 °C with formation of anatase-type TiO2 film, as studied by TGA, XRD, SEM and AES. Between 650 and 850 °C both rutile and anatase are formed, rapidly becoming protecting films and giving rise to slow formation of SiO2 and more TiO2. The oxidation kinetics is slower than for TiC, owing to a protecting effect of silica. By increasing the temperature, both oxidation processes (i.e. direct reaction and diffusion through oxide layers) are activated and an almost total oxidation is achieved between 1050 and 1250 °C resulting in titania (rutile) and silica (cristobalite).

Hydrothermal synthesis of biocompatible whiskers

Abstract: The preparation of non-toxic and biocompatible fibres or whiskers is one of the most urgent tasks today, because most of the fibrous materials which have been used (including asbestos which has been used for many years) are thought to be biohazardous. Whiskers of hydroxyapatite (Ca10(PO4)6(OH)2∶HAp), which is expected to be one of the best biocompatible materials, have been successfully synthesized by hydrothermal treatments of beta-tricalcium phosphate (beta-Ca3(PO4)2: beta-TCP) with citric acid. These whiskers were single crystals, elongated along the c-axis, with a length of 20–30 μm and a width of 0.1–1 μm. They were slightly calcium deficient (Ca/P molar ratio = 1.63) and they contained a trace of CO 3 2− in their structure.

Weathering of polymers: mechanisms of degradation and stabilization, testing strategies and modelling

Abstract: The weathering of polymers is reviewed with attention concentrated on the mechanisms of degradation and stabilization, the methods of testing weatherability, and the predictive modelling of weathering behaviour. An introduction to the chemical mechanisms of degradation and stabilization is given and reference made to some of the many reviews available in the literature. Significant emphasis is placed here on engineering aspects, such as the way that weathering influences fracture mechanisms. The difficulties associated with relating accelerated laboratory tests with outdoor service behaviour are discussed. The complexities of the degradation processes limit modelling of the rate of degradation to rather specific systems and the predictions cannot be generalized.

Role of oxygen diffusion in polymer ageing: kinetic and mechanical aspects

Abstract: For an ageing process involving the consumption of a small molecule (typically O2 or H2O) by reaction with the polymer, there are critical conditions of reaction rate and/or thickness above which the process becomes kinetically controlled by the diffusion of the small molecule in the polymer. Suitable lifetime prediction models must then involve the thickness distribution of reaction products. This latter can be predicted from Fick's law, modified by a term relative to the rate of consumption of the diffusing species by the chemical reaction. Some problems related to the use of this approach are examined here. It appears that, in the most frequent case, the thickness of the degraded layer is of the order of magnitude of D/k, where D is the diffusion coefficient and k the pseudo-first-order rate constant for reactant consumption. Some examples of application related to photochemical, radiochemical and thermochemical ageing are examined. It can, for instance, be shown that in photochemical or radiochemical ageing, the thickness of the oxidized layer (TOL) is proportional to the reciprocal of lβ, where l is the radiation intensity and Β an exponent depending essentially on the radical chain mechanism. It is generally expected that in the case of thermal ageing, the TOL is a decreasing function of the temperature. Some consequences of diffusion control on accelerated and natural ageing methods are briefly examined. The consequences of the ageing-induced “skin-core” structure due to the diffusion control are examined. The main features of the observed polymer embrittlement can be explained in terms of fracture mechanics.

The extent of microcracking and the morphology of microcracks in damaged bone

Abstract: Strain-induced damage in bovine laminar bone has been examined using laser scanning confocal microscopy (LSCM) The specimens were loaded in a fluorescein solution, which penetrated the newly formed cracks in the specimen The microcracking, and the larger cracking, induced by strain were very clearly visible The microcracking occurred diffusely in regions of high strain (stress), but was particularly obvious in the vicinity of large machined stressconcentrators The microcracking could be shown not to be artefactual, that is, it was produced by strain, and not by specimen preparation The microcracking interacted with the structure of the bone, often having a wavy appearance related to the histology Microcracks seemed to be particularly associated with the most highly mineralized parts of the bone LSCM is a technique holding great promise for the investigation of the initiation and development of damage in mineralized hard tissues, and other translucent materials

Bismuth oxide-based solid electrolytes for fuel cells

Abstract: During the last three decades, a large number of investigations has been reported pertaining to the science and technology of solid oxide fuel cells (SOFCs) based mainly on the yttria-stabilized zirconia (YSZ) electrolyte. Because of the problems associated with the high temperature of operation (~ 1000°C) of the YSZ-based cells, there has been a substantial effort to develop alternative electrolytes with ionic conductivity comparable to that of YSZ at relatively lower temperatures. This review presents a systematic evolution in the area of the development of new electrolytes based on bismuth sesquioxide for fuel cell applications at moderate temperatures.

Processing of molybdenum disilicide

Abstract: Inspection of the scientific literature reveals that intermetallic compounds have, in recent years, attracted considerable interest as a result of their unique elevated temperature characteristics. Among the wide range of intermetallic compounds that are actively being studied, MoSi2 has been singled out as a result of its unique combination of properties, which include an excellent oxidation resistance, a high modulus of elasticity, and an elevated melting point (2030°C). In view of this interest, the present work was undertaken with the objective of providing the reader with a comprehensive review of the mechanical and oxidation behaviour of MoSi2, paying particular attention to the synergism between processing and microstructure. Accordingly, synthesis techniques, including powder metallurgy, self-propagating hightemperature synthesis, spray processing, solid-state displacement reactions, and exothermic dispersion, are critically reviewed and discussed. In addition, recent efforts aimed at using MoSi2 as a matrix material in metal-matrix composites are also critically reviewed and discussed.

Thermoplastic-toughened epoxy polymers

Abstract: The microstructure and properties of two epoxy-resin systems which have been modified with varying amounts of a thermoplastic to improve the toughness of the thermosetting epoxy polymers, have been studied. The curing agent was 4,4′ diaminodiphenylsulphone and the thermoplastic was a reactively terminated poly (ether sulphone) copolymer. Different microstructures were found to occur as the concentration of the thermoplastic component was steadily increased. In particular, the relationships between the microstructures and values of stress-intensity factor, KIc, and fracture energy, GIc, were explored.

An approach to the formation and growth of new phases with application to polymer crystallization: effect of finite size, metastability, and Ostwald's rule of stages

Abstract: This article aims to link the mainstream subject of chain-folded polymer crystallization with the rather speciality field of extended-chain crystallization, the latter typified by the crystallization of polyethylene (PE) under pressure. Issues of wider generality are also raised for crystal growth, and beyond for phase transformations. The underlying new experimental material comprises the prominent role of metastable phases, specifically the mobile hexagonal phase in polyethylene which can arise in preference to the orthorhombic phase in the phase regime where the latter is the stable regime, and the recognition of “thickening growth” as a primary growth process, as opposed to the traditionally considered secondary process of thickening. The scheme relies on considerations of crystal size as a thermodynamic variable, namely on melting-point depression, which is, in general, different for different polymorphs. It is shown that under specifiable conditions phase stabilities can invert with size; that is a phase which is metastable for infinite size can become the stable phase when the crystal is sufficiently small. As applied to crystal growth, it follows that a crystal can appear and grow in a phase that is different from that in its state of ultimate stability, maintaining this in a metastable form when it may or may not transform into the ultimate stable state in the course of growth according to circumstances. For polymers this intermediate initial state is one with high-chain mobility capable of “thickening growth” which in turn ceases (or slows down) upon transformation, when and if such occurs, thus “locking in” a finite lamellar thickness. The complete situation can be represented by a P, T, 1/l (l ≡ crystal thickness) phase-stability diagram which, coupled with kinetic considerations, embodies all recognized modes of crystallization including chain-folded and extended-chain type ones. The task that remains is to assess which applies under given conditions of P and T. A numerical assessment of the most widely explored case of crystallization of PE under atmospheric pressure indicates that there is a strong likelihood (critically dependent on the choice of input parameters) that crystallization may proceed via a metastable, mobile, hexagonal phase, which is transiently stable at the smallest size where the crystal first appears, with potentially profound consequences for the current picture of such crystallization. Crystallization of PE from solution, however, would, by such computations, proceed directly into the final stage of stability, upholding the validity of the existing treatments of chain-folded crystallization. The above treatment, in its wider applicability, provides a previously unsuspected thermodynamic foundation of Ostwald's rule of stages by stating that phase transformation will always start with the phase (polymorph) which is stable down to the smallest size, irrespective of whether this is stable or metastable when fully grown. In the case where the phase transformation is nucleation controlled, a ready connection between the kinetic and thermodynamic considerations presents itself, including previously invoked kinetic explanations of the stage rule. To justify the statement that the crystal size can control the transformation between two polymorphs, a recent result on 1 -4-poly-trans-butadiene is invoked. Furthermore, phase-stability conditions for wedge-shaped geometries are considered, as raised by current experimental material on PE. It is found that inversion of phase stabilities (as compared to the conditions pertaining for parallel-sided systems) can arise, with consequences for our scheme of polymer crystallization and with wider implications for phase transformations in tapering spaces in general. In addition, in two of the Appendices two themes of overall generality (arising from present considerations for polymers) are developed analytically; namely, the competition of nucleation-controlled phase growth of polymorphs as a function of input parameters, and the effect of phase size on the triple point in phase diagrams. The latter case leads, inter alia to the recognition of previously unsuspected singularities, with consequences which are yet to be assessed.

Crystal structures of TiO2 thin coatings prepared from the alkoxide solution via the dip-coating technique affecting the photocatalytic decomposition of aqueous acetic acid

Abstract: TiO2 coatings with different crystal structures were prepared from alkoxide solutions via the dip-coating technique. The physical properties, except the crystal structure, were adjusted to distinguish the effect of crystal structure on their photocatalytic property. The results of photocatalytic measurements using TiO2 coatings with different crystal structures showed that the decomposition of aqueous acetic acid was enhanced by the content of anatase phase.

Size effects due to Cosserat elasticity and surface damage in closed-cell polymethacrylimide foam

Abstract: This article describes the experimental investigation of Cosserat (or micropolar) elasticity and surface damage effects in closed-cell polymethacrylimide foams of different densities. The method of size effects was used to find the degree of Cosserat behaviour for both cylindrical and square cross-section specimens in bending and torsion. The foams were found to behave as Cosserat materials in which slender specimens appear less stiff than thick ones, provided sufficient care is taken when machining the specimens. Surface damage caused by the machining process may cause the apparent stiffness to decrease with decreasing specimen size, giving an opposite softening size effect.

A model for particle cavitation in rubber-toughened plastics

Abstract: An energy-balance criterion for cavitation of rubber particles, which was proposed in an earlier paper [A. Lazzeri and C. B. Bucknall, J. Mater. Sci.28 (1993) 6799], is developed by including a term for the energy stored in the matrix and released during expansion of the voids. The model relates the critical volume strain at cavitation to the radius of the rubber particle, and to the shear modulus, surface energy and failure strain of the rubber. The effects of temperature, strain rate and type of stress field upon cavitation behaviour and the resulting toughness of the two-phase polymer are discussed in terms of the model.

An assessment of expressions for the apparent thermal conductivity of cellular materials

Abstract: Diverse expressions for the thermal conductivity of cellular materials are reviewed. Most expressions address only the conductive contribution to heat transfer; some expressions also consider the radiative contribution. Convection is considered to be negligible for cell diameters less than 4 mm. The predicted results are compared with measured conductivities for materials ranging from fine-pore foams to coarse packaging materials. The dependencies of the predicted conductivities on the material parameters which are most open to intervention are presented graphically for the various models.

Microstructure and crystallization behaviour of TiO2 precursor prepared by the sol-gel method using metal alkoxide

Abstract: TiO2 precursors were prepared by the sol-gel method using titanium tetraisopropoxide. The effects of water and hydrochloric acid addition for the hydrolysis and polymerization reaction on the crystal lattice or the molecular structure, and on the crystallization behaviour of prepared precursors, were examined by using Raman, and infrared spectroscopy, X-ray diffraction, DTA-TG and carbon analysis. The quantity of unhydrolysed alkyls in precursors decreased with increasing amounts of H2O and HCl addition, but the alkyls always remained. Raman spectroscopy revealed that the molecular structure of prepared amorphous precursors resembled that of the anatase phase with increased H2O and HCl addition. On the crystallization process of these precursors, the ratio of anatase formed at 310 °C was influenced by residual unhydrolysed alkyls. Steric hindrance by the residual alkyls preventing crystallization to crystalline anatase was observed, and there was estimated to be 2.5 mol of octahedral coordination of Ti-O per 1 mol of residual alkyls. It is noted that crystallization of the anatase phase and the transformation from anatase to the rutile phase were also observed on long-term ageing of precursors at room temperature.

Precipitate Structure in a Cu-Ni-Si Alloy

Abstract: The precipitates responsible for the age-hardening effect in dilute Cu-Ni-Si alloys based on Cu2NiSi have been studied using transmission electron microscopy. The work has confirmed the findings of an earlier study by Teplitskiy et al., which showed that the precipitates formed as discs on {1 1 0} planes and indicated that they had a structure which corresponded to that of δ-Ni2Si. The orientation relationship of the precipitate and matrix found in the present work differs from that found in the earlier work, and a high degree of coherency across the precipitate-matrix interface, has been shown to exist. In establishing that the precipitate is δ-Ni2Si, EDX analysis has been employed and ring patterns from extracted precipitates have also been analysed. Absences from such ring patterns which were noted in the earlier work and which were thought to cast doubt on the precipitate identification have been accounted for in terms of preferred orientation effects. At peak strength the deformation of the material is believed to involve Orowan looping around the precipitates.

Rare-earth chalcogenides — an emerging class of optical materials

Abstract: Sulphide compounds belong to the family of chalcogenides and are well known for their optical and electronic properties. They possess good optical properties because of their ability to transmit into the infrared (IR) region. Several sulphide glasses are known to exist which exhibit far infrared transmission and are also useful semiconductors. In recent years, there has been an increasing interest in IR materials to be used on surveillance equipment. This led to the identification of several new crystalline sulphide materials which can transmit very far into the IR region (up to a wavelength of 14 Μm). Crystalline and amorphous rare-earth sulphides emerged as a new class of materials, which possess several unique optical and electronic properties. This paper reviews the status of these rare-earth sulphide amorphous and polycrystalline materials, the techniques used to process these materials and discusses their structure, thermal, mechanical and optical properties. Conventional and emergent novel chemical processing techniques that are used for synthesizing these materials are reviewed in detail. The use of metallorganic precursors and the modification of their chemistry to tailor the composition of the final ceramic are illustrated. The potential of these chemical techniques and their advantages over the conventional solid state techniques used for processing sulphide ceramics is discussed, particularly in light of their successful applications in processing novel electronic and optical oxide ceramics.

Recent development in the fabrication of metal matrix-particulate composites using powder metallurgy techniques

Abstract: It is advantageous to fabricate metal matrix-particulate composites (MMPCs) using powder metallurgy (PM) because the fabricated composites possess a higher dislocation density, a small sub-grain size and limited segregation of particles, which, when combined, result in superior mechanical properties. The various PM-related processes currently in use in the fabrication of MMPCs, are reviewed, outlining the common problems encountered in each of these fabrication processes. The more recently developed PM techniques to fabricate MMPCs are also discussed.

Reduction of residual stress in montmorillonite/epoxy compounds

Abstract: An epoxy resin was cured while in intimate contact with small amounts of epoxyphilic montmorillonites. It was determined that cured epoxy exists within the montmorillonite interlayer by the observation of very high interlayer spacings, even greater than 8 nm, Generally, epoxy compounds containing montmorillonites that had been swollen in the curing agent prior to curing exhibited larger interlayer spacings, especially among the non-dispersed montmorillonite layers. The maximum observed residual stress was reduced by greater than 50% in the epoxyphilic montmorillonite/epoxy compounds over that of the pure epoxy. The epoxyphilic montmorillonite/epoxy compounds generally exhibited higher values of glass transition temperature, flexural modulus, and ultimate flexural strength than the pure epoxy. The tyramine-montmorillonite compounds typically had the highest values overall.

Particulate silicon nitride-based composites

Abstract: In an attempt to optimize the structure and properties of silicon nitride ceramics, a variety of novel processing techniques and materials compositions have evolved over the last 15 years. Among the most important, was the development of various silicon nitride-based composites. A review of particulate, silicon nitride-based composites other than whisker- or platelets-reinforced, is presented. Materials based on silicon nitride and SiAlONs, with additions of carbides, nitrides and borides of transition metals are described. Special emphasis is placed on TiN- and TiC-containing ceramics. The manufacture of composites by hot pressing, reaction sintering, pressureless and gas-pressure sintering is discussed. The data on properties, including conductivity, density, Young's modulus, strength, fracture toughness, hardness, thermal expansion, wear, creep and oxidation resistance are presented. Analysis of actual and potential uses of the selected composites demonstrates that the particulate composites are very promising as tool, structural and electronic materials.

Fabrication and mechanical behaviour of Al2O3/Mo nanocomposites

Abstract: Two types of Al2O3/Mo composites were fabricated by hot-pressing a mixture of γ- or α-Al2O3 powder and a fine molybdenum powder. For Al2O3/5 vol% Mo composite using γ-Al2O3 as a starting powder, the elongated molybdenum layers were observed to surround a part of the Al2O3 grains, which resulted in an apparent high value of fracture toughness (7.1 Mpa m1/2). In the system using α-Al2O3 as a starting powder, nanometre sized molybdenum particles were dispersed within the Al2O3 grains and at the grain boundaries. Thus, it was confirmed that ceramic/metal nanocomposite was successfully fabricated in the Al2O3/Mo composite system. With increasing molybdenum content, the elongated molybdenum particles were formed at Al2O3 grain boundaries. Considerable improvements of mechanical properties were observed, such as hardness of 19.2 GPa, fracture strength of 884 MPa and toughness of 7.6 MPa m1/2 in the composites containing 5, 7.5, 20 vol% Mo, respectively; however, they were not enhanced simultaneously. The relationships between microstructure and mechanical properties are also discussed.

Effect of atmosphere on the PTCR properties of BaTiO3 ceramics

Abstract: The influence of low-temperature annealing, at < 360 °C, in various reducing and oxidizing atmospheres for a series of BaTiO3 ceramics with a positive temperature coefficient of resistance (PTCR) is discussed. Combined impedance and modulus spectroscopy is used to analyse a.c. impedance data and shows that the total resistance of the sample can be composed of up to three components, dependent on the cooling rate from the sintering temperature. For quickly cooled samples the PTCR response is dominated by an outer shell on individuals grains, whereas for slowly cooled samples the grain boundary resistance dominates. Annealing in reducing atmospheres destroys the grain boundary PTCR effect whereas the outer-shell grain PTCR effect is relatively insensitive to the reducing atmosphere. It is proposed that the acceptor states responsible for the outer-grain and grain-boundary PTCR effects are predominantly intrinsic metal vacancies, i.e. Ba and/or Ti, and adsorbed oxygen, respectively.

The setting of gypsum plaster

Abstract: The α- and Β-forms of calcium sulphate hemihydrate, CaSO4·1/2H2O, were characterized using thermal, X-ray diffraction, optical and electron microscopy techniques. The differences between the two forms arose from their mode of production, resulting in different crystal size and habit, and not from different crystal structures. The hydration of CaSO4·1/2H2O to CaSO4·2H2O was investigated using calorimetry, phase analysis and the changes in electrical resistance of the plaster slurries. The reaction occurred in three steps: dissolution, nucleation and precipitation, and finally completion due to depletion of reactants. The differences in reactivity of the α- and Β-forms with water again appeared to be due to their different physical states.

Pressureless sintering of β-SiC with Al2O3 additions

Abstract: β-SiC was pressureless sintered to 98% theoretical density using Al2O3 as a liquid-phase forming additive. The reaction between SiC and Al2O3 which results in gaseous products, was inhibited by using a pressurized CO gas or, alternatively, a sealed crucible. The densification behaviour and microstructural development of this material are described. The microstructure consists of fine elongated α-SiC grains (maximum length ≈ 10 μm and width 2–3 μm) in a matrix of fine equi-axed grains (2–3 μm) and plate-like grains (2–5μm). The densification behaviour, composition and phases in the sintered product were studied as a function of the sintering parameters and the initial composition. Typically, 50% of the β-phase was transformed to the α-phase.

Effect of transcrystalline morphology on interfacial adhesion in cellulose/polypropylene composites

Abstract: A study was conducted on the effect of cotton cellulose fibres on the crystallization behaviour of isotactic polypropylene (PP) from the melt and the resulting morphology. When the PP was allowed to crystallize isothermally at 131° C, the cotton fibres acted as nucleating agents and a transcrystalline phase was created around the fibres. Quench cooling of the melt prevented the occurrence of such a phase. Transcrystalline layers of different thicknesses were created by interrupting the isothermal crystallization at certain intervals and quenching the melt. The effect of these morphologies on interfacial shear stress transfer was investigated using the single-fibre fragmentation test. It was found that the transcrystalline morphology at the fibre/matrix interface improved the shear transfer considerably when a tensile load was applied in the fibre direction. One mechanism is proposed to be particularly responsible for this increase: slow cooling favours the kinetics of the approach of PP molecules, and hence interfacial adsorption, which yields an ordered transcrystalline PP interphase having a high density of intermolecular secondary bonds with the cellulose surface. An increase in the shear transfer efficiency with increasing thickness of the transcrystalline layer was also observed.

Thermally induced phase transformations of 12-tungstophosphoric acid 29-hydrate : synthesis and characterization of PW8O26-type bronzes

Abstract: The phase transformations of 12-tungstophosphoric H3PW12O40-29H2O (29-WPA) acid in the temperature range from ambient temperature to 1150°C were investigated and characterized by differential thermal analysis (DTA), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), X-ray powder diffraction (XRPD), scanning electron microscopy (SEM), infrared (ir) and Raman spectroscopies From room temperature to 550°C, 29-WPA passes through a dehydration process, which characterizes the formation of different crystallohydrates, in anhydrous form as well as “denuded” Keggin's anions, the D-phase (PW12O38) During these processes, Keggin's anions are not disturbed too much and they are preserved up to about 550°C The “D” phase is transformed by solid-solid recrystallization at about 600°C in a new monophosphate bronze type compound PW8O26 Unit cell dimensions were calculated from XRPD data (a o=07515 nm) With the temperature increasing up to 1150°C, novel synthesized cubic bronze passed through three polymorphous phase transitions According to a general formula for monophosphate tungsten bronzes (WO3)2m (PO4)4 all four polymorphs have m=16

The influence of vanadium on fracture toughness and abrasion resistance in high chromium white cast irons

Abstract: The influence of vanadium on wear resistance under low-stress conditions and on the dynamic fracture toughness of high chromium white cast iron was examined in both the ascast condition and after heat treatment at 500 °C. A vanadium content varying from 0.12 to 4.73% was added to a basic Fe-C-Cr alloy containing 2.9 or 19% Cr. By increasing the content of vanadium in the alloy, the structure became finer, i.e. the spacing between austenite dendrite arms and the size of massive M7C3 carbides was reduced. The distance between carbide particles was also reduced, while the volume fraction of eutectic M7C3 and V6C5 carbides increased. The morphology of eutectic colonies also changed. In addition, the amount of very fine M23C6 carbide particles precipitated in austenite and the degree of martensitic transformation depended on the content of vanadium in the alloy. Because this strong carbide-forming element changed the microstructure characteristics of high chromium white iron, it was expected to influence wear resistance and fracture toughness. By adding 1.19% vanadium, toughness was expected to improve by approximately 20% and wear resistance by 10%. The higher fracture toughness was attributed to strain-induced strengthening during fracture, and thereby an additional increment of energy, since very fine secondary carbide particles were present in a mainly austenitic matrix. An Fe-C-Cr-V alloy containing 3.28% V showed the highest abrasion resistance, 27% higher than a basic Fe-C-Cr alloy. A higher carbide phase volume fraction, a finer and more uniform structure, a smaller distance between M7C3 carbide particles and a change in the morphology of eutectic colonies were primarily responsible for improving wear resistance.

Phase II to phase I crystal transformation in polybutene-1 single crystals: a reinvestigation

Abstract: The spontaneous phase II to phase I crystal-crystal transformation of polybutene-1 was investigated by electron diffraction and bright- and dark-field imaging of solution and thinfilm grown single crystals. Whole single crystals were observed to transform with a single phase II to phase I orientational relationship and, in the case of multiple orientations, the transformed areas were not dependent on growth sectors. These results do not support the views of Holland and Miller of a “twinned” phase II to I transformation determined by growth sectors but are consistent with a transformation scheme introduced by Fujiwara. Nucleation and growth of the transformation are further discussed.