Solid state phase transformation of BaB2O4 during the isothermal annealing process

Solid-state phase transformation of BaB₂O₄ during the isothermal annealing process for both to and to were investigated using a platinum crucible. For the -phase crystal at the -phase stable temperature (> 925 °C), the phase transforms to the phase perfectly below the melting temperature of 1100 °C. Meanwhile, for the -phase crystal at the -phase stable temperature (< 925 °C), the phase transforms to the phase perfectly above 800 °C. There is some difference in phase transformation behaviour between bulk-shape crystals and the powder, caused by thermal stress.     

The θ projection method and small creep strain interpolations in a commercial Titanium alloy

The paper describes the early and final uniaxial creep behaviour of a Titanium alloy used for manufacturing intermediate power compressor disks and blades. Tests were conducted at the operating temperature (773 K) for such components and for rupture lives up to 3600 hours. Creep curves were fitted using either the conventional 4 model or the recently developed 6 equation. Parameters allowing the interpolation of times to small strains were produced and their accuracy checked against experimental values using distributions found to be most supported by the data. At strains above 0.75% both methods yielded zero mean interpolation errors. At strains above 0.27% and below 0.75% the 4 equation produced systematic errors in interpolation but the 6 function gave errors which were not statistically different from zero. For strains below 0.27% both techniques produced systematic interpolation errors but the 6 interpolations were always significantly better than their 4 counterparts. Both the 6 and 4 techniques produced systematic errors when predicting the failure time using interpolated rupture strains. Unlike the 4 function, the 6 equation produced unbiased predictions of the minimum creep rate and so produced failure time interpolations from the Monkman–Grant relation that were indistinguishable from zero.     

Room temperature age-hardening in a β titanium alloy

A metastable titanium alloy containing 10 wt % Zr and 12 wt % V has been found to undergo a substantial age-hardening reaction at temperatures as low as 20° C. The reaction involves continued growth of athermal-phase particles produced during water quenching from the-phase field. The morphology of the as-quenched is retained, implying the absence of long-range diffusion during ageing: this is consistent with the low value of the activation energy measured (93 kJ kg mol^–1). It is suggested that the growth is caused by unpinning of/ interfaces as a result of the short-range motion of interstitials present in the alloy. The age-hardening produces a severe loss in tensile ductility and inhibition of stress-induced martensite formation.     

Microwave dielectric properties of ceramics with compositions along the La2/3TiO3(STAB)–LaAlO3 tie line

The stabilization of La2/3TiO3 in the perovskite structure by 4 mol% LaAlO3 resulted in maximum values for the relative permittivity (k72) and temperature coefficient of resonant frequency (f=123 ppm K-1). The Q-value quality factor of these ceramics were over 5300 measured at 4.5 GHz. Higher contents of LaAlO3 cause a decrease in both the permittivity and f values, accompanied by an increase in the Q-value. Typical values measured at 6.0 GHz for ceramics with the composition 70 mol% La2/3TiO3 – 30 mol% LaAlO3 are k>44, f=+7 ppm K-1 and Q>5500. The results depend on the processing parameters. The sintering temperature increases with increasing LaAlO3 content. An oxidizing sintering atmosphere improves the Q-value, whereas the permittivity and f are not significantly affected.     

Flux-line cutting in type II superconductors

Experimental evidence for flux-line cutting in superconductors (intersection and cross-joining of singly quantized vortices) is briefly reviewed. The interaction energy between two straight vortices tilted at an angle ( 0)is then shown to be finite in the London model, i.e., in the limit of vanishing core radius. Next, the activation energy and maximum interaction force are calculated for the vortices in an analytic approximation to the Ginzburg-Landau theory. Here two competing interactions determine the behavior. Electromagnetic repulsion (0 < < /2) varies as cos and decays over distances scaled by the penetration depth , while core attraction is independent of and varies over distances scaled by the coherence length . The force is always repulsive at large flux-line separation (0 < < /2) and its maximum decreases rapidly as decreases, so that flux-line cutting isexpected to be more probable in low- materials. The calculations provide a basis for explaining longitudinal flux-flow resistance as well as some intriguing magnetization behavior in the same configuration.This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Materials Sciences Division, and by the Deutsche Forschungsgemeinschaft.On leave from Max-Planck-Institut für Metallforschung, Institut für Physik, Stuttgart, West Germany.On leave from New University of Ulster, Coleraine, Northern Ireland.     

Relationship between the structure and mechanical properties in ß III titanium alloy

The ageing reactions that take place during the heat-treatment of solution-treated III titanium (11.5 wt% Mo, 6 wt% Zr, 4.5 wt% Sn, balance Ti) were followed by detailed structure characterization using electron microscopy. The variations in mechanical properties with heat treatment were also followed systematically. The electron microscopy investigations indicated that the omega phase formed on quenching. The size and volume fraction of the omega phase increased on subsequent ageing,, and phases were found to co-exist at ageing temperatures between 800 and 900° F (427 and 482° C) for short ageing times. From the observations of interfacial dislocations at the/ interface and the precipitation of fine alpha near the omega particles, with a morphology that is characteristic of the prior morphology, it is suggested that the-phase forms directly from the omega phase. The observed increase in yield strength over the solution-treated condition, due to the precipitation of phase, was found to agree well with that predicted by the Orowan hardening mechanism. Since the precipitation of fine ellipsoidal-phase was found to increase the yield strength of the alloy with reasonable ductility, it is suggested that the optimum heat treatment to produce high strength with good ductility in III titanium is to age at 900° F (482° C) for 10 to 25 h.     

Thermodynamic and structural factors determining the wear resistance of aluminum cast irons

When the carbon content in aluminum cast iron containing about 2.5% Al is reduced (to about 2.5%),. carbon is concentrated near the dendritic branches where a specific variety of pearlite (containing a finely dispersed carbon-rich -phase) is formed, the interdendritic regions remaining ferritic. Modifying cast irons of this kind with cerium leads not only to spherodization of graphite but also to the formation of -phase dendrites with a corresponding reduction in the carbon content in the ferritic matrix surrounding -phase dendrites and pearlite. This has a beneficial effect on the wear resistance of cast iron.     

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.     

Ageing characteristics of cast Zn-Al based alloy (ZnAl7Cu3)

Microstructure and ageing characteristics of a cast Zn-Al based alloy (ZnAl7Cu3) were studied using X-ray diffraction, electron scanning microscopy and back-scattered diffraction techniques. Two stages of phase transformation, i.e., decomposition of zinc rich phase and four phase transformation, + T + , were detected during ageing at 150°C. Electron back-scattered diffraction technique was applied in distinguishing both zinc rich and phases.     

The influence of ageing on the stability of β-γ/γ′ derived microstructures in Ni-Al-Cr-(Co) alloys

Multiphase Ni-Al-(Fe)-(Cr)-(Co)-based intermetallics with (B2)- (A1)/(L1₂), - or - microstructures can exhibit significant room-temperature tensile ductility. In the case of Ni-Al-Cr-based alloys, microstructural development is complicated by the precipitation of -Cr, which can supplant the -phase during ageing of three-phase -/ microstructures. An investigation of the stability, during ageing, of cast Ni-Al-Cr-(Co) alloys with microstructures derived from -/ is reported. In the as-cast condition, the materials investigated consisted of a dendritic matrix containing L1₀ type martensite and interdendritic /. Extensive intra- and interdendritic -Cr precipitation was also observed. The stability during ageing of the interdendritic / microstructure is also considered and transformation of the L1₀ martensite is examined.     

Heating of “thin” bodies in liquid media

Heating of thin (Bi0.25) bodies in liquid media is examined, taking into account crystallization and fusion at the body-medium interface.     

Equations of generalized thermoelasticity of cosserat medium

Dynamic equations of generalized thermoelasticity are derived for the Cosserat continuum, the theorem of uniqueness of solution of the problem is proved, and an expression is derived which represents the content of a theorem analogous to the reciprocal theorem.Notation absolute temperature - d surface element - Euler radius vector of a point - r Lagrange radius vector of a point - n vector of the external normal to the surface - E unit tensor - s entropy per unit volume - q vector of heat flux - w density of volumetric heat release - =(–_o) _o ¹ relative deviation of temperature from the initial value - _o initial absolute temperature of the medium - asymmetric strain tensor - tensor of flexure and torsion - _o constant characterizing the rate of heat propagation - k coefficient of thermal conductivity - A mechanical power of external surface forces - L mechanical power of external body forces - W kinetic energy of strain - potential energy of strain - X dissipation function - temperature potential - U thermal analog of the power of internal sources - Q thermal analog of the power of the surface of sources Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, Wo. 4, pp. 716–723, October, 1980.     

Microstructural study of ε-iron-carbonitrides formed by a glow discharge technique

The microstructure and phase transformations occurring in-iron-carbonitrides have been studied by means of X-ray and electron diffraction, electron microscopy and Mössbauer spectrometry. Ordering of the interstitial atoms, N or C, results in a hexagonal unit cell for Fe₃(C, N) with parametersa=a3 andc=c wherea andc are the lattice parameters of the hexagonal close-packed (h c p) iron unit cell. Stacking faults on (0001) planes and partial dislocations with Burgers vector b=1/31 0 ¯1 0 are observed in quenched-Fe₄ (C, N). After quench-aging, the carbonitrides show a structural hardening due to the precipitation of a metastable phase. Slow cooling of-carbonitrides with less than 25 at% interstitials leads to the precipitation of-carbonitride and ferrite in-phase grains which allows the orientation relationships between the-,- and-phases to be defined and a model of the-phase--phase transformation to be proposed.     

Size-strength effects in sapphire and silicon nitride whiskers at 20° C

Tensile tests at 20° C have been carried out on seventy-three sapphire whiskers and on seventeen silicon nitride whiskers. The sapphire whiskers were of 0001, 1¯120, 10¯10, and 10¯11 orientations, while the silicon nitride whiskers were 0001, 11¯20, and 10¯13. Tensile strengths were in the range 45 to 1500 kg/mm², and deformation was found to be purely elastic. The tensile strength data have been analysed and fitted to empirical equations describing the effect of size on strength for different orientations. These empirical equations have been used to deduce possible fracture nucleation mechanisms. It is concluded that, in the case of 0001 sapphire whiskers, fracture nucleation may be due to dislocation pile-ups or interactions, while in the other cases a Griffith flaw mechanism is probably applicable.     

Low-frequency dielectric spectroscopy and electrical properties of Ca[B<Subscript>3</Subscript>O<Subscript>4</Subscript>(OH)<Subscript>3</Subscript>]·H<Subscript>2</Subscript>O single crystals near the phase transition

The dielectric properties and polarization of ferroelectric Ca[B₃O₄(OH)₃]·H₂O (colemanite) crystals are studied near the Curie temperature. The real part of dielectric permittivity (), dielectric losses (tan), pyroelectric coefficient (), and thermally stimulated depolarization currents are measured from -50 to 50°C. To assess the detailed nature of the phase transition (Curie temperature t_C~-7°C), the temperature dependences of and tan and are measured at frequencies f = 0.8-20 kHz. The results are used to determine () (where =f ) and (T) and evaluate the activation energy. The (t) data indicate that colemanite undergoes a diffuse phase transition.Translated from Neorganicheskie Materialy, Vol. 40, No. 12, 2004, pp. 1489–1494.Original Russian Text Copyright © 2004 by Slabkaya, Lotonov, Gavrilova.     

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.     

Suppression of Anomalous Allotropic Transformations at High Pressures

Anomalous allotropic transformations accompanied by a reduction in the density of the substance can be suppressed by applying a high pressure equal to the ratio of the heat of transition to the difference in the volumes of the phases involved. The estimated pressures necessary to suppress the (ZrO₂), (Fe), and (Sn) phase transitions are 4.34, 3.00, and 0.495 GPa, respectively.     

TEM study of metastable β-phase decomposition in rapidly solidified Ti-6Al-4V alloy

A new rationale is presented for various decomposition products obtained from the metastable -phase found in Ti-6A1-4V alloy produced by hot isostatic pressing comminuted melt-spun fibres and cooled to room temperature by furnace cooling. This alloy has an -matrix with about 8 vol% retained -phase, which is supersaturated with -stabilizers to such an extent that the martensitic transformation has been suppressed. The metastable -phase decomposes by different modes during continuous cooling, depending on the actual composition of individual -grains. Less enrichment of vanadium and iron favours the direct formation of the equilibrium -phase from the -matrix, while greater enrichment of vanadium and iron leads to a spinodal decomposition of the metastable -phase, resulting in a + two-phase structure. During further continuous cooling, the -phase which is lean in -stabilizers will transform into isothermal -phase. In addition, an unknown phase has also been observed in the -phase, which is typified by the appearance of 1/2{112} reflections in the SAD patterns.     

Characterization of α ? β PbF2 phase transition by several techniques

X-ray Diffraction (XRD), Dynamic-Mechanical Analysis (DMA), Impedance Spectroscopy (IS), and Scanning Electron Microscopy (SEM) have been used to the study of phase transition in PbF₂ pellets pressed uniaxially in the 37 MPa–480 MPa range. A mixture of and phases is detected and the -phase content at room temperature is found to be dependent on the applied load. The dilation results on DMA, the phase identification by XRD, the ionic conductivity results by IS analysis and the SEM micrographs of -PbF₂ pellets show evidences for the increase in -phase content at the expense of the -phase. SEM analysis provided further evidence for specimen sintering under heating at approximately 498 K.     

Structural and constitutional studies of some cerium-praseodymium alloys

Room temperature X-ray diffraction studies on some powdered Ce-Pr alloys indicate that the face-centred-cubic (f c c) structure extends from 0 to 65% Pr and the double hexagonal (d h c p) structure from 66 to 100% Pr, after a heat treatment of 600° C for 2 h and quickly cooling to room temperature. Variations of atomic volume with composition in both ranges indicate that the volume difference between the f c c form of praseodymium and the d h c p form is similar to that observed for (d h c p) and (f c c) lanthanum, whereas extrapolation to 100% Ce from the d h c p range gave an atomic volume for the d h c p Ce appreciably in excess of the atomic volume of f c c -Ce. This volume expansion is consistent with a slight change of the effective valency of the cerium atoms in the d h c p solid solutions when compared with the -Ce but there is uncertainty as to the precise atomic volume of d h c p -Ce. The DTA studies indicate a narrow liquidus/solidus separation and the electrical resistivity and DTA measurements indicate a regular change with composition in the transition temperature of the high temperature b c c phase. For the d h c pf c c transition there is a marked variation in the width of the hysteresis loop across the Ce-Pr system which can be correlated with the degree of plastic deformation involved in the transformation. There is a marked increase in the slope of the transition temperature with composition for the Pr-rich alloys and no such transition is observed for the praseodymium samples after one heating cycle. A possible d h c pfc c transition, however, is indicated by the DTA traces of the commercially pure praseodymium sample on cycling with temperature and this has been attributed to the influence of interstitial impurities.This praseodymium was kindly loaned by Dr D. Fort of the Centre of Materials Science, University of Birmingham, and the metal had been purified by solid state electrolysis to give a resistance ratio of 200.