Showing papers in "Journal of Materials Science in 1978"

Constrained cracking in glass fibre-reinforced epoxy cross-ply laminates

Abstract: Specimens of 90° cross-ply glass-reinforced epoxy resins were tested in tension parallel to the direction of reinforcement in the outer plies. The thickness of the inner ply was varied and cracking constraint was observed at small thicknesses. At large inner-ply thicknesses the specimens showed uniform transverse cracking, and at very low inner-ply thicknesses this transverse cracking could be suppressed completely prior to total specimen failure. Fracture toughness values were determined on transverse unidirectional laminates of the same volume fraction. It was found that the cracking constraint observed can be accounted for, using the theory of Aveston and Kelly.

Effect of stress-induced phase transformation on the properties of polycrystalline zirconia containing metastable tetragonal phase

Abstract: Polycrystalline zirconia containing a high content of metastable tetragonal phase shows high strength (∼ 700 MPa), high fracture toughness (Kc = 6 to 9 MN m−3/2) and small grain size (<0.3jμm). The strength and grain size remain nearly constant over a wide range of tetragonal phase content (100 to 30%). At a low concentration of tetragonal phase <30%, there is a rapid decrease in strength accompanied by a rapid increase in grain size. These results are explained by means of a stress-induced phase transformation in the metastable tetragonal phase.

Synthesis of continuous silicon carbide fibre with high tensile strength and high Young's modulus

Abstract: Polycarbosilane as the precursor of continuous SiC fibre was synthesized by thermal decomposition of polydimethylsilane. The structure of the polycarbosilane is concluded to be similar to that of polysilapropylene by the measurements of i.r. spectra, NMR spectra and chemical analyses. Its formation mechanisms are initially the formation of carbosilane by thermal decomposition of polydimethylsilane and then the increase in molecular weight by dehydrogenation-condensation of the carbosilane. Molecular structure and molecular weight distribution of the polycarbosilane depend on the reaction temperature.

A sintered magnetoelectric composite material BaTiO3-Ni(Co, Mn) Fe2O4

Abstract: Magnetoelectric composite materials have been made by sintering a mixture of a piezoelectric and piezomagnetic phase. This paper deals with the preparation and some properties of the combination BaTiO3-Ni(Co, Mn)Fe2O4, and investigates the influence of the cooling rate after sintering, TiO2 additions, the mole ratio of both phases and the crystallite size of the grains of both phases on some physical properties of the composite material. The maximum value of the conversion factor (δE/δH)max found for this material up till now is about 80 mV cm−1 Oe−1.

Analysis of the alternating current properties of ionic conductors

Abstract: A method of analysis of inclined semicircular complex impedance (Z) diagrams for ionic conductors has been developed on the basis of the concept of “non-Debye” behaviour of solid dielectrics. This approach replaces the rather arbitrary distribution of relaxation times by the physically much simpler concept of a frequency-independent ratio of energy lost per cycle to energy stored. This criterion of dielectric behaviour leads directly to complex admittance (Y) plots in the form of straight lines inclined to the vertical which then transform to inclined semicircles in Z. The use of Y diagrams gives better accuracy of representation and it is shown how this can be further enhanced by plotting log Yi − log Yr instead of the usual linear representation. Different parts of the graphs are assigned to “bulk” and “barrier” regions on the basis of the value of the dielectric permittivity and it is shown that a hitherto unrecognized physical process can manifest itself as a strong dispersion at frequencies intermediate between the high-frequency bulk response and the low-frequency barrier-dominated behaviour.

Synthesis of continuous silicon carbide fibre with high tensile strength and high Young's modulus: Part 1 Synthesis of polycarbosilane as precursor

Abstract: Polycarbosilane as the precursor of continuous SiC fibre was synthesized by thermal decomposition of polydimethylsilane. The structure of the polycarbosilane is concluded to be similar to that of polysilapropylene by the measurements of i.r. spectra, NMR spectra and chemical analyses. Its formation mechanisms are initially the formation of carbosilane by thermal decomposition of polydimethylsilane and then the increase in molecular weight by dehydrogenation-condensation of the carbosilane. Molecular structure and molecular weight distribution of the polycarbosilane depend on the reaction temperature.

Structural relaxation in amorphous Fe40Ni40P14 B6 studied by energy dispersive X-ray diffraction

Abstract: The atomic structure and the structural relaxation of amorphous Fe40Ni40P14B6 alloy were studied using the energy dispersive X-ray diffraction method. It was demonstrated that the structure of the amorphous alloy can be determined self-consistently with high accuracy by this method. The results indicated that the structural relaxation is a highly collective process involving many atoms, and can be described in terms of the redistribution and transformation of the structural defects.

Hybrid effects in composites: conditions for positive or negative effects versus rule-of-mixtures behaviour

Abstract: A positive or negative hybrid effect in hybrid composites is defined as a positive or negative deviation of a certain mechanical property from the rule-of-mixtures behaviour. The question of hybrid effects is first examined with special hybrids which have been chosen so that the effect of the fibre-matrix interface is minimized. The hybrids examined consisted of two types of carbon fibres with different mechanical properties but similar surface treatments. The results of all the mechanical properties examined (modulus, strength, stress intensity factor, fracture energies) under quasi-static and fast testing conditions do not show any synergism. In view of these results a second hybrid system of E-glass fibre/AS carbon fibre-reinforced epoxy has been chosen. In this system both the mechanical properties of the fibres and the interface which they form with the resin are entirely different. None of the mechanical properties, excluding the fracture energies, show any signs of a hybrid effect. The fracture energy results, however, show the existence of a negative hybrid effect. A theory which sets upper and lower bounds for the hybrid effect is proposed, and the conditions for the occurrence of either a positive or a negative effect are discussed.

Synthesis of glasses by hot-pressing of gels

Abstract: The synthesis of glasses by hot-pressing of gels was studied for pure SiO2, as well as for La2O3-SiO2 and B2O3-SiO2 systems. It was recognized that gels prepared by hydrolysis and polycondensation of organometallic compounds crystallize less quickly than those obtained from hydrosols, and are, therefore, particularly suited for hot-pressing. Different methods of pressing were investigated; they influence the residual OH− content of resultant glasses. Murrayet al.'s densification model based on viscous flow was found to apply in the case of the gels under study and a dynamic viscosity evaluation is proposed to determine rapidly the pressing parameters for a given gel-glass conversion.

The stiffness and strength of a polyamide thermoplastic reinforced with glass and carbon fibres

Abstract: It has been established that the optimum degree of mechanical property enhancement by fibre reinforcement of a typical thermoplastic material (polyamide 6.6) is achieved if comparatively long fibres are used, the fibre length required being determined by the properties of the interface between the fibre and the thermoplastic matrix. The extent of stiffness improvement at low strains is described by simple modifications to the law of mixtures to allow for fibre orientation and length. The strength enhancement is limited by an embrittlement effect which reduces the strain to fracture as the stiffness of the composite is improved. The cause of this effect has been identified as matrix crack formation at the ends of the reinforcing fibres. At strains of between 0.5% and 1.0%, according to fibre type, length andVf, cracks form at the tips of the longest fibres aligned in the straining direction. Subsequently as the strain is increased more cracks form progressively at the ends of shorter and/or more misaligned fibres. It has been shown that initially this cracking can be accommodated by load transfer to adjacent fibres which “bridge” the cracked region. Final failure occurs when the extent of cracking across the weakest section reaches a critical level when the surrounding fibres and matrix can no longer support the increasing load.

On multiple transverse cracking in glass fibre epoxy cross-ply laminates

Abstract: An investigation has been made of multiple transverse cracking in glass fibre epoxy cross-ply laminates. Four laminates of differing transverse ply thicknesses were investigated. Transverse crack spacing was found to decrease with increasing applied stress and decreasing transverse ply thickness. Very close agreement has been found between the experimental results and a multiple cracking theory based on shear lag analysis in which the plies remain essentially elastically bonded. In these composites a small modulus change is observed at a strain lower than that at which cracking initiated. This phenomenon is associated with a visual, under some circumstances reversible, whitening effect.

Microstructural characterization of cast nickel aluminium bronze

Abstract: The morphology and chemical analysis of the complex phases present in cast nickel aluminium bronze, of nominal composition 10% aluminium, 5% nickel and 5% iron, have been investigated using optical and electron microscopy techniques and energy dispersive analysis. It has been shown that α, β and four forms of κ can exist in the ascast microstructure of this alloy. Heat treatment can lead to the precipitation of a further κ phase which differs in morphology and chemical composition to those present in ascast structures.

Grain rearrangements during superplastic deformation

Abstract: Current models for obtaining large superplastic flow without change of grain size are two-dimensional; they therefore involve rearrangement of grains without increasing the surface area of the specimen as it deforms. A new model is proposed in which grainboundary sliding (GBS) in a group of grains is accommodated by a grain emerging from the next layer of grains, giving the correct increase in surface area. This also produces curved grain boundaries and there is some rotation of grains involving plastic flow in a zone along grain boundaries (the “mantle”) of predictable width. Grains do not have to be uniform and regular for the process. Characteristic configurations of marker lines are produced by the deformation. All these features are shown to have been observed in the literature. The model does not predict a threshold stress. It can be linked with a previous constitutive equation based on the climb and glide of dislocations in the grain mantles.

Hot-pressing and the ?-? phase transformation in silicon nitride

Abstract: The kinetics of densification and the kinetics of theα-β phase transformation have been measured during the hot-pressing of silicon nitride ceramics using magnesia as additive. Two mechanisms of densification have been identified. The first is a very rapid particle rearrangement, liquid-enhanced above 1550° C, which operates up to relative densities of about 0.65. The kinetics of the much slower decelerating second stage obey the Coble hot-pressing equation and the rate of densification is found to be proportional to the amount of additive. The controlling mechanism is believed to be diffusion in a boundary second phase, and values for the diffusion coefficient,D b, of the rate-controlling species in the boundary phase for temperatures above and below 1550° C are given. The kinetics of theα toβ transformation, the greater part of which occurs after densification is complete, are described by a first order reaction; the dependence of rate on the quantity of additive and on temperature is similar to that found for densification, and a similar controlling mechanism is believed to be responsible for the two processes.

Brittle fracture in a ductile material with application to hydrogen embrittlement

Abstract: A physical model of fracture in materials is developed which features a brittle crack imbedded in a plastically deformed medium. This model is presented as an alternative to fully ductile failure by hole growth, and general criteria for the two alternatives are discussed. One of these criteria for the existence of an atomically sharp crack is that the dislocation content near the crack tip be limited by the inhomogeneous character of dislocation slip in the crystal. With the dislocation distribution characteristic of Mode III fracture, we derive expressions for the fracture toughness as a function of material parameters. We have extended the theory to the case of hydrogen embrittlement in steels and compare our theoretical predictions with experimental work by others.

Preparation of a gel from metal alkoxide and its properties as a precursor of oxide glass

Abstract: Basic research on the formation of monolithic gel as the precursor of oxide glass has been done concerning the hydrolysis and gelling of silicon tetramethoxide and the properties of dry gel. Disks of dried gel were prepared without fracture by adding the necessary amount of water for hydrolysis into a methanol solution of silicon tetramethoxide, followed by drying the formed gel while keeping the vapour pressure of the methanol under control. The dried gel was porous with micropores ranging from 15 to 100 A in diameter. It had a specific surface area of more than 300 m2 g−1 and about 30% porosity. Monolithic gel was also prepared by using dilute ammonia water for hydrolysis. The time taken to gel became shorter with increasing ammonium ion concentration, but resulted in a gel of coarser structure. Dehydration polymerization of 60 to 75% occurred at room temperature. The polymerization was more pronounced in gels made with higher ammonium concentrations, but the amount of unreacted ethoxy group was also larger in these gels.

Growth morphology of the tetragonal phase in partially stabilized zirconia

Abstract: The zirconia-rich, metastable tetragonal phase in partially stabilized zirconia—magnesia, zirconia—calcia and zirconia—yttria is examined using electron microscopy and electron and X-ray diffraction. The tetragonal phase precipitate distribution is that normally associated with homogeneously nucleated coherent precipitation. An attempt is made to explain the growth morphology of the tetragonal phase in terms of the cubic—tetragonal lattice parameter mismatch. It is found that the tetragonal phase is retained at room temperature provided coherency with the cubic matrix is retained. Once coherency is lost, due to growth strains or mechanical influences, the precipitate reverts to the room temperature stable monoclinic form.

The α/β silicon nitride phase transformation

Abstract: The literature on the α/β silicon nitride transformation is reviewed briefly. Data are presented on the kinetics of the tranformation of 1600° C on low and high purity silicon nitride powders. The addition of magnesia increased the rate of transformation while the addition of yttria had no effect. Scanning electron photomicrographs show clearly the morphology changes that accompany the transformation. It is concluded that the transformation occurs via a solution-precipitation mechanism and that α and β are probably low and high temperature forms of silicon nitride.

A study of the effect of injection speed on fibre orientation in simple mouldings of short glass fibre-filled polypropylene

Abstract: The pattern of fibre orientation in injection moulded strips of glass fibre reinforced polypropylene has been studied using the technique of contact micro-radiography. It has been found that the fibre orientation in the core of the mouldings is very dependent on injection speed. High injection speed gives alignment of fibres transverse to the flow direction, while for very low speeds the fibres align parallel to the flow. The associated changes in topography of the mouldings have been studied using scanning electron microscopy. The rheological properties of both glass fibre-filled and unfilled polypropylene have been studied in a capillary rheometer. At low shear rates, the fibres cause a significant increase in viscosity, but at the shear rates likely to be encountered in injection moulding, the filled and unfilled melts have very similar viscosities. The rheological data can be used to interpret the pattern of fibre orientation in the mouldings.

The role of recovery forces in the deformation of linear polyethylene

Abstract: In the light of the finding that the deformation of linear polyethylene is associated with the development of recoverable strain, a technique has been developed to determine the magnitude of the recovery forces. The difference between the applied force and the recovery force represents the effective force which acts on the anelastic processes and a consideration of the kinetics of deformation suggests that the anelastic process consists of the co-operative movement of a number of molecular segments. The extrapolated yield point appears to be associated with the effective force and has no particular structural significance, in that it corresponds merely to the point of maximum curvature in the relationship between effective stress and rate of deformation.

The wetting of alumina and vitreous carbon by copper-tin-titanium alloys

Abstract: The wetting behaviour of copper-rich copper-tin-titanium alloys on alumina and vitreous carbon substrates at 1050 to 1150° C has been determined using the sessile drop technique. Substantial additions of titanium induce copper to wet, but tin has no significant effect. However, the simultaneous addition of tin and titanium is markedly beneficial, particularly with vitreous carbon. Metallographic and EPMA studies showed that titanium-rich reaction products were formed at the interfaces. The wetting and reactivity data are interpreted in terms of surface enrichment of the alloys by tin and of a disproportionately greater activity of titanium in tin.

The crystallization kinetics of Fe-Ni based metallic glasses

Abstract: Differential scanning calorimetry is used to study the crystallization kinetics of two commerical Fe-Ni metallic galsses near their glass transition point. For 0.01

A mechanism for the nitridation of Fe-contaminated silicon

Abstract: The influence of iron impurity on both the oxidation and nitridation of high purity silicon has been investigated. It is shown that iron is effective in rapidly removing the protective silica film which normally covers silicon. Experimental evidence suggests that the removal is achieved by iron-induced devitrification and disruption of the silica, thus allowing the SiO (g) generated by the Si/SiO2 interface reaction to escape. During the nitridation of iron-contaminated silicon powder compacts it is found that iron significantly enhances the extent of reaction for contamination levels of <1000 p.p.m. Fe (by weight). Above this level there is a decrease in the rate of formation of extra nitride. At all levels of contamination the percentage of silicon converted to β-Si3N4 was observed to be directly proportional to the iron concentration, and it is shown that this β-growth occurs within an FeSix liquid phase. The possible implications of the findings for the optimization of strength of reaction-bonded silicon nitride are briefly discussed.

Microstructural characterization of ?REFEL? (reaction-bonded) silicon carbides

Abstract: Quantitative characterization of the microstructure of a number of samples of reactionbonded (REFEL) silicon carbide has been undertaken employing transmission and scanning electron microscopy, optical microscopy, and electron and X-ray diffraction techniques. Impurity-controlled secondary electron SEM image contrast has proved particularly useful in differentiating between the SiC present in the initial compact and that formed during the reaction-bonding process, and, in contrast to previous descriptions of the microstructure, it has been found that the newly-formed SiC is deposited from the supersaturated solution of carbon in molten silicon both epitaxially on the original SiC grains, maintaining the sameα-polytypic stacking sequences, and by nucleation of fine cubicβ-SiC elsewhere. The relative quantities of material occurring by these two mechanisms have been found to vary from sample to sample, although the epitaxial growth on the original grains always occurs to some extent and is responsible for the bulk cohesion of the material. Some conclusions have been drawn concerning the reaction model and the process parameters controlling the microstructure of this type of material.

A study on titanium diffusion into LiNbO3 waveguides by electron probe analysis and X-ray diffraction methods

Abstract: The diffusion of titanium into lithium niobate has been studied by X-ray rocking curve and topographic methods as well as electron probe microanalysis in an attempt to determine the diffusion mechanism and evaluate the crystallinity of the diffused layer. It was found that the titanium concentration along the depth in the diffused layer approximates to a Gaussian distribution, the activation energy for diffusion being 2.18 eV. We also found that the diffusion of titanium caused a marked lattice contraction along the a-axis (Δa/a ≈ −10−3) resulting in the generation of misfit dislocations and cracks in the diffused layer. The high activation energy and the lattice contraction suggest that the titanium ions diffuse substitutionally into the lithium niobate crystal. Mechanisms causing refractive index changes in the diffused layer are also discussed.

The structure of lead silicate glasses determined by vibrational spectroscopy

Abstract: The polarized Raman spectra and infrared powder absorption spectra for PbO ·xSiO2 glasses, with 0.5⩽x⩽2, were measured and interpreted in terms of the structure of the glasses. Comparison of the spectra of the glasses with the spectra of numerous stable and metastable crystalline phases was also made. It was found that the glasses are composed of a number of silicate species. In glasses for whichx is less than 1, the presence of ionic oxygen (oxygen not bonded to silicon) is confidently inferred from the spectra. A new way of deconvoluting the spectra is also reported.

Crack propagation in an amine-cured epoxide resin

Abstract: Crack propagation has been studied in a DGEBA epoxide resin cured with two different series of amine hardeners. Both series produced a cured resin in which cracks propagated in a stick-slip manner in a series of jumps. With a primary diamine hardener, the initiation energy depended on the molecular weight of the hardener, but with a mixed amine hardener the initiation energy was independent of molecular weight. There were also significant differences in fracture surface morphologies between the two series. The effect of temperature and testing rate on the fracture energy and fracture surface morphology of ethylene diamine cured resin has also been studied. The fracture initiation energy increased with increasing temperature and decreasing testing rate and there is some indication of a peak occurring in the 22° C data. Long term static loading experiments have failed to show the occurrence of a static fatigue effect. Different batches of nominally identical resin produced different results, leading to problems of irreproducibility of behaviour.

Interfacial slip and damping in fibre reinforced composites

Abstract: The effect of interfacial slip on the stress—strain properties of a rubbery polymer reinforced with short fibres is discussed with particular reference to hysteresis. The amount of energy dissipated by interface sliding and by the viscoelastic response of the matrix is calculated in the light of a simple model. This is compared with the dynamic properties of the composites, both with a view to designing composites with a useful combination of stiffness and damping, and also with the possibility of evaluating the integrity of the fibre-matrix interface from dynamic measurements.

Evaluation of biaxial stress failure surfaces for a glass fabric reinforced polyester resin under static and fatigue loading

Abstract: In an attempt to evaluate failure theories for a glass fabric reinforced polyester resin over 370 tests have been conducted on thin-walled tubes under combined axial loading and internal pressure, both for static and fatigue loading. For plane stress the results are considered in relation to imaginary failure surfaces inσ1,σ2,σ6 space. A limited measure of agreement between theories and results can be obtained after highly subjective selection of data. Only those theories which involve complex stress properties provide a reasonable fit. The behaviour of tubular specimens is strongly influenced by the presence of joints in the reinforcements.

Cordierite glass-ceramics-effect of TiO2 and ZrO2 content on phase sequence during heat treatment

Abstract: A series of glasses of stoichiometric composition with varying proportions of TiO2 and/or ZrO2 as nucleating agent have been studied to examine the effect of nucleation addition on the sequence of crystallization and transformation to the stable phase, cordierite. It is shown that TiO2 is the most effective nucleating agent and that if large amounts of ZrO2 are substituted for TiO2 cristobalite forms as an intermediate phase and is associated with rapid volume changes and consequently with weak porous products. Substitution of ZrO2 for TiO2 also causes other changes in phase development, especially in the relative proportions of β-quartz solid solution and magnesian petalite produced during the early stages of crystallization. The use of a combination of experimental techniques (dilatometry, DTA, X-ray diffraction and electron microscopy) has proved most effective in studying phase development and the relationship between processing characteristics and composition.