Showing papers in "Journal of Materials Science in 1969"

The formation of omega phase in titanium and zirconium alloys: A review

Abstract: The omega phase is a metastable phase which forms in alloys of titanium and zirconium with most transition metals. In this paper the available data from both alloy systems on the occurrence, the structure, the mechanism of formation, and the morphology of the phase are reviewed and compared. The effect of omega phase on the mechanical behaviour and on the superconducting properties is then discussed. It is concluded that uncertainties still exist on the mechanism of the omega transformation during quenching and on the mechanism of the effect of omega on the mechanical properties.

Transparent glass-ceramics

Abstract: Glass-ceramic materials may transmit visible light if either of the following conditions are operative: (i) the crystallites of all species are much smaller than the wavelength of visible light, or (ii) the optical anisotropy (birefringence) within the crystals and refractive index difference between crystals and glass are very small. These conditions are achieved in several aluminosilicate glass-ceramic systems. Solid solutions (ss) of β-quartz, spinel, and mullite are the major crystalline phases in these transparent glass-ceramics. The transparent β-quartz solid solution glass-ceramics can be divided into three areas: ultra-low expansion materials, refractory and colourless materials, and high electrical resistivity—low dielectric loss materials. These three material groups, though chemically distinct, are all characterised by high crystallinity, thermal shock resistance, excellent chemical durability, and susceptibility to mechanical strengthening by ion-exchange techniques. The transparent spinel and mullite glass-ceramics contain considerable glassy phase, but nevertheless are characterised by excellent transparency, even after thermal exposure for long periods above 1000° C.

Microtopography of Surfaces Eroded by Ion-Bombardment

Abstract: Conical protrusions have been observed in the scanning electron microscope on surfaces that have been eroded by sputtering under 5 to 8 keV A+ ion bombardment. It is shown why these have a half-angle corresponding to the principal peak in the graph of sputtering rate versus angle of ion incidence. These lead to some general conclusions on the microtopography ofion-eroded surfaces.

Dislocation morphology in graded heterojunctions: GaAs1−xPx

Abstract: The details of the formation, propagation, interaction, and densities of misfit dislocations are combined into a simple model quantitatively predicting dislocation densities for both abrupt and graded heterojunctions. Three key concepts are introduced: (1) misfit dislocations are segmented; (2) accordingly, they must give rise to a density of inclined dislocations, n I, that propagate through the growing layer; and (3) these inclined dislocations can bend in and out of any subsequently formed misfit plane to relieve the strain, and when bent in, serve as strain-relieving misfit dislocations. Thus, the value of n I is expected to remain constant with thickness. Also, n I is predicted to vary directly with the compositional gradient at the heterojunction. It is pointed out that there are two general classes of misfit dislocations, pure-edge and mixed and that their intersections, which cause the misfit dislocations to appear to bend within their plane, can be simply classified into three general types. Transmission electron microscopy was used for a comprehensive study of dislocations in a series of GaAs1−xPx heterojunctions prepared by a vapour phase growth technique. The main features of the above model were corroborated. The value of n I was found to be constant with growth distance as postulated, and in quantitative agreement with prediction, n I decreased from 4 × 107 cm−2 to 106 cm−2 as the compositional gradient decreased from 5% phosphorus/μm to 0.2% phosphorus/μm. Note that these values can far exceed the dislocation density of the substrates. Of particular significance, the inclined dislocations n I were found to propagate through a constant-composition region grown on top of a compositionally graded region, so that formation of the heterojunction must affect subsequently grown layers. Finally, it is shown that the misfit dislocations are, indeed, a combination of pure-edge and mixed, and all three postulated general interactions between these dislocations are shown to occur.

Wetting of Aluminium Oxide by Molten Aluminium and Other Metals

Abstract: The sessile drop technique has been used to measure the contact angle of molten aluminium, aluminium-nickel and aluminium-copper alloys, copper and gold, with sapphire, ruby and recrystallised alumina. Measurements were madein vacuo, and as a function of time and temperature over the range 800 to 1500° C. Cinematography and time-lapse photography were used. At temperatures below 950° C, sessile drops of aluminium reached equilibrium only after a period of time which increased with decrease in temperature and could be in excess of one hour. A rapid increase in contact area occurred around 900° C. Above 1150° C drops of aluminium and of the aluminium alloys were observed to spread and contract repeatedly. Contractions were observed with both polycrystalline and single-crystal alumina, although they were much more pronounced with the latter, and were associated with the formation of a series of reaction rings on the plaque. Ruby and sapphire behaved similarly. The shape of the rings depended on the crystallographic orientation of the plaque: the reaction profile tended to terminate in certain low index directions. Neither contractions nor reaction was observed with copper or gold. The observations are discussed in terms of the combined effects of evaporation, chemical reactivity and interfacial tensions in the system.

Lattice self-potentials and madelung constants for some compounds

Abstract: The computerised potential and electrostatic energy calculations described in Part 1 can be used as the first step in more sophisticated calculations, four types of which are described. (i) The results obtained by calculating the actual crystal field in a sodium vacancy in NaCl are compared to the results of the familiar approximation for that situation. (ii) CaF2 and β-alumina are used as examples of compounds in which there is remarkable similarity between some of the interstitial and some of the lattice sites. A complete calculation of all the self-potentials and the Madelung constant of β-alumina is included. (iii) In a discussion of p- and n-type semiconduction in simple binary compounds, self-potentials are used to show that many structures have a built-in preference for a deviation from stoichiometry, although other factors (not considered in this paper) can obscure this preference. (iv) The concept of a solid-state energy storage using compounds with two different cations in non-interacting sublattices is explored, using the mineral quenselite (PbMnO2(OH)), as an example. The computer program is used to evaluate this mineral with respect to its energy density.

Temperature coefficients of capacitance of solids

Abstract: An earlier treatment of temperature coefficient of capacitance, γ c [1] has been extended to include most solids. Materials are divided into those with given ranges of permittivity, e, and temperature coefficient of polarisability. It appears that, for low dielectric loss, high permittivity glasses, like simple ionic compounds, always have a positive γ c, whereas paraelectrics and polymers have negative γ c. Ferroelectrics can have any value of γ c. Limitations in γ c and e for given classes of solid are discussed.

The tensile strength and ductility of continuous fibre composites

Abstract: The plastic instability approach has been applied to the tensile behaviour of a continuous fibre composite. It is shown that the combination of two components with different strengths and degrees of work-hardening produces a new material with a new degree of work-hardening, which may be determined by the present analysis. Expressions for the elongation at rupture and the strength of a composite have been obtained and the results of the calculation are compared with some experimental data.

The equilibrium topography of sputtered amorphous solids

Abstract: A theory is developed for the sputtering of amorphous solid by an ion beam and the changes in surface topography to which this sputtering leads. It is shown that surfaces which have pinning regions, such as vertical steps and included impurities, reach an equilibrium in which cones are developed on the surface. The theory is compared with several experimental observations of such conical development.

Precipitate strengthening mechanisms in magnesium zinc alloy single crystals

Abstract: The mechanical behaviour of Mg-5.1 wt % Zn alloy single crystals was studied in the 4.2 to 300° K temperature range. Quenched crystals have activation energies and volumes best associated with the cutting of small clusters of Zn atoms by dislocations. Fully hardened crystals contain fine β 1′ and occasionalβ 2′ precipitates with an averageβ 1′ interparticle spacing of 330 to 660 A. Strengthening in these crystals is mainly ascribed to the cutting of β 1′ particles by dislocations. In the overaged condition β 1′, β 2′ and equilibrium β particles are present and lead to a considerable temperature-dependence unusual for an overaged condition. Analysis of this temperature-dependence suggests that below 77° K the relatively easy cutting of β 1′ particles by dislocations takes place in addition to the cutting of β 2′ and β particles. Above 77° K the difficult cutting of β 2′ and β particles alone controls the deformation, β 1′ being more easily cut with the aid of thermal fluctuations.

Anisotropy in oriented fibres from synthetic polymers

Abstract: On the assumption that synthetic fibres at low strains can be considered as transversely isotropic elastic bodies the five independent elastic compliances have been measured for filaments of low and high density polyethylene, nylon 66, polyethylene terephthalate and polypropylene, all at room temperature Experimental values are compared with those predicted using an aggregate theory which assumes that the partially oriented fibre can be considered as an aggregate of elastic units which are aligned by the drawing process, and whose properties are those of the fully oriented fibre

Low endurance fatigue in metals and polymers

Abstract: Studies of crack growth in fatigue of annealed aluminium, Nylon 66 and Araldite epoxy resin have revealed two mechanisms, shear and tensile tearing. The former applies both to fatigue and to tensile straining under approximately plane strain conditions and requires a capacity for plastic flow within the material. The latter mechanism occurs where flow is either restricted or impossible. A model for crack propagation in low endurance fatigue is described, and a rigid-plastic analysis is shown to be consistent with the observations.

Facetting and optical perfection in Czochralski grown garnets and ruby

Abstract: Studies of ruby (Al2O3/Cr3+) and rare-earth aluminium garnet single crystals, in particular the mixed garnets formed between Y3Al5O12 (YAG) and Dy3Al5O12 (DyAG), have shown that the formation of facets on the solid/liquid interface, which give rise to a strained central core within the crystals, is dependent upon the shape of the solid/liquid interface. Development of the strained and facetted core can be prevented by modifying the growth conditions to produce a flat solid/liquid interface and as a result the optical perfection of the crystals is greatly improved. Certain crystals, e.g. DyAG, grow naturally with aflat interface, and in the present work this has been shown to be due to the optical absorption characteristics of this material. In other materials, e.g. YAG and ruby, the interface shape can be controlled by the rate at which the growing crystal is rotated. The changes in temperature gradient produced in a YAG melt by changes of crystal rotation rate have been measured, and their effect upon crystal perfection is described.

Eutectic solidification in the system Al2O3/Y3Al5O12

Abstract: The eutectic solidification in the system Al2O3/Y2Al5O12 has been investigated. A Bridgman-type crystal-growing furnace was used in this investigation. A temperature gradient of 190° C/cm and growth rates which were varied between 2 and 12 cm/h were employed in the directional solidification studies. Three types of microstructure were observed depending upon the composition and the growth rate. At a growth rate of 4 cm/h and at compositions removed from the eutectic composition, a mixture of primary phase and fine eutectic dispersion was found. At growth rates between 2 and 12 cm/h at the eutectic composition, a colony type microstructure was most commonly observed. At growth rates above 4 cm/h at the eutectic composition, regions in the solidified ingot were found to have a highly oriented eutectic microstructure consisting of both rods and platelets. These eutectic microstructures indicate that coupled growth can occur in this system. The method of Sunquist and Mondolfo [15] was used to determine whether Y3Al5O12 was the first phase to nucleate at the eutectic.

Some observations on the deformation characteristics of bulk polycrystalline zirconium hydrides

Abstract: The compressive deformation of polycrystalline ɛ-zirconium hydride has been investigated within the temperature range 22 to ∼ 400° C and over the composition range ZrH1.71 to ZrH1.92. The observed deformation modes included slip, creation of new lamellae of different orientations to those of the main transformation lamellae, and movement of lamellar boundaries. The results are considered in terms of the contribution of these three processes to the overall deformation behaviour.

Recrystallisation mechanism and annealing texture in aluminium-copper alloys

Abstract: The correlation between the mechanism of recrystallisation and the annealing texture of aluminium-copper alloys was investigated by transmission electron microscopy and selected area diffraction, and pole figure determination by X-rays. Continuous recrystallisation by sub-grain growth leads to preservation of the rolling texture, while recrystallisation by motion of a high-angle boundary produces a cube texture as in pure aluminium. The conditions under which the different modes of recrystallisation occur and the reasons for the formation of the two types of textures are discussed on the basis of microscopic mechanisms.

Mechanical properties of glass fibre-reinforced gypsum

Abstract: Gypsum plaster, like other inorganic cements, is strong in compression but weak in tension exhibiting brittle behaviour. Commercialy available E glass fibres can be used to reinforce the gypsum plaster matrix and produce a strong composite material having improved tensile and impact properties. This paper describes the effect of glass content on the flexural, tensile, compressive and impact strength of this glass fibre plaster composite which is made by a simple spray/suction technique.

Review: Radiography with neutrons

Abstract: The use of neutron beams for radiographic purposes is a relatively new method of non-destructive testing. This paper is a general review of the technique. The various sources of neutrons and image recorders which may be used are discussed with particular emphasis on photographic recorders. Problems in which neutron radiography is expected to show advantages over conventional radiography are outlined, and some results are presented.

The determination of the diffusion constants of oxygen in nickel and α-iron by an internal oxidation method

Abstract: The diffusion constants of oxygen in nickel and α-iron are calculated from experimentally determined internal oxidation rates in dilute silicon-bearing alloys. A single crystal alloy of composition 0.058 wt % silicon in nickel and polycrystalline alloys of composition 0.48 wt % silicon in nickel and 0.072 wt % silicon in iron were oxidised and allowance is made for the absorption of oxygen in the reaction with silicon. For nickel the values of activation energy Q=73.9 kcal/g atom in the range 800 to 1200° C and Q=74.4 kcal/g atom in the range 900 to 1300° C are in good agreement with a previous result. The activation energy required for oxygen diffusion in α-iron was found to be 39.9 kcal/g atom in the range 700 to 850° C.

Preparation and phase studies of the ternary semiconducting compounds ZnSnP2, ZnGeP2, ZnSiP2, CdGeP2, and CdSiP2

Abstract: ZnSnP2, ZnGeP2, ZnSiP2, CdGeP2 and CdSiP2 have been prepared in single or polycrystalline form by direct synthesis from the elements, combination of the group IV element with the II-V2 phosphide, vapour transport or solution growth. Differential thermal analysis and X-ray powder diffraction studies have been used to determine the temperatures of phase transitions in each of these compounds. Of particular interest is the observation of a phase transition in CdSiP2 at 1098° C some 22° C below the melting point. The thermal analysis studies of ZnSnP2 indicate a peak, the cause of which is uncertain, at 720° C which is well separated from the melting temperature of 930° C; further ZnSnP2 does not appear to be congruently melting and has a freezing point at 970° C. c/a ratios have been determined for all five compounds to 1 part in 2000.

Micro-indentation hardness variation as a function of composition for polycrystalline solutions in the systems PbS/PbTe, PbSe/PbTe, and PbS/PbSe

Abstract: Vickers micro-indentation hardness as a function of composition has been measured for polycrystalline solutions of lead chalcogenide systems and results compared to exsolution tendencies as implied by subsolidus features of the phase diagrams. Each system exhibited a positive deviation from a linear hardness relation between the end member compounds. The most pronounced hardening was observed for PbS/PbTe solutions; the maximum hardness occurs at about 30 mole % PbTe, coincident with a large solubility gap with a solvus maximum (critical point) at about 30 mol % PbTe and approximately 805° C. The least amount of hardening was observed for the system PbS/PbSe, which exhibits no exsolution at temperatures as low as 300° C. The hardness versus composition curve was approximately symmetric about the 50 mol % composition. Intermediate between these two systems, the PbSe/PbTe crystalline solutions exhibited an asymmetric hardness/composition curve with maximum hardness at about 30 mol % PbTe. Partial phase studies indicate the possibility of a solvus maximum at 500 to 600° C on the PbSe-rich side of the diagram.

Sintering of aluminium nitride: Particle size dependence of sintering kinetics

Abstract: The effect of particle size (0.78 ∼ 4.4 μm) on the sintering kinetics of AIN powder was investigated in the temperature range from 1600 to 2000° C and the results were analysed on the basis of vacancy diffusion models. The mechanisms of sintering are discussed. Fractional shrinkage is proportional to the nth power of soaking time with n = 0.20 for 4.4 μm and 1.5 μm powders and 0.33 for 0.78 μm powder. For the 0.78 μm powder at 1900° C, however, n decreases gradually as grain growth proceeds. The experimental activation energy for sintering is between 92 kcal/mole for 4.4 μm and 129 kcal/mole for 0.78 μm powder. Unlike this activated energy, the rate of sintering and the diffusion constant calculated from it increase drastically with decrease of particle size; the derived diffusion constant for 1.5 μm powder is 101 to 102 times larger than that of 4.4 μm powder, and for 0.78 μm powder the diffusion constant is estimated to be still higher. The particle-size dependence of parameter n and the diffusion constant seems to be caused by a variation in predominant diffusion mechanisms; namely, bulk diffusion in coarse powder and surface or grain-boundary diffusion in fine powder.

Plastic deformation of polypropylene

Abstract: In a wide range of drawing temperatures (20 to 150° C) and draw-ratiosλ (5 to 20) the axial long periodL T is a unique function of temperature. It is completely independent of the long period of the starting material which was varied within wide limits (125 to 350 A). The gradual transformation of the long period observed at small draw-ratio (λ between 1 and 5) from the valueL 0 of an undrawn sample to the limiting valueL T>L 0 of highly drawn polypropylene (λ ≥ 5) could be demonstrated on samples withL 0 larger thanL T. The change of long period is abrupt, indicating a discontinuous step in the transformation from the original microspherulitic into the fibre structure. These results may be interpreted in the same manner as in the case of polyethylene. During plastic deformation in the neck the lamellae are broken into small folded chain blocks which are then incorporated into the microfibrils, the basic building element of the fibre structure. The work of deformational forces must so mobilise the chains in the blocks that they get rearranged with a new long period, corresponding to the temperature of drawing.

Crystal clamping in PbTiO3 glass-ceramics

Abstract: Lead titanate crystals precipitated in borate glasses by the glass-ceramic process have been found by X-ray diffraction to have abnormally low axial ratios. This has been shown to be due primarily to the stress applied by the surrounding glass matrix.

Structural transformation in thin films of binary alloys

Abstract: Structural transformation in thin films of binary alloys of 1st and 6th group elements, prepared by vacuum evaporation, has been studied by the author in detail by electron diffraction. The alloys chosen for the investigation were Ag2Te, Ag2Se, Ag2S, Cu2Te and Cu2Se. The study of thin films of these alloys revealed that there occurs a transformation in structure and on heating, the temperatures of transformation are higher compared to the alloys in the bulk state. It has also been observed that the temperatures of structural transformation during heating and cooling are different. No quantitative explanation of the result has been given and it is expected that detailed discussion of the results will be published later.

The crystallisation of glasses based on the eutectic compositions in the system Li2O-Al2O3-SiO2

Abstract: Structural transformations have been studied in glasses related in composition to the binary eutectic between lithium metasilicate and β-spodumene. Crystallisation processes and changes in microstructure during the controlled heating of the glasses have been followed using X-ray diffraction, electron microscopy, high temperature microscopy, thermal analysis and electron spin resonance spectroscopy. The influence exerted by titanium dioxide on the phase relationships, crystal growth rates and micromorphology of the polycrystalline products of heat-treatment has been investigated and the findings used as a basis for proposals on the role of TiO2 during nucleation and crystal growth.

Compositional study and properties characterisation of alkaline earth feldspar glasses and glass-ceramics

Abstract: Fields in which metastable crystallisation of feldspars may occur as a result of heterogeneous nucleation were defined at 1000° C for the systems BaO-Al2O3-SiO2, SrO-Al2O3-SiO2, and CaO-Al2O3-SiO2. The metastable fields of the above barium and calcium systems differ considerably from the respective primary liquidus fields of celsian and anorthite with the tendency for the stoichiometric compositions to be situated at the centre of the metastable region. Various physical and dielectric properties of the glasses and glass-ceramics of the three systems are presented and compared. High linear thermal expansion, high electrical resistivity, and low loss tangent and dielectric constant characterise these glass-ceramics. One of them (No. 2, table I) has, in addition, high heat resistance and translucency at high temperatures.

The elastic constants and interatomic binding in yttria-stabilised zirconia

Abstract: Zirconia (ZrO2) can be forced into a cubic fluorite structure by the addition of more than 7 mol % yttria (Y2O3). The elastic stiffness constants of cubic zirconia single crystals containing 8 and 12 mol % yttria have been determined between 77 and 300° K by an ultrasonic pulse echo technique. Elastic constants are almost temperature-independent and at 77° K are for the 8 and 12 mol % respectively: C 11=2.04, C 12=0.87, C 44=1.58; C 11=2.23, C 12=0.973, C 44=1.54 (units: 1012 dynes cm−2). Compressibilities and elastic compliances are also presented. The data provide a basis for discussion of crystalline stability and the nature of the interatomic forces. The decreasing cubic lattice stability as the yttria content is reduced is demonstrated quantitatively. An ionic model closely characterises the binding forces. The repulsive energy is about 12% of the Madelung attractive energy. Debye temperatures calculated from the elastic constant data extrapolated to 0° K are 595 and 604° K for the 8 and 12 mol % respectively.

Low endurance fatigue in metals and polymers: Part 1 Stress/strain relationships

Abstract: The low endurance fatigue behaviour of commercial purity aluminium, Nylon 66 and an epoxy resin is examined and compared. Both aluminium and nylon obey a plastic strain criterion for failure and this behaviour is independent of frequency. No satisfactory criterion for failure could be found for the epoxy resin whose behaviour is strongly frequencydependent.