Nucleation and growth of β′magnesium sialons and their composition limits

Previous reports on the solubility of magnesium in -sialons have been conflicting. The present work shows conclusively that crystalline magnesium sialons with the silicon nitride structure do exist. They are formed by crystallization of magnesium sialon glasses at low temperatures. -magnesium sialon crystals nucleate on -sialon nuclei which are themselves formed by precipitation from Mg-Si-Al-O-N liquids at high temperatures. The current results suggest that -magnesium sialons exist only over a limited composition range within the 3M/4X plane of the Mg-Si-Al-O-N system, which is indicative of some form of ordering within the structure. Although the compositions investigated in the present study are unstable with respect to forsterite above about 1000° C, the possibility of producing -magnesium sialons which are stable at much higher temperatures cannot be fully discounted at present.     

Microstructure and mechanical properties of rapidly quenched L20 and L20+L12 alloys in Ni-Al-Fe and Ni-Al-Co systems

Ductile L2₀-type wires and+L1₂-type duplex wires with high strengths and large elongation in the Ni-Al-Fe and Ni-Al-Co ternary systems have been manufactured directly from the liquid state by an in-rotating-water spinning method. The wire diameter was in the range 80 to 180m and the average grain size was 2 to 4m for the wires and 0.2 to 1.0m for the+ wires. _y, _f and _p of the wires were found to be about 360 to 760 MPa, 560 to 960 MPa, and 0.2 to 5.5%, respectively, for the Ni-Al-Fe system, those of the+ wires were about 395 to 660 MPa, 670 to 1285 MPa, and 3.5 to 17%, respectively, for the Ni-Al-Fe system, and about 260 to 365 MPa, 600 to 870 MPa, and 4.0 to 7.0%, respectively, for the Ni-Al-Co system. Cold-drawing caused a significant increase in _y and _f and the values attained were about 1850 and 2500 MPa, respectively, for Ni-20Al-30Fe and Ni-25Al-30Co wires drawn to about 90% reduction in area. The high strengths, large elongation and good cold-workability of the melt-quenched and+ compound wires have been inferred to be due to the structural change into a low-degree ordered state containing a high density of phase boundaries, suppression of grain-boundary segregation and refinement of grain size.     

Mapping co-word structures: A comparison of multidimensional scaling and leximappe

The LEXIMAPPE method and Multidimensional Scaling (MDS) are discussed as methods to visualize (map) characteristics of structures of word-occurrence (co-word) relations. Utilization of MDS is proposed as an alternative mapping method able to circumvent problematic features of LEXIMAPPE maps of the total co-word structure. A comparison of both methods on the same real-life co-word matrix demonstrates topological advantages of an extended MDS-mapping.     

Irreversibilites in science and technology networks: An empirical and analytical approach

The theory of autopoiesis, i.e., self-referentiality in the operation of the system, provides us with a production rule for change in the structure of the network. Using information theory, a model system is developed to study the relative likelihood of dynamic transitions: various senses of irreversibility (emergence, and path dependency) are disinguished. A test for path dependency is applied to two sets of empirical data which supposedly reflect historical discontinuities: the budget of theFraunhofer Gesellschaft, and the citation network among AIDS research related journals. The model for the interaction between self-referential developments and goal-referential boundary conditions is further specified, using the example of technological trajectories and selection environments.     

The constitution of the Ni-Al-Ru system

The constitution of the Ni-Al-Ru system has been investigated in the range 0 to 50 at% Al. Isothermal sections at 1523 and 1273 K have been determined using microstructural observations, electron probe microanalysis and X-ray diffraction. The phases present were: nickel-based solid solution (); (based on Ni₃Al); solid solutions based on NiAl and RuAl, respectively (designated ₁ and ₂), and ruthenium-based solid solution (Ru). The maximum solubility of Ru in was 5 at%. ₁, and ₂ show extensive range of solubilities, namely up to 20at% Ru in ₁ and up to 25 to 35 at% Ni in ₂. Three-phase equilibrium between , ₂ and (Ru) existed at 1523 and 1273 K. Also at 1523 K, three-phase equilibria existed between , and ₁ and ,₁ and ₂, while at 1273 K, the equilibria were between , ₁,₂ and , , ₂ indicating the occurrence of a reaction +₁, +₂ at a temperature between 1523 and 1273 K. Liquidus features have been deduced from data on as-solidified structures. Lattice parameter data and hardnesses are also reported.     

Microstructural evolution in rapidly solidified Al-Cu-Si ternary alloys

Several Al-Cu-Si alloys were melt spun to produce stable, fine scale microstructures suitable for superplastic deformation and consolidation. Scanning electron microscopy of the ribbon cross-sections reveal two distinct alternating microstructural morphologies, suggesting transitions in solidification behavior. One structure consists of intimately interlocked -Al and (Al₂Cu) phases with dispersed spheroids of (Si). The other structure consists of equiaxed or cellular-dendritic -Al with interdendritic and (Si). The latter was found in the middle portion of the ribbon cross-section when cast at a low speed, and throughout the ribbon cross-section when cast at high speed. The dendritic structure appears to result from independent nucleation events in the undercooled liquid ahead of the solid-liquid interface. The solidification mechanism for the interlocked structure appears to involve multiple nucleation of the phase followed by its cooperative growth with the -Al phase. This cooperative growth is unlike that which forms a lamellar structure, as it results in a branched, randomly oriented network. We postulate that the (Si) phase is the first phase to form from the undercooled liquid, and it is uniformly dispersed throughout the undercooled melt. The (Si) spheroids provide nucleation sites for the phase because of its observed association with the phase. The -Al grain size varies from 1 m near the wheel side surface of the ribbon to 8 m with sub-grains near the free surface. The size of the and (Si) phases is on the order of a m and less. The microstructural size scale appears to be small enough for this material to exhibit superplastic behavior when deformed.     

Temperature and randomness dependence of the pair-breaking parameter in superconducting aluminum films

Magnetoconductance and excess conductance due to superconducting fluctuations in aluminum films are measured in order to study the temperature dependence of the pair-breaking parameter at temperatures nearT _c . The parameter _M is estimated from the relation =/8k _B T_in, where _in is the inelastic scattering time deduced from the analysis of the magnetoconductance. The parameter _F is determined by fitting theories to data on the excess conductance at zero magnetic field. It is shown that: (1) For films with a wide range of the sheet resistanceR , 12R 200 /, the temperature dependence of _M nearT _c agrees well with the theory of Brenig et al. (2) For clean films withR 100 /, the value of _F analyzed with theories including the correction term to the Maki-Thompson contribution shows almost the same temperature dependence as _M . In a film withR 200 /, however, a discrepancy between _M and _F remains.On leave from College of General Education, Kyushu University, Ropponmatsu, Fukuoka, Japan.     

Growth of Gray Tin Crystals

A method is proposed for growing large -Sn single crystals without their disintegration as a result of the phase transition. -Sn crystals are prepared by freezing the closed system water–Sn–seed in such a way that the pressure exerted by the expanding ice minimizes the amount of -Sn nuclei and reduces the growth rate.     

Eutectoid decomposition mechanisms in hypoeutectoid Ti-X alloys

A TEM study has been made of the bainite reaction in five hypoeutectoid Ti-X alloys, where X was successively cobalt, chromium, copper, iron and nickel. Rational orientation relationships were demonstrated amongst eutectoid , eutectoid intermetallic compound and the matrix in Ti-Ni, Ti-Co and Ti-Cr. Formation of Ti₂Co at : boundaries was observed. Eutectoid in bainite was found to be slightly misoriented with respect to proeutectoid , indicating that it is separately nucleated, perhaps sympathetically, rather than the result of the continued growth of proeutectoid . Eutectoid Ti₂Co and Ti₂Cu crystals in bainite were approximately equiaxed whereas Ti-Cr₂ crystals were elongated, a result ascribed to a ledge height-to-spacing ratio / at intermetallic compound crystal: boundaries approaching that of eutectoid (: boundaries in Ti-Cr but not in the other two systems. In the Ti-Fe alloy, eutectoid and eutectoid TiFe were directly observed to have ledged interphase boundaries with their matrix, but with different inter-ledge spacings and growth directions. Observation of pearlite lamellae growing normal to the broad faces of proeutectoid plates in the Ti-Ni alloy indicates that this mode of eutectoid decomposition, like that of bainite, can develop from partially coherent interphase boundaries. The suggestion was offered that pearlite forms when approachesh at the nucleating proeutectoid : interface and that bainite develops when h at this interface.     

Microstructural changes during ageing of Cu-2.5 wt% Ti

Microstructural changes in Cu-2.5wt % Ti during ageing at 573 and 773 K have been studied by transmission electron microscopy. Ageing times ranged from 60 sec to 200 h. Ordering of the precipitates was observed after very short ageing treatments at 773 K, and coarsening according tot ^1/3 was also observed from very early times. The particles were observed to become increasingly aligned into rod-like groups along 10 0 as ageing progressed. These results permit a new interpretation of the strengthening mechanisms in this alloy.     

Computational thermography: Numerical modeling of the thermal regimes of building structures

We consider numerical methods of simulating thermal regimes of building structures that make it possible to create optimum structures as regards power consumption by using more accurate calculations than those available in existing construction specifications and regulations. Possible means of reducing energy expenditures for formation of an optimum microclimate in living quarters are described.Notation R thermal resistance to heat transfer - _i thermal conductivity - c _i heat capacity - _i moisture content - i number of a layer - S thermal inertia of the material - density of the substance - frequency of harmonic vibrations - t time - Fo Fourier number - thermal diffusivity - t time step - x spatial step - Bi Biot number - h _c coefficient of convective heat transfer - k thermal conductivity - T ambient temperature - T _w wall temperature - Nu Nusselt number - Ra _l Rayleigh number - Gr _l Grashof number - Pr Prandtl number - g free fall acceleration - coefficient of thermal volumetric expansion of air - l characteristic length - coefficient of kinematic viscosity of air - determining temperature - thermal conductivity of the material - q heat flux - s area of the heat transfer surface - perimeter of the heat transfer surface - T free stream velocity - air viscosity Academic Scientific Complex A. V. Luikov Heat and Mass Transfer Institute of the Academy of Sciences of Belarus, Minsk. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 66 No. 6, pp. 733–738, June, 1994.     

Thermodynamic investigations of liquid Bi-Na and Sn-Na alloys by coulometric titration using β″-alumina

The following double galvanic cell was assembled and the thermodynamic properties of liquid Bi-Na and Sn-Na alloys, and the ion selectivity of -alumina during coulometric titration, were investigated. Mo, Na(I)¦-alumina¦M-Na(I), Mo [I] M-Na(I)¦-alumina¦Au + Au₂Na, Mo [II] (M = Bi or Sn) where M-Na(1) and Au + Au₂Na were used as the common electrode and reference electrode, respectively. Sodium was coulometrically titrated through the -alumina electrolyte of cell I both ways, and the EMFs were measured. It was found that no ion-exchange reaction occurs between the liquid alloys and the -alumina, and only Na was transferred in the -alumina during coulometric titrations. The thermodynamic properties of liquid Sn-Na and Bi-Na alloys were found to be in agreement with the literature.     

Structural Changes in the Surface Layers of PdHx Alloys Caused by the Hydride Phase Decomposition

By the method of optical microscopy, we perform the in-situ investigation of the plane polished surfaces of PdH _x alloys (with video recording of the processes of transformation) in the case of their rapid cooling at a rate of 11–20°C·sec^–1 from T _c = 292°C ( = 1.97MPa) with simultaneous lowering of the pressure of hydrogen within the temperature range 100–160°C. In the course of decomposition of the PdH_0.24 alloy according to a scheme ₀ + , the formation of nuclei of the - and -phases and their growth were not detected. However, we observe a simultaneous coherent transformation of the entire surface accompanied by the formation of a surface topography similar to the modulated structures formed as a result of the spinodal decomposition. We discuss the possibility of realization of spinodal decomposition under the described experimental conditions.     

Tensile strength of hybrid composites

This paper presents an approximate, statistical analysis for the tensile strength of unidirectional hybrid composite materials consisting of two-dimensional arrays of alternating low elongation and high elongation fibres in a common matrix. Expressions for ineffective length and fibre strain concentration factor in hybrid composites are developed. The analysis identifies a number of important material properties that affect the failure process in hybrids: statistical fibre tensile failure strain characteristics, and fibre extensional moduli and cross-sectional areas. The influence of these parameters on the failure process is examined and differences between failure mechanisms in hybrids and composites containing only one type of fibre are considered. The analysis predicts that, in general, the failure strain of a hybrid should be different from those of composites reinforced with either of the parent fibres alone. It is found that the theory can explain the hybrid effect that has been observed by several authors: hybrids made by combining high elongation and low elongation fibres, such as graphite and glass, often display tensile failure strains greater than those of composites made from the low elongation fibres alone. Predicted failure strains are compared with available experimental data. Suggestions for future work in the area are presented.     

Phase transformation and densification during pressureless sintering of Si3N4 with MgO and Y3Al5O12 additives

The - transformation of Si₃N₄ during liquid-phase sintering appears to be controlled by the growth of the -Si₃N₄ grains in the direction perpendicular to thec-axis in the case of MgO additive. The diffusion through the liquid is the rate-controlling step in the case of the Y₄Al₅O₁₂ additive. The density of the sintered body at the solid skeleton stage was influenced by the change in the - transformation rate and/or by a change of the transformation mechanism. The indirect proportionality between the -phase content in the starting powder and the density at the solid skeleton stage was found. The microstructure of the sintered body is influenced by both the -phase content in the starting powder and the chemical composition of the additive. Fine, uniform microstructure with a high aspect ratio of -grains is obtained, when the -phase content in the starting powder is as small as possible and when the - transformation is controlled by grain growth.     

Development of solidification microstructures in a fibre reinforced alloy

The solidification behaviour of a fibre reinforced Al-6 wt% Cu alloy, containing 30 vol% of 3 m diameter, semi-continuous, aligned alumina fibres has been studied. Results are presented to show the influence of fibres on the microstructural development of and microsegregation in the matrix during freezing. The effect of total solidification time, _t, on solidification behaviour was examined for 1<_t<520 S. By using interrupted solidification experiments microstructural development was studied in detail. It was found that -Al begins to grow within interfibre regions, and grows towards the Al₂O₃ fibres, avoiding them where possible. Consequently fibres are located in the last regions to solidify. When _t>10 s the final microstructure is non-dendritic, and CuAl₂ is located predominantly at the fibre-matrix interface. When _t 1 s it was observed that the final microstructure is dendritic with a periodic segregation pattern, and the CuAl₂ is more dispersed. The matrix composition becomes more uniform, and the minimum matrix composition rises as _t increases. The growth and microsegregation are analysed and discussed using simple semi-analytical models. The implications are that fibres significantly influence solidification behaviour if _f/_s<1, where _f is the average interfibre spacing and _s the secondary dendrite arm spacing which would develop in the unreinforced alloy.     

Effect of martensitic transformation in Ti–15 at %V β-phase particles on lamellar boundary decohesion in γ-TiAl Part II Finite element analysis of crack-bridging phenomenon

The commercial finite element package ABAQUS has been used to analyse the crack bridging process by Ti-15 at%V -phase particles dispersed in -TiAl matrix in the presence of particle–matrix decohesion. Both the particle–matrix decohesion potential and the -phase materials constitutive relations are found to have a major effect on the ductility, fracture toughness and failure mode of the – two-phase material. The interface potential is found to primarily affect the distribution of the normal interface strength ahead of the advancing interfacial crack and the mode (gradual versus sudden) of decohesion. The -phase materials constitutive relations are found to influence the location of nucleation of the interfacial cracks and, in turn, the mode of decohesion. A metastable -phase that can plastically deform at low stress levels by undergoing a stress-assisted martensitic transformation, but experience a high rate of strain hardening is found to give rise to the largest levels of ductility and fracture toughness is the – two-phase material. © 1998 Kluwer Academic Publishers     

The morphology of α′ martensite in a two-phase (α+γ) Fe-Cr-Ni stainless steel

Both lath-shaped and martensites are induced by tensile deformation within of a two-phase ( + ) Fe-Cr-Ni stainless steel, forms from the through the at an intersection of two crystals. These are observed both when is surrounded by and when borders . The amount of strain at which both and nucleate, increases with test temperatures in the range –196 to 50° C. Adjacent laths are either twin-related, or 5, 9, 15 or 19° off the twin relationship, as found by analysing electron diffraction patterns.     

CR-39 as a gamma dosimeter: Dielectric and infrared studies

The relative permittivity, , and dielectric loss factor, tan , were measured in the frequency range 50–10⁵ Hz for the solid state track detector CR-39 irradiated with -rays (5–100 kGy). The variation of and tan with -dose have been interpreted in terms of structural changes as emphasized from infrared spectra. A mathematical model has been introduced describing the use of CR-39 as a dielectric dosimeter.     

Carbon fibre adhesion to organic matrices

It has been possible to measure carbon fibre adhesion to organic resins on single fibres by the pulling-out method. Tests were concerned with fibres as-received and following surface treatment. In spite of its limitations, this method provided values in good agreement with the computations of various authors. It can be useful for determining the fibre/ interface strength and testing the efficiency of surface treatments.