Mechanical properties and microstructure of β-SiAION-β-SiC composites by pressureless sintering

-SiAION--SiC composites containing up to 12 wt% -SiC were prepared by pressureless sintering. The strength of composites at room temperature remained relatively unchanged, whereas strength at 1200 °C increased for composites. The fracture toughness (K _IC) for composites was higher than that for -SiAION ceramics. The maximum value was 5.4 MPa m^1/2 for 6 wt% -SiC, and this was an improvement of 15% in comparison with -SiAION ceramics. From SEM observations, an improvement inK _IC values was attributed to crack deflections and branching-off of cracks. Intra-granular fractures were frequently observed in -SiAION. From TEM observations, -SiAION crystals were nanocomposites, within which existed the fine crystals in -SiAION crystal. For composite, -SiAION and -SiC crystals were directly in contact. The mismatching zone was observed in -SiC.     

Li+ ion mobility in eucryptite phases

The two polymorphs of LiAlSiO₄, and eucryptite, were studied by X-ray and thermal methods. The-eucryptite originated from different precursors that obtained from LiA zeolite is presented. It is shown that the electric conductivity of-eucryptites increased as the unit cell parameter,c ₀, increased. These results correlate well with chemical compositions and thermal investigations. The conductivity in mixed systems depends on the ratio/ eucryptites which indicates a higher Li⁺ mobility in the-phase.     

Hall-Effect in LuNi2B2C and YNi2B2C Borocarbides in Normal and Superconducting Mixed States

It was shown that the Hall resistivity _xy for LuNi ₂ B ₂ C and YNi ₂ B ₂ C is negative in the normal and mixed states and has no sign reversal below T _c . In the mixed state the scaling relation _{xy xx} (_xx is the longitudinal resistivity) was found for both compounds with 2.0. In the normal state a distinct nonlinearity in the _xy(H) dependence, accompanied by a large magnetoresistance, was found below 40 K only for LuNi ₂ B ₂ C. The difference in the behaviour of Lu- and Y-based borocarbides seems to be connected with the difference in the Fermi surfaces of these compounds.     

Supercarrier Effective Mass Isotope Effect by Interband Scattering

Supercarrier effective mass isotope effect (exponent ) is investigated using a two-band model with interband pair scattering. The corresponding repulsive interaction incorporates besides the dominating electronic (Coulomb) part an electron-phonon contribution inversely proportional to the ionic mass factor. Calculations illustrating the behaviour of T _c , its isotope exponent , and with doping in La _2–x Sr _x CuO ₄ type underdoped system reflect the observed tendencies. ² Both and diminish with doping, the sign of is opposite to . A typical estimation gives || 0.2.     

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.     

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.     

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.     

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.     

Preparation of silicon carbide powders by chemical vapour deposition of the SiH4-CH4-H2 system

Chemical vapour deposition (CVD) of the SiH₄ + CH₄ + H₂ system was applied to synthesize-silicon carbide powders in the temperature range 1523 to 1673 K. The powders obtained at 1673 K were single-phase-SiC containing neither free silicon nor free carbon. The powders obtained below 1623 K were composite powders containing free silicon. The carburization ratio (SiC/(SiC + Si)) increased with increasing reaction temperature and total gas flow rate, and with decreasing reactant concentration. The average particle sizes measured by TEM ranged from 46 to 114nm, The particle size increased with the reaction temperature and gas concentration but decreased with gas flow rate. The-SiC particles obtained below 1623 K consisted of a silicon core and a-SiC shell, as opposed to the-SiC particles obtained at 1673 K which were hollow. Infrared absorption peaks were observed at 940 and 810 cm^–1 for particles containing a silicon core; whereas a single peak at about 830 cm^–1 with a shoulder at about 930 cm^–1 was observed for the-SiC hollow particles. The lattice parameter of-SiC having a carburization ratio lower than 70 wt%, was larger than that of bulk-SiC and decreased with the increasing carburization ratio. However, when the carburization ratio exceeded 70 wt%, the lattice parameter became approximately equal to that of bulk-SiC.     

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.     

Structural transformation during sintering and annealing of beta alumina

Changes in the phase content, microstructure and lattice parameter are observed in stabilized/ alumina specimens following extended sintering and annealing treatments. The resulting state is dependent on composition of the starting powder and on temperature and duration of heat treatment; the kinetics of transformation between and alumina are generally slow and certain/ ceramics remain in a metastable state even after a prolonged high temperature anneal. Following post-sinter heat treatment, splitting of X-ray diffraction peaks reveals a segregation of the phase into two components of differing lattice parameter. With sintering schedules of a long duration the splitting may even be present in the as-fired condition as recently reported by Harbach [1]. The splitting is attributed to a structural change resulting from the expulsion of Na₂O from supersaturated grains.     

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.     

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.     

Compressibility measurements of β- and β″ -alumina

The compressibilities of the a- and c-axes for sodium - and -aluminas were determined up to 10 GPa from the pressure dependence of powder X-ray diffraction measured at room temperature using synchrotron radiation as an X-ray source. Powders of sodium - and -aluminas which were prepared from grinding synthesized single crystals were used as the specimens for X-ray diffraction. The compressibilities of - and -aluminas are 1.5 ± 0.2 ×10^–12 and 1.7 ± 0.2 × 10^–12 Pa^–1 for the a-axis and 2.9 ± 0.2x10^–12 and 1.6 ± 0.2 ×10^–12 Pa^–1 for the c-axis, respectively. For the c-axis, the compressibility of -alumina is larger than that of -alumina. This experimental fact is explained by the different stacking of oxygen layers and the different content in sodium ion between - and -aluminas.     

Theory of RF SQUIDs Operating in the Presence of Large Thermal Fluctuations

A comprehensive analytical theory is presented for non-hysteretic RF SQUIDs operating in the adiabatic mode in the presence of large thermal fluctuations. When 1 ( = 2LI_c/₀ is the hysteresis parameter, L is the SQUID inductance, I_c is the critical current of the Josephson junction, and ₀ is the flux quantum) the theory is applicable also for RF SQUIDs operating in the non-adiabatic mode. In contrast to previous theories in which the noise is treated perturbatively and which therefore are applicable only if the product 1 ( = 2k_BT/ ₀ I_c is the noise parameter, k_B is the Boltzmann constant, and T is the absolute temperature)—the case of small thermal fluctuations—the present theory is valid for around unity or higher. In the limit 0 the theory reproduces the results of small thermal fluctuations theories. It has been found that in the presence of large thermal fluctuations the screening current in the SQUID inductance is suppressed by a factor that increases with increasing . Taking into account this new basic fact, all SQUID characteristics (output signal, transfer function, noise spectral density and energy sensitivity) have been recalculated and a good agreement with experimental data has been obtained. It has been also found that RF SQUIDs can be operated with substantially higher values of the inductance and of the noise parameter than DC SQUIDs. These two aspects, which are of particular importance at liquid nitrogen temperature, make high T_c RF SQUIDs very attractive.     

Distribution of α-AlFeSi and β-AlFeSi particles in surface layer of AA6063 alloy billets after heat treatment

We developed the EPMA mapping method of small -AlFeSi(Al_8.3Fe₂Si) and -AlFeSi(Al_8.9Fe₂Si₂) particles in the billets of Al-Mg-Si alloys such as AA6063 alloys. To discriminate between -AlFeSi and -AlFeSi particles we used the relative X-ray intensities of Fe/Si ratio, the I _Fe/I _Si ratio, instead of the Fe/Si mass ratio. To obtain the I _Fe/I _Si ratio, we used a Monte Carlo method. In this study, using this method the mapping of -AlFeSi and -AlFeSi particles in the surface layer of AA6063 billets after the heat treatment (for 2 h at 580°C) was done. Namely, the distribution of -AlFeSi and -AlFeSi particles of zones from the billet surface to a depth of 800 m was measured. Results showed the zone from the surface to a depth of 200 m was occupied mainly by -AlFeSi particles and the zone from a depth of 200 m toward the center was occupied mainly by -AlFeSi particles. From these results, it was found that if we remove zones from the surface to a depth of 200 m, we can remove the majority of the -AlFeSi particles, and thus improve the quality of anodizing performance of Al-Mg-Si alloys extrusions.     

Observations on the formation of β-Al5FeSi phase in 319 type Al-Si alloys

In Al-Si alloys, the properties are influenced by the shape and distribution of the eutectic silicon particles in the matrix, as also by the iron intermetallics and copper phases that occur upon solidification. The -Al₅FeSi iron intermetallic phase, in particular, is known for its detrimental effect on the properties, and is controlled variously by the iron content and the melt/solidification conditions of the alloy. The formation of the iron intermetallics has been observed in commercial 319 alloy end-chilled castings, obtained from non-treated and treated melts, where the effects of cooling rate, strontium modification and grain refinement have been studied. The volume fraction of -phase formed was seen to increase with the decrease in cooling rate (i.e. with increasing distance from the chill) in the untreated alloy. Sympathetic (preferential) nucleation of the -iron needles was also observed, in which the branching of -needles from a parent needle resulted in the formation of large -needle entities that can cover distances of 1200 m across the matrix surface. The beneficial effect of modification, i.e. strontium addition to the melt, was manifested through its influence on the fragmentation and dissolution of the -needles. The strontium poisons the sites where sympathetic nucleation takes place. Dissolution was accelerated with increasing strontium content, the optimum level being 300 p.p.m. Grain refining, on the other hand, negated the beneficial effect of modification, in that the -needles underwent thickening and the sympathetic nucleation/branching also occurred. The modified alloy was found to possess the lowest volume fraction of -Al₅FeSi phase among the unmodified, modified, grainrefined, and modified/grain-refined alloys.     

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.     

Crystallization of some spodumene-lithium zinc orthosilicate glasses

By using thermal analysis, X-ray diffraction, polarizing and electron microscopy the effect of compositional variation, thermal treatment and nucleation catalysts TiO₂ and ZrO₂ on the nature, type and stability fields of the crystallizing phases, as well as the resulting microstructures, is described for some stoichiometric glass compositions within the system LiAlSi₂O₆-Li₂ZnSiO₄. Intense uniform volume crystallization was achieved. Transparent glass-ceramics with ultrafine microstructures could also be obtained at temperatures near 700 °C. Crystallization begins with the formation of a proto - and/or _II-Li₂ZnSiO₄ followed by, or concomitant with, -eucryptite ss. The proto -phase was formed over a narrow temperature range and rapidly transformed into its _II-modification which, although showing a wider stability range, ultimately transformed into the stable _o-modification. The metastable -eucryptite ss starts its transformation into -spodumene around 800 °C. By prolonged heating at temperatures as high as 1000–1040 °C, -spodumene and _o-Li₂ZnSiO₄ were the main stable end products. TiO₂ and ZrO₂ have contrasting effects on the stability of the LiZn orthosilicates and -eucryptite ss -spodumene transformations. The former exhibits a catalytic effect and the latter showed a retarding effect on these processes.     

Reversible colour change in Cu-Al-Ni alloy ribbon associated with phase transformation

The colour change behaviour and its relation to phase transformation of Cu-14 wt% Al-4 wt% Ni alloy ribbon produced by the twin-roller type melt-quenching method were investigated by spectral reflectivity and X-ray diffraction, respectively. This ribbon turns copper-coloured around room temperature on quenching it from a temperature above 1020 K, and turns gold-coloured on ageing it between 670 and 970 K. By repeating these heat treatments, either of the two colours can be acquired interchangeably. The spectral reflectivity also changes with respect to the colour change. The copper-coloured alloy shows a DO₃ structure which is the ₁, phase. The gold-coloured alloy shows a mixed phase of ₂ (cubic, Cu₉Al₄ compound), and (fcc, Cu-Al-Ni solid solution) and/or ₁ which is a martensite of the ₁ phase having a (1 21) twinning structure. Therefore, the colour change between copper and gold is due to the solid-state phase transformation between ₁ and ₂ + ( and/or ₁) on heat treatment.