Showing papers in "Journal of Materials Science in 1970"

The “multiple-sample concept” in materials research: Synthesis, compositional analysis and testing of entire multicomponent systems

Abstract: This paper questions the efficiency of the present approach to the search for new materials which consists of studying only one composition at a time. A new concept of materials research is introduced which consists of synthesising, analysing, testing and evaluating of large parts of multicomponent systems in single steps. Implementation of this concept has been possible through the development of a one-cathode, multiple-target, radio-frequency co-sputtering method of synthesis and of a novel, simple method of compositional analysis of any co-sputtered film, based on film thickness measurements. An example is given of the use of such an approach in studying new binary super-conducting compositions, which resulted in a thirty-fold increase in the rate of finding new materials. A several hundred-fold increase in research productivity is predicted for higher-component systems.

Thin foils of non-metals made for electron microscopy by sputter-etching

Abstract: Established techniques of making thin foils of non-metallic materials are reviewed briefly and their inadequacies outlined. The preparation of foils by sputter-etching is described and the effects on the sample of ion current density, and angle of ion incidence are discussed. Results on specimens of ceramics, glasses and minerals are presented. From these it is concluded that sputter-etching is a valuable and nearuniversal thinning technique for non-metals.

The effect of hydrostatic pressure on the shear yield behaviour of polymers

Abstract: The torsional stress-strain behaviour of isotropic poly(methylmethacrylate) (P M M A), poly(ethylene terephthalate) (P E T) and polyethylene has been studied under hydrostatic pressures up to 7 kbar. In P M M A the following important features were observed. First, there is a monotonic increase in the initial slope of the stress-strain curve with increasing pressure. Secondly, there is a substantial increase in the yield stress and the strain to yield as pressure is raised. Thirdly, there is a transition in the mode of failure at elevated pressure, the specimens fracturing in the high pressure region before a drop in stress occurs. Finally, in the high pressure region the fracture stress increases with increasing pressure but the strain at fracture decreases.

The strain-rate, temperature and pressure dependence of yield of isotropic poly(methylmethacrylate) and poly(ethylene terephthalate)

Abstract: The yield behaviour of poly(methylmethacrylate) (PMMA) has been investigated in tension and compression over a range of testing temperatures and strain-rates. Both tensile and compressive yield stresses were found to increase monotonically with increasing strainrate and decreasing temperatures. Compressive yield stresses were in general found to be more dependent on strain-rate. The results of this investigation have been correlated with previous published data for the dependence of the torsional yield stress of PMMA on hydrostatic pressure. This was done by a modification of a theory proposed by Robertson which uses the internal viscosity approach to yield in glassy polymers. The modified theory clearly explains the temperature and strain-rate dependence of the yield stress and provides a quantitative explanation of the differences in behaviour between tension and compression in terms of the dependence of yield on the hydrostatic component of the applied stress. The tensile yield behaviour of isotropic amorphous poly(ethylene terephthalate) (PET) sheets has also been investigated over a wide range of temperatures and strain-rates. No torsion or compressive yield stresses are available because of the sheet form of the PET, but the results obtained in tension are shown to be fully consistent with the above theory, and with other published work.

The wetting of carbon by copper and copper alloys

Abstract: The effects of alloying additions on the wetting of two types of carbon, HX30 graphite and vitreous carbon, have been investigated, with a view to developing a system for metal impregnation of carbon fibre assemblies. Pure copper is inert and non-wetting but two additions caused the copper to wet: Cr on both substrates and V on the vitreous carbon only. Many of the additions formed a carbide reaction layer at the interface, and in the two wetting systems this was a thin, continuous, coherent layer. The data have been analysed in terms of the chemical and physical properties of the system and it was found that the wetting behaviour of copper alloys on HX30 graphite and vitreous carbon could be explained in terms of the behaviour of the reaction product carbides of the alloying addition when these existed.

Superplasticity: A review

Abstract: Superplastic behaviour in torsion was first reported by Saveur [2] Pearson [3] first presented detailed observations of tensile superplasticity.

Hypothetical mechanism of crazing in glassy plastics

Abstract: Crazing in glassy plastics is attributed to a stress-activated devitrification of a small amount of material at the tip of a chance nick or flaw, to a softer rubbery state. Subsequent cavitation of the softened material is then assumed to take place under the action of the same dilatant stress responsible for its formation. A transition to ductile yielding is proposed to occur when the material in the tip region undergoes large deformations before softening. The proposed mechanism of crazing is shown to provide quantitative predictions for the magnitude of tensile stress at which crazing occurs, the increase in crazing stress with hydrostatic pressure, the transition at high pressures to a yielding process without crazing, the reduction in crazing stress in the presence of certain liquids and vapours and, to some extent, for the effects of temperature and pre-orientation. These theoretical predictions are found to be in reasonably satisfactory agreement with experiment. In view of the limited number of adjustable parameters in the theory (the principal one being the stress-magnification factor associated with a typical nick or flaw), this general agreement over a wide range of experimental conditions and variables suggests that the proposed mechanism of stress-crazing is basically correct.

The cold-drawing of high density polyethylene

Abstract: The cold-drawing of high density polyethylene has been studied over a wide range of temperatures and strain rates for a series of samples of very different molecular weight distribution. A natural draw-ratio was observed which was almost independent of temperature and strain rate, but was sensitive to the weight average molecular weight. A good correlation was observed between the Young's modulus of the drawn mono-filaments and the natural draw-ratio, and it is proposed that the Young's modulus is primarily related to molecular orientation rather than to interlamellar ties.

The strength and oxidation of reaction-sintered silicon nitride

Abstract: The structure of reaction-sintered silicon nitride is studied using scanning electron and optical microscopy at various stages during nitriding, for a range of nitriding and compacting conditions. The strength is then evaluated and interpreted in terms of the microstructure. It is found that fracture always occurs in a brittle manner by the extension of the largest pores. The effects of prolonged annealing in air above 1000† C on both the structure and strength are investigated. At 1400† C, cristobalite is formed. If the temperature is then maintained above 250† C, the strength is enhanced, but below this temperature the oxide layer cracks and reduces the strength.

Experimental work on the niobium-tin constitution diagram and related studies

Abstract: Three intermetallic phases Nb3Sn, Nb6Sn5 and NbSn2 are formed in the niobium-tin system. Nb6Sn5 and NbSn2 appear to be stoichiometric with narrow homogeneity range, but Nb3Sn can exist over a wide composition range from about 73 to 83 at. % niobium although the niobium-rich compositions are not formed on annealing below 1400° C. Nb6Sn5 and NbSn2 form peritectically at 930 and 845° C respectively. The three phases appear to be stable to low temperatures, but Nb3Sn and Nb6Sn5 are slow to form below about 800° C. The solubility of niobium in liquid tin is small at temperatures below 1000° C and the solid solubility of tin in niobium decreases from about 9 at. % tin at the peritectic temperature of Nb3Sn to about 1 at. % tin at 1495° C and is negligible below 1000° C. An equilibrium diagram is constructed from the present data and from other published information.

Theoretical estimation of fracture toughness of fibrous composites

Abstract: A method of estimating the fracture surface energy of fibre-reinforced materials is discussed. The surface work is shown to increase with increasing fibre content, strength and diameter, and decrease with increasing fibre modulus and matrix flow stress (or hardness). Relatively short fibres should be used if high toughness is required, and the maximum toughness that can be achieved is limited by the amount of crack opening that can be permitted. Under certain conditions, incorporation of fibres into a material can lead to embrittlement.

Kinetic study on the hexacelsian-celsian phase transformation

Abstract: The hexacelsian-celsian transformation from grains of 1/4 in. (0.635 cm) size is slow and erratic. This is because the rate of heterogeneous nucleation in such grains is low and is influenced by contamination present in the furnace. When the grain size is reduced to a −200 mesh powder, heterogeneous nucleation becomes a dominant factor and the transformation is accelerated. The transformation has three stages: the first is controlled by the rate of crystal growth, the second is controlled by the rates of nucleation and crystal growth, and the third is controlled by the rate of nucleation. The activation energy of crystal growth in the first stage of transformation is 20.1 kcal mole−1 ±20%. This suggests that the hexacelsian-celsian transformation does not involve Si-0 and Al-O bond openings, since this would be expected to involve an activation energy of at least 60 kcal mole−1.

The activation areas for creep deformation

Abstract: The activation areas for creep deformation are collected and examined in the light of many material and deformation variables. The activation area is A *= (kT/b) (∂ In $$\dot \in $$ /∂τ*) T where k is Boltzmann's constant, T the absolute temperature, b the Burgers vector, $$\dot \in $$ the steady state creep rate, and τ * the effective shear stress. It is found that within a factor of 5, there is a general correlation between activation area and stress for all metals, alloys, semiconductors and ionic crystals. A jog-limited dislocation motion with a distribution of jog spacings is suggested as a possible mechanism for this behaviour. Some limitations for the jog mechanism are discussed.

Steady state creep of a composite material with short fibres

Abstract: The shear within a matrix volume is assumed to be an important process during the creep of composite material reinforced with short rigid fibres. The rate of elongation of such a composite with certain fibre distributions can be estimated. The agreement with a few experimental data is reasonably good.

Oriented eutectic microstructures in the system Al2O3/ZrO2

Abstract: Oriented eutectic microstructures have been produced in the system Al2O3/ZrO2 using a Bridgman-type crystal-growing furnace. Ingots consisted of elongated columnar grains or colonies. Inside the colonies a rod-type eutectic microstructure consisting of rods of ZrO2 surrounded by an Al2O3 matrix was observed. The eutectic point was re-established at 63 mol % Al2O3/37.0 mol % ZrO2 and 1870±5° C. Al2O3 is the first phase to nucleate when eutectic growth occurs.

The fracture toughness of composites reinforced with weakened fibres

Abstract: Fibre fractures which occur near, but not at, the plane of matrix failure in a composite, lead to fibre pull-out during fracture. Energy absorbed in this process contributes directly to the work of fracture and hence to the toughness of the composite. Factors which determine the mean length of fibre pulled out during fracture are discussed for the case of composites reinforced with continuous fibres having variously spaced points of weakness. The presence of such weak points also affects the strength of the composite, but not all composites of the same strength have the same toughness. The greatest toughness for a given strength is always found in composites reinforced with discontinuous fibres.

Czochralski-type crystal growth in transverse magnetic fields

Abstract: Te-doped inSb single crystal growth in transverse magnetic fields, using Czochralski crystal puller

Fatigue crack propagation in polymeric materials

Abstract: In order to gain a better understanding of matrix-controlled fatigue failure processes in non-metallic materials a series of fatigue tests were performed on several different polymer materials representing different classes of mechanical response Fatigue crack propagation rates between 5×10−6 in cycle−1 (127 nm cycle−1) and 4×10−4 in cycle−1 (10 300 nm cycle−1) were measured in nylon, polycarbonate, ABS resin, low-density polyethylene and polymethyl methacrylate A strong correlation was found between the fatigue crack propagation rate and the stress intensity factor range prevailing at the advancing crack tip Whereas metals exhibit comparable fatigue growth rates for a given stress intensity range when normalised with respect to their static elastic modulus, the polymer materials exhibited a 1300-fold difference in crack growth rate for a given normalised stress intensity range This observation dramatically illustrates the importance of understanding molecular motion and energy dissipation processes in polymer materials as related to their chemistry and architecture The relative behaviour of the different polymer materials could be generally correlated with their reported damping characteristics

Effect of crazes on the propagation of cracks in polystyrene

Abstract: A study of the fracture surfaces of polystyrene has revealed a direct connection between the fracture path and the shape and distribution of crazes. The effect is demonstrated by two examples; firstly, the propagation of a crack through initially uncrazed material and, secondly, the propagation of a crack through material containing an array of planar crazes parallel to the plane of propagation. It is shown that cracks propagate preferentially along crazes and that the formation of crazes in the stress field of a propagating crack results in the “hackle” surface normally associated with the propagation of cracks at high velocities.

Oxide crystal growth using gas lasers

Abstract: The use of CO2-N2-He gas lasers as a heat source in a floating-zone recrystallisation technique of crystal growth is described. So far, single crystals of Y2O3, CaZrO3, MgAl2O4 and Al2O3 have been grown in this manner and factors controlling their quality are discussed. The problems associated with this technique of crystal growth are reviewed.

Structure and properties of tantalum carbide crystals

Abstract: Single crystals of tantalum carbide, up to 2 mm in size have been grown from solution in a bath of molten iron. The slip plane was found to be {111} using a two-surface analysis on etch-pitted crystals deformed by microindentation at room temperature. Observations of etch-pit patterns around inclusions suggest that slip occurs on other planes at elevated temperatures. Maximum microhardness values between 3800 and 5200 Knoop (100 gm load) were found at a composition TaC0.83±0.01. In regions of crystals with a carbon content less than TaC0.83 a phase transformation was seen close to microhardness indentations in samples decarburised below 2200† C. The mechanical behaviour of tantalum carbide is discussed with reference to a general model for the electronic structure of carbides.

The effect of fibre length and interfacial bond in glass fibre-epoxy resin composites

Abstract: The effect of fibre length on the strength of glass fibre-epoxy resin composites has been examined by beam bending experiments on uniaxially aligned material. The results agree well with theoretical predictions and the critical fibre length is found to be 12.7 mm (0.5 in.). A method of measuring the interfacial shear strength of the fibre-matrix interface has been developed and the measured value of the interfacial shear strength found to be 9.5 N mm−2. The mechanism of shear failure is examined and discussed in detail.

Solidification of aluminium-germanium alloys at high cooling rates

Abstract: Aluminium-germanium alloys, covering the entire composition range, were subjected to rapid solidification by different techniques ranging from the gun technique of splat cooling to simple quenching of the melt in water. The constitution, microstructure and crystal structure of the alloys thus solidified under cooling rates varying from 103 to 107° C/sec were studied by standard optical, electron- and X-ray metallographic techniques. The solid solubility of Ge in Al could be increased from the equilibrium value of 2.8 at. % to a maximum of 7.2 at. %. In addition, two new non-equilibrium tetragonal phasesγ 1(a=12.91 to 13.11 A,c=12.00 to 12.10 A) andγ 2(a=14.98 A,c= 16.03 A) could be obtained in alloys containing 15 to 40 and 40 to 80 at. % Ge respectively. In relation to the face-centred cubicα solid solution, these appear to be made up of 30 and 52 unit cells, respectively, with the relations $$a_{\gamma _1 } \simeq \sqrt {10a_\alpha } $$ , $$C_{\gamma _1 } \simeq 3a_\alpha $$ and $$a_{\gamma _2 } \simeq \sqrt {13a_\alpha } $$ , $$C_{\gamma _2 } \simeq 4a_\alpha $$ . A new non-equilibrium constitution diagram connecting cooling rate with constitution at room temperature has been proposed and seems to explain all observations satisfactorily.

Preparation and characterisation of high surface area semiconducting oxides

Abstract: The conventional method of preparing doped semiconducting oxides involves solid state sintering at elevated temperatures and the resultant products have a very low surface area (< 1 m2 g−1). This severely limits the activity of semiconducting oxide catalysts, such as lithiated NiO. A cryochemical method based on the freeze drying of mixed salt solutions, followed by low temperature calcining has yielded lithiated NiO of over 60 m2 g−1. DTA, conductivity measurements and chemical analyses confirmed that under these conditions, very uniform mixing between Li2O and NiO is achieved and that Li2O can diffuse into the NiO lattice at about 400° C, as compared to 950 to 1000° C for products prepared by conventional means.

Order-induced strengthening in Ni4Mo

Abstract: Polycrystalline samples of Ni4Mo have been examined at various stages of isothermal ageing at 700° C. The mechanical properties were measured on initially disordered samples at various stages of the ordering process. These properties were correlated with microstructural observations such as domain size, degree of order and fracture characteristics. The correlations are rationalised in terms of a model for the ordering process and its effect on dislocation motion.

Crystallisation of lithium zinc silicates

Abstract: Phase relations on the Li4SiO4-Zn2SiO4 join of the Li2O-ZnO-SiO2 system have been studied. Extensive ranges of solid solutions form, and many of these have structures which are related to those of Li3PO4; the mechanism of lithium ⇌ zinc replacement is discussed. Ten binary phases have been found; seven probably occur at equilibrium. In addition, the high-temperature polymorphism of Li4SiO4 has been studied. Physical data are presented to characterise the thermodynamically stable phases, and their stability relations depicted on a temperature-composition equilibrium diagram.

Fluorine self-diffusion in CaF2 and BaF2

Abstract: The self-diffusion of F-18 in single crystals of BaF2 and CaF2 was measured between 350 and 940°C. The results were in agreement with diffusion coefficients deduced from ionic conductivity studies, and yieldedΔH's of 1,6 eV for BaF2 and of 2.0 eV for CaF2 for intrinsic diffusion. Below 700°C, extrinsic diffusion behaviour was observed in CaF2 withΔH=0.9 eV. The formation energy for anion Frenkel defects was estimated to be in the range 2.2 to 3.1 eV. Both anion and cation diffusion rates were increased in CaF2 doped with SmF3 as compared with pure CaF2, indicating an interrelation between anion and cation defects, as observed previously in the related substances, UO2 and UO2+x , The migration energy of cations was estimated to be roughly in the range 1.5±0.6 eV. The implications of the results in terms of diffusion mechanisms are discussed and the remaining uncertainties are indicated.

Transitions in creep behaviour

Abstract: Attempts to identify the mechanisms operating during creep are often made by examining plots which yield apparent activation energies, or the stress or grain size-dependences of creep-rate The forms of such plots are here examined and the ambiguities which arise near transitions from one regime to another are noted The ranges of temperature, stress and grain size commonly used are inadequate and serious errors in interpreting the results of creep tests will continue to be made until a better understanding of the interaction of the basic processes is developed, so as to enable the positions of transitions to be predicted

Thin films of semiconductors and dielectrics produced by laser evaporation

Abstract: Thin films of several III–V and II–VI compounds as well as of some dielectrics have been vacuum-deposited using a focused beam of a CO2 or ruby laser to evaporate these materials. The crystallinity, morphology and the chemical composition of the produced thin films have been examined by various analytical methods. Films produced by the ruby laser were in most cases polycrystalline and stoichiometric, while films produced by the CO2 laser were amorphous and non-stoichiometric. Different mechanisms of evaporation leading to the observed differences in characteristics of thin films are discussed.