Showing papers in "Journal of Materials Science in 1968"

The strength of two-phase ceramic/glass materials

Abstract: The strengths of various glasses, with a range of expansion coefficients, containing 10 vol % thoria spheres, of diameter 50 to 700μm, have been measured. Stresses occur around the spheres, due to differences in the expansion coefficients of the glass and the spheres, on cooling from the fabrication temperature. Stress magnification occurs near the spheres, due to differences in elastic properties, in the presence of an applied stress. When the expansion coefficient of the sphere is greater than that of the glass, circumferential cracks form around the spheres but only when the sphere diameter is greater than a critical value. An approximate value for the critical diameter may be obtained by an energy balance criterion. Cracks may form around spheres smaller than the critical diameter under application of applied stress at stresses below the macroscopic fracture stress. In these cases the strength is governed by a Griffith relationship with the crack size equal to the sphere diameter. When the expansion coefficients of the spheres and glass are similar, the strength of the glass is reduced only when large spheres (⪞300μm diameter) are present. When the expansion coefficient of the spheres is less than that of the glass, linking radial cracks form between the spheres and the material has very low strength.

The effective surface energy of brittle materials

Abstract: The effective surface energy of four brittle materials, alumina, poly(methylmethacrylate), glass, and graphite, is calculated from load/deflection curves of notched bars deformed in three-point bending. Two of the methods, which are commonly used in fracture mechanics studies,viz the modified Griffith treatment and the compliance analysis method, are concerned with the effective surface energy at the initiation of fracture,γ I . The third method, the work of fracture test, is concerned with the mean effective surface energy over the whole fracture process,γ F . The two estimates ofγ I give consistent values, and there is no systematic variation ofγ I with notch depth. Values ofγ F decrease with increasing notch depth as the fracture process becomes more controlled. For aluminaγ I ∼γ F . For PMMA and glassγ I > γγ F because of a multiplicity of crack sources during fracture initiation. For graphiteγ I <γ F because of subsidiary cracking as fracture proceeds.

Self-diffusion of Ti, Zr, and Hf in their hcp phases, and diffusion of Nb95 in hcp Zr

Abstract: Preliminary values for the self-diffusion constants of α-(hexagonal cp) Hf and new values for the self-diffusion constants of α-(hcp) Zr are given. A new determination of the latter was considered necessary since existing values disagree widely and the experimental methods employed in their determination are not considered to be sound. Values obtained by the authors for the self-diffusion constants of α-Ti are reviewed.

Hardness and low-temperature deformation of silica glass.

Abstract: Vickers diamond pyramid hardness measurements have been made on silica glass with varying thermal history using loads up to 1000 g. Hardness was independent of load and source. From interference photographs and subsequent anneal of the indentations at temperatures belowT g, it was concluded that indentation leads primarily to densification of a volume of glass in the vicinity of the indenter. A portion of the densification which is recoverable at relatively low temperatures is attributed to molecular entanglement of the glassy network due to high pressure and shear. The other portion which is not recoverable belowT g represents an approach to the final equilibrium density of the glass. Hardness of silica glass as determined by this method is thus defined as a resistance of the material to densification.

Microstructural Development During the Liquid-Phase Sintering of Two-Phase Alloys, with Special Reference to the NbC/Co System

Abstract: An investigation was made of the grain growth and other microstructural changes occurring during the liquid-phase sintering of NbC alloys with ∼20 wt % cobalt. The effects of sintering time, sintering temperature, and small alloying additions were studied. It was found that the grain growth of NbC in liquid cobalt, at 1420° C, can be described by the equation: $$\bar d^3 - \bar d_0 ^3 = {\text{K}}t$$ where $$\bar d$$ is the mean linear intercept of the grains after time t, and $$\bar d_0$$ the initial mean intercept, K being a temperature-dependent constant with an “activation energy” of 95±15 kcal/mole. This equation suggests that grain growth occurs by a solution/ precipitation process controlled by diffusion in the liquid phase. Small alloying additions of WC, TiC or NbB2 inhibit the growth and/or alter the growth process, as well as affecting such properties as the shape and contiguity of the carbide grains. The relative significance of grain coalescence to grain growth in a liquid phase is discussed. By examining theoretically the effect of anisotropy of interface energy on the cube ⇌ sphere grain-shape change, it has been possible to explain the observed sensitivity of grain shape towards sintering conditions.

The mechanism of reactive sputtering

Abstract: The reactive sputtering of tantalum in mixed argon/oxygen atmospheres at a total pressure of 3.0×10−4 torr has been investigated by means of measurements on deposition and growth rates, density, electrical properties and electron diffraction. The main controlling factor on all of the parameters was found to be the partial pressure of oxygen.

The plastic yield behaviour of polymethylmethacrylate

Abstract: The yield behaviour of an amorphous glassy polymer has been investigated under a system of combined stress in an attempt to define a criterion for yield. Sheets of polymethylmethacrylate were compressed in plane strain and the compressive yield stress was determined as a function of the tension applied in the plane of the sheet. The compressive yield stress was found to decrease with applied tension more rapidly than would be expected if the shear yield stress of the material were independent of pressure. The results have been analysed in terms of a Coulomb yield criterion where the shear yield stress is expressed as a constant plus a friction term proportional to the pressure on the shear plane. Birefringent shear zones were observed in the deformed region after the load was removed and these zones were inclined at 52.9° to the plane of the sheet. It was found that if the stresses at yield were expressed as nominal stresses then the inclination of the shear planes predicted by the yield stress data coincided with the observed inclination. It also appears that it may be possible to define a fracture criterion in terms of the applied stress system.

The Thermal Diffusivity of Ice and Water Between -40 and + 60 ~ C

Abstract: The thermal diffusivity αs of triply-distilled deionised water, and αL of single-crystal ice along the c-axis, have been measured by Angstrom's method. The temperature range covered was −40 to +60° C. The results for water compare well with published data for the thermal conductivity, but for ice there are unexplained discrepancies. The linear relationships αs=(8.43−0.101 T) 10−3 cm2/sec and αL=(1.35+0.002 T) 10−3 cm2/sec where T° C is the temperature, fit the data obtained.

Crystal growth and magnetic susceptibility of some rare-earth compounds

Abstract: Magnetic susceptibility measurements have been carried out on five series of rare-earth compounds using an ac mutual inductance technique. The compounds satisfy the general formulae RNbO4, R3NbO7, R2TiO5, R2Ti2O7, or RAIO3 where R is a rare-earth ion. The measurements were mainly carried out in the helium range 1 to 4.2° K, and the results analysed in terms of the general Van Vleck formula χ= C/(T+θ) +α. Many of the compounds were observed to undergo antiferromagnetic transitions, and in some cases showed evidence of extremely anisotropic behaviour above the Neel temperature.

The strength of metal/alumina interfaces

Abstract: The room-temperature strengths of interfaces formed by melting aluminium, cobalt, gold, iron, nickel, palladium, silver, and uranium in contact with alumina have been measured. Correlations were sought between the strengths and the physical properties of the systems. Metals which formed void-free interfaces and were not subject to martensitic transformations bonded well to alumina. Metals subject to martensitic transformations (cobalt and uranium) were virtually non-adherent at room temperature.

An X-ray diffraction topographic study of single crystals of melt-grown yttrium aluminium garnet

Abstract: Melt-grown crystals of yttrium aluminium garnet usually exhibit an optically imperfect central core region associated with the formation of facets on the solid/liquid interface during growth. The nature of the optical inhomogeneity of the core region, and the general defect structure of the crystals, was investigated by the technique of X-ray diffraction topography. The core region, which showed an elastic strain effect, contained straight growth striations reflecting the development of facets. Comparison was made between undoped and neodymium-doped crystals, and between crystals grown on and growth axes. Impurity striations were thought to be caused by temperature oscillations in the melt ahead of the growing interface. Dislocations, which were observed in the outer regions of the crystals, were sometimes nucleated at inclusions.

Self-diffusion in simple oxides (A bibliography)

Abstract: The paper is directed towards the materials scientist who wishes to employ published values of diffusion coefficients in his investigations. A brief review of self-diffusion behaviour and the likely inaccuracies of the various techniques used in the measurement of diffusion is followed by a selected bibliography and tabulation of coefficients for oxides.

Modifications to the Zener formula for limitation of grain size

Abstract: An experimental investigation of the ratio of grain boundary curvature to grain size is reported, from which it is deduced that the terminal grain size determined by inclusions should be about an order of magnitude less than that predicted by the Zener formula in its usual form. It is demonstrated that the Zener formula does not necessarily include the condition that each boundary is impeded by an inclusion, and a supplementary condition is derived to cover this possibility.

Recovery creep in materials hardened by a second phase

Abstract: A recovery creep model, based upon previous theories by McLean and co-workers, has been developed for creep in materials hardened by a second phase. According to the model the increased creep strength in these materials is caused by a decrease in the recovery rate, and this in turn is due to a decrease of the driving force for the recovery process and of the mobility of the climbing dislocations involved in the process. It is shown that the model can account for the very large stress-dependence of the creep-rate often found for alloys hardened by a second phase. Another support for the model is the observation that changes in the creep-rate for materials in different states of precipitation-hardening are entirely due to changes in the recovery rate.

The Adhesion of Metal/Alumina Interfaces

Abstract: Cylinders of copper and nickel have been melted under various conditions to form sessile drops on alumina plaques. The resultant metal/ceramic adhesion at room temperature has been measured using the commonly adopted test in which the drops are pushed off the ceramic plaques. The stress system involved in the test has been analysed and it has been shown that the standard interpretation of the test, as a measure of interfacial shear strength, is not valid; the revised interpretation makes it a measure of adhesion in tension. Results for the Cu/Al2O3 and Ni/Al2O3 systems show that non-wetted interfaces can be strong and have strengths that are independent of contact-angle changes caused by wetting-temperature variations.

Crystal growth and magnetic susceptibility of some rare-earth compounds

Abstract: Single crystals of the following types of rare-earth compounds have been obtained by the fluxed-melt method of crystal growth: ROF, R2Ti2O7, R2TiO5, RNbO4, R3NbO7 (R = rare-earth metal). The experimental conditions are described and X-ray diffraction data are presented for compounds not previously reported.

The dependence of grain-boundary sliding on shear stress

Abstract: Polycrystalline specimens of four materials: aluminium, a lead/thallium alloy,β-brass and Magnox AL80 were subjected to slow creep at a temperature of approximately 0.5Tm (whereTm is the melting temperature in degrees Kelvin) and measurements made of the dependence of grain-boundary sliding on the orientation of the boundary with respect to the applied stress. The results were analysed in detail on the basis of the hypothesis that sliding is governed by the component of the applied stress resolved in the boundary plane. Agreement between theory and experiment leads to the conclusion that this is a most important factor governing sliding. The component of sliding revealed by steps on a free surface is seen to contain an extra term due to the unbalanced pressure from grains below the surface.

The inter-relation of some mechanical properties with molecular weight and crystallinity in poly(ethylene terephthalate)

Abstract: Some mechanical properties of poly(ethylene terephthalate) were determined as a function of molecular weight, temperature and crystallinity. The effect of talc, a nucleating agent, on mechanical properties was also considered.

Origin of the equiaxed zone in small ingots

Abstract: It is deduced by thermal analysis that no entirely new crystals are nucleated in the interior of an ingot at any time after pouring. By comparing the conditions for columnar growth in ingots and in unidirectional solidification experiments, it is concluded that in the ingot, sufficient constitutional supercooling for nucleation might occur if it were not prevented by the presence of crystals in the liquid. At low pouring temperatures, most of the equiaxed grains are nucleated as separate crystals at the time of pouring. When the superheat is higher, the main process for the origin of the equiaxed zone appears to be a multiplication mechanism operating by local remeiting. Dynamic procedures used during unidirectional solidification and conventional ingot freezing permit one to conclude that the main mechanism operating to refine the equiaxed structure, as a function of imposed disturbances (mechanical or forced fluid flow) is a multiplication mechanism, most likely the process of detachment of dendritic arms by remeiting.

Stacking-faults in tellurium-doped gallium arsenide

Abstract: Samples of bulk-grown gallium arsenide single crystals, taken from both static freeze, and Czochralski ingots doped to a high level with tellurium, have been examined using transmission electron microscopy. Observation of single and multiple stacking-fault layers which have fault vectors of the kindR=a/3〈111〉 is reported. It is shown by diffraction contrast experiments that the stacking-fault defects are extrinsic. They are thought to be rafts of tellurium substituting for arsenic in {111} planes. The prevalence of the observed layer defects correlates well with the increase in carrier concentration in certain regions of the crystals.

Mechanical anisotropy of oriented polymers

Abstract: Measurements of tensile and shear yield stress have been made on strips cut at various angles to the draw direction from films of drawn amorphous poly(ethylene terephthalate). The data were well fitted by a criterion of the von Mises type but with modifications to allow for the anisotropy of the samples and also for a built-in compressive stress in the draw direction. In the tensile experiments a sharp neck is usually formed at an angle to the tensile direction and this angle is predicted with good accuracy by an application of the theory of plastic potential.

Surface-diffusion studies on UO2 and MgO

Abstract: An optical interferometric technique has been used to study the growth of grain boundary grooves and the decay of surface scratches on UO2 and MgO at temperatures in the range 1100 to 1700° C. The results were interpreted using equations derived by W. W. Mullins and it was found that surface-diffusion was the predominant material transport process for both oxides under the experimental conditions used. Surface-diffusion coefficients and activation energies were calculated, and gave the following equations for the variation of the mass transfer surface-diffusion coefficientDs with temperature. $$\begin{gathered} UO_{2.005, } {\text{ }}D_s = 1.3 x 10^8 exp^{ - 11000 \pm 15000} /RT[1200{\text{ to }}1{\text{400}}^\circ {\text{ C]}} \hfill \\ MgO, D_s = 8 x 10^4 exp^{ - 88500 \pm 15000} /RT[1200{\text{ to }}15{\text{00}}^\circ {\text{ C]}} \hfill \\ \end{gathered}$$

The fatigue of fibre-reinforced aluminium

Abstract: In order to make use of the available strength of strong fibres, the metal matrix of a composite will have to undergo plastic deformation The effect that this will have on the fatigue behaviour and damping capacity is discussed, with reference to aluminium reinforced with silica fibres and with stainless steel wires

The powder metallurgy of n-type Bi2Te2.55Se0.45 thermoelectric material

Abstract: The effects of fabrication by powder processes on the density and thermoelectric properties of an n-type pseudo-binary Bi2Te2.55Se0.45 alloy have been examined. The large effects on the thermoelectric properties are ascribed primarily to the generation and removal of point defects, together with the doping effects of atmospheric contamination in finer powders (−50 mesh).

Electron microscope examination of the surface topography of ion-bombarded copper

Abstract: Transmission electron microscopy of thin, polycrystalline copper foils has revealed the formation of a sputtering structure after bombardment at normal incidence with Xe+, Cu+, or Pb+ ions of 50 to 80 keV energy. In general, the surface of each individual grain degenerates into a “hill and valley” structure, so as to develop those {100} faces lying nearest to the plane of the surface. The sputtering structure has been found to be independent of any prior electropolishing surface structure. Similarity between the spacing and crystallographic direction of the sputtering structure, and the general appearance of the dislocation arrays produced by ion bombardment, suggests that these dislocations could influence the initiation of the structure. A tentative model based on this hypothesis is proposed to account for the periodicity of the observed sputtering effect.

Thermally activated deformation in dilute zirconium--oxygen alloys.

Abstract: The effect of temperature and strain-rate on the yield and flow stress of zirconium, containing oxygen in the range of 0.09 to about 2.5 at. % has been examined at temperatures between 77 and 700° K. The above measurements were coupled with strain rate change tests and stress-relaxation experiments, so that all the deformation parameters associated with thermal activation could be evaluated. The results of these measurements showed that the thermal component of flow stress increases with increasing oxygen content of the alloy. Two stages of activation were found to occur for all these alloys. The first stage was associated with the unpinning of dislocation loops from impurity pinning points. At higher temperatures, because of the unpinning of many impurity pinning points, a second stage of activation associated with a different mechanism was found to occur. The variation of certain parameters associated with this stage of deformation strongly suggests the Peierls-Nabarro force as the controlling criterion.

Mixed oxides prepared with an induction plasma torch

Abstract: Pure and mixed aluminium and chromium oxides have been prepared by passage of the halides through a radio-frequency oxidising plasma. The nature of the materials produced has been assessed by electron microscopy, electron diffraction, X-ray diffraction, magnetic susceptibility, and electron spin resonance. The products ware finely divided solids, containing mostly spherical particles in the size ranga 0.01 to 0.15μm; however, the chromia particles were often hexagonal platelets up to 0.4μm in diameter. The X-ray diffraction patterns were well defined, showingδ-alumina andα-chromia as major phases, andθ-alumina as a minor phase. The maximum degree of solid solution of chromium ions in theδ-alumina was limited to about 6% of the total cation content; solid solution of aluminium ions in theα-chromia phase was below the detection limit of 10%. The nature of the particles, the phases produced, and the degree of solid solution are shown to be consistent with a sequence of condensation which can be predicted from thermodynamic data.

Cathodoluminescent studies of laser quality GaAs

Abstract: Using an ion-pumped scanning electron microscope, cathodoluminescent (CL) studies of striations, dislocation-induced defects, and the doping behaviour of Te, Se, and Si in GaAs, have been carried out. The striations in heavily Te-doped Czochralski material were different from those previously reported in that they possessed a very regular spacing which agreed well with the growth/revolution of the ingot. They were studied as a function of temperature, excitation and position on the crystal face. It is thought that these bright striations in highly-doped material correspond to regions of decreased Te concentration in agreement with earlier work. However, infra-red studies of striations at lower concentrations have shown that the bright striations correspond to an increase in Te concentration. These results are consistent with earlier work relating CL efficiency to doping level. Studies of the dislocation-induced defects and their reactions with the striations in Te-doped material are all consistent with the formation of a Cottrell atmosphere at the dislocations at the expense of the immediate surrounding volume. Comparisons between Te-, Se-, and Si-doped material prepared by three different methods have revealed differences, especially in the case of Si where direct observation of its amphoteric nature were recorded.

The phase diagram of the system tellurium/arsenic

Abstract: Thermal analysis, metallographic and X-ray procedures have been used to investigate the system tellurium/arsenic. As2Te3 is the only binary compound present. It melts at 381° C and forms a eutectic with tellurium at 18.5 wt % As/81.5 wt % Te and at a temperature of 363° C. The As2Te3 and As also produce a eutectic at 31.5 wt % As/68.5 wt % Te, with a melting point of 380°C. The solid solubilities at the As and Te ends of ths phase diagram are too small to detect by the methods used, and the compositional range of the As2Te3 is very restricted.