Continuous ordering below the tricritical point: A critical test for the kinetic pathway in Fe-12at.% Si alloy

A critical test has been presented to establish the nature of the kinetic pathways for the decomposition of Fe-12 at.% Si alloy below the metastable tricritical point. The results, based on the measurements of saturation magnetization, establish that a congruent ordering from B2 --> D0(3) precedes the development of a B2 + D0(3) two-phase field, consistent with the predictions in 1976 of Allen and Cahn.

Mechanical alloying—a novel synthesis route for amorphous phases

Mechanical alloying (MA) pioneered by Benjamin is a technique for the extension of solid solubility in systems where the equilibrium solid solubility is limited. This technique has, in recent years, emerged as a novel alternate route for rapid solidification processing (RSP) for the production of metastable crystalline, quasicrystalline, amorphous phases and nanocrystalline materials. The glass-forming composition range (GFR), in general, is found to be much wider in case of MA in comparison with RSP. The amorphous powders produced by MA can be compacted to bulk shapes and sizes and can be used as precursors to obtain high strength materials. This paper reports the work done on solid state amorphization by MA in Ti-Ni-Cu and Al-Ti systems where a wide GFR has been obtained. Al-Ti is a classic case where no glass formation has been observed by RSP, while a GFR of 25–90 at.% Ti has been obtained in this system, thus demonstrating the superiority of MA over RSP. The free energy calculations made to explain GFR are also presented.

Phase Transformation Introduced by Mechanical and Chemical Surface Preparations of Tetragonal Zirconia Polycrystals

Cutting of Y2O3-doped TZP rods by a low-speed diamond saw introduces an unidentified, metastable phase X (x-ZrO2) coexisting with the tetragonal (t-ZrO2) and the monoclinic (m-ZrO2) phases initially present in the sample. Further mechanical deformation of the cut surface by indentation or polishing sustains the x-ZrO2. Chemical etching removes the x-ZrO2 and increases the m-ZrO2content.

Role of population transfer under strong probe conditions in electromagnetically induced transparency

We analyze theoretically the phenomenon of electromagnetically induced transparency (UT) under conditions where the probe laser is not in the usual weak limit. We consider the effects in both three-level and four-level systems, which are either closed or open (due to losses to an external metastable level). We find that the EIT dip almost disappears in a closed three-level system but survives in an open system. In four-level systems, there is a narrow enhanced-absorption peak (EITA) at line center, which has applications as an optical clock. The peak converts to an EIT dip in a closed system, but again survives in an open system. (C) 2010 Elsevier B.V. All rights reserved.

Edge Stabilities of Hexagonal Boron Nitride Nanoribbons: A First-Principles Study

We investigate the comparative stability of sp(2) bonded planar hexagonal boron nitride (h-BN) nanoribbon (BNNR) edges, using first principles calculations. We find that the pristine armchair edges have the highest degree of stability. Pristine zigzag edges are metastable, favoring planar reconstructions in the form of 5-7 rings] that minimizes the energy. Our investigation further reveals that the pristine zigzag edges can be stabilized against 5-7 reconstructions by passivating the dangling bonds at the edges by other elements, such as hydrogen (H) atoms. Electronic and magnetic properties of nanoribbons depend on the edge shapes and are strongly affected by edge reconstructions.

Chemical Approaches to the Design of Oxide Materials.

Chemical methods of synthesis play a crucial role in designing and discovering new and novel materials and in providing less cumbersome methods for preparing known materials. Chemical methods also enable the synthesis of metastable materials which are otherwise difficult to prepare. In this presentation, the various innovative chemical methods of synthesising oxide materials will be briefly reviewed with emphasis on soft-chemical routes. Electrochemical synthesis, ion-exchange method, alkali-flux method and some of the interaction reactions will be highlighted, besides topochemical aspects of solid state synthesis. Cuprate superconductors as well as intergrowth structures will also be examined.

Intermetallic Ti2Ni/TiNi nanocomposite by rapid solidification

Nanoscale dispersions of intermetallic Ti2Ni particles in an ordered TiNi intermetallic matrix have been produced by rapid solidification processing of near equiatomic TiNi alloys containing small amount of Si utilising the principle of kinetic competition in the undercooled liquid, A detailed characterisation of the microstructures obtained by different processing conditions was carried out to establish the trend of the refinement of the scale of microstructure. The observed microstructural conditions are rationalised in terms of a metastable phase diagram and the Uhlmann, Chalmers and Jackson theory of the trapping of second phase particles by a moving interface.

Wet chemical syntheses of ultrafine multicomponent ceramic powders through gel to crystallite conversion

Coarse (BOn/2)-O-n+/xH(2)O (10metastable phases, such as cubic BaTiO3 at room temperature or superparamagnetic hexaferrites. Through the same route, luminescent phosphors of aluminates doped with rare-earth metals could be prepared. The present results indicate the general features of the gel-crystallite (G-C) conversion involving the instability of the metal hydroxide gel brought about by the disruption of the ionic pressure in the gel as a result of the faster diffusion of A(2+) ions through the solvent cavities within the gel frame work. This is accompanied by the splitting of the bridging groups like B-(OH)-B or B-O-B, leading to the breakdown of the gel into crystallites. G-C conversion has advantages as a method of synthesis of ceramics in terms of operational cost and procedural simplicity.

Collapse Behavior of an Artificially Cemented Clayey Silt

The present study examines the role of interparticle cementation in the collapse behavior of two partly saturated (S-r = 4 to 12%) and very highly porous (initial void ratio = 1.5 to 2) laboratory-desiccated clayey silt specimens containing varying amounts (5 and 15% by dry weight of the respective specimens) of the cementitious iron oxides hematite and goethite, which are generally encountered in tropical residual soils. Kaolinite is the representative clay mineral of the soil matrix used for this research. Interparticle cementation by the crystalline iron oxides was generated in the laboratory by repeated (six times) wetting and drying of the iron-hydroxide-admixed clayey silt specimens under ambient conditions of temperature and humidity. Results showed that, for a given laboratory-desiccated clayey silt specimen (i.e., a specimen containing 5 or 15% of iron oxide on a dry weight basis), the amount of collapse (represented by Delta epsilon, the change in vertical strain upon wetting under constant pressure) increases with an increase in the experimental loading under which the specimen is inundated. The laboratory results also show that the desiccated specimen with a higher iron oxide content (containing 15% iron oxide by dry weight of the desiccated specimen) in spite of a lower dry unit weight (gamma(d) = 8.8 kN/m(3)) undergoes a lesser amount of collapse on soaking under a constant external stress (50 or 100 kPa) than the desiccated specimen with a lower iron oxide content (i.e., containing 5% iron oxide by dry weight of the desiccated specimen, gamma(d) = 10.4 KN/m(3)). Based on the X-ray diffraction results and the stress-strain relationships obtained from isotropically consolidated undrained triaxial tests, it is suggested that the laboratory-desiccated specimens are characterized by a metastable bonding provided by capillary suction and the crystalline iron oxides. On soaking under load owing to the loss of the metastable bonding, collapse of the laboratory-desiccated specimens occurs. Also, in the case of the laboratory-desiccated specimen with a higher iron oxide content, the presence of a stronger interparticle cementation (due to a greater abundance of crystalline iron oxides) and a higher initial moisture content are considered responsible for the specimen exhibiting a lower amount of collapse in comparison to that exhibited by the desiccated specimen with a lesser iron oxide content.

Quasicrystals, crystals and multiple twins in rapidly solidified Al-Cr-Si, Al-Mn-Si and Al-Mn-Cr-Si alloys

The coexistence of quasicrystals and rational approximant structures (RAS) has been observed in melt-spun Al80Cr14Si6, Al80Mn14Si6 and Al75Mn10Cr5Si10 alloys. The presence of a b.c.c. alpha-AlMnSi phase in Al-Mn-Si and alpha-AlMnSi(Cr) phase in Al-Mn-Cr-Si has been seen. A multiple twinning around an irrational axis of the RAS has been reported in an aggregate of fine size cubic crystallites in all three alloys. Selected area diffraction patterns show that the crystalline aggregate symmetry is linked to the icosahedral point group symmetry (m35). Various ways of expressing the twin relationship in the cubic crystalline aggregates have been discussed. The thermal stability of the icosahedral phase at high temperatures reveals that the icosahedral phase in Al-Mn-Si and Al-Mn-Cr-Si alloys transforms to alpha-AlMnSi at temperatures of 690 and 670 K, respectively. In Al-Cr-Si alloy, heating to a high temperature (615 K) leads to the transformation of the icosahedral phase into a new metastable phase having an ordered cubic structure equivalent to alpha-AlMnSi. The occurrence of multiple twinning leading to icosahedral symmetry in the as-spun Al-Cr-Si alloy is presumably due to this metastable phase. Copyright (C) 1996 Acta Metallurgica Inc.

Time scales for transitions between free-energy minima of a dense hard-sphere system

Time scales associated with activated transitions between glassy metastable states of a free-energy functional appropriate for a dense hard-sphere system are calculated by using a new Monte Carlo method for the local density variables. In particular, we calculate the time the system, initially placed in a shallow glassy minimum of the free-energy, spends in the neighborhood of this minimum before making a transition to the basin of attraction of another free-energy minimum. This time scale is found to increase as the average density is increased. We find a crossover density near which this time scale increases very sharply and becomes longer than the longest times accessible in our simulation. This time scale does not show any evidence of increasing with sample size

Electron microscopy and diffraction of ordering in Ni---W alloys

Electron microscopy and diffraction studies of ordering in stoichiometric Ni-20%W and off-stoichiometric Ni-15%W alloys have been carried out. The specimens of Ni-20%W were first disordered at 1398 K for 4 h and then quenched rapidly into water. Short range order (SRO) spots were observed at {1 1/2 0}* positions. Two hitherto unknown metastable phases: D-2h(25)-Ni2W and DO22-Ni3W were observed in the diffraction patterns. Long range order (LRO) transformations were studied at 1103 and 1213 K. Kinetics and mechanism of transformations have been identified. Ni-15%W specimens were solution treated at 1523 K for 1 h followed by quenching in water. SRO spots similar to those found in Ni-20%W were observed in this alloy as well. The transition to LRO was studied at 1093 K. Distinct Ni4W precipitates could be observed after 5 h of annealing at this temperature. After 100 h of annealing precipitates were found to grow into faceted shape coherent with the disordered matrix. After prolonged annealing for over 150 h the Ni4W precipitates began to lose coherency by the generation of misfit dislocations. The microstructural observations have been compared for the stoichiometric and off-stoichiometric alloys.

On the decomposition of ? phase in some rapidly quenched titanium-eutectoid alloys

The decomposition of the beta phase in rapidly quenched Ti-2.8 at. pet Co, Ti-5.4 at. pet Ni, Ti-4.5 at. pet, and 5.5 at. pet Cu alloys has been investigated by electron microscopy. During rapid quenching, two competitive phase transformations, namely martensitic and eutectoid transformation, have occurred, and the region of eutectoid transformation is extended due to the high cooling rates involved. The beta phase decomposed into nonlamellar eutectoid product (bainite) having a globular morphology in Ti-2.8 pet Co and Ti-4.5 pet Cu (hypoeutectoid) alloys. In the near-eutectoid Ti-5.5 pet Cu alloy, the decomposition occurred by a lamellar (pearlite) type, whereas in Ti-5.4 pct Ni (hypereutectoid), both morphologies were observed. The interfaces between the proeutectoid alpha and the intermetallic compound in the nonlamellar type as well as between the proeutectoid alpha and the pearlite were often found to be partially coherent. These findings are in agreement with the Lee and Aaronson model proposed recently for the evolution of bainite and pearlite structures during the solid-state transformations of some titanium-eutectoid alloys. The evolution of the Ti2Cu phase during rapid quenching involved the formation of a metastable phase closely related to an ''omega-type'' phase before the equilibrium phase formed. Further, the lamellar intermetallic compound Ti2Cu was found to evolve by a sympathetic nucleation process. Evidence is established for the sympathetic nucleation of the proeutectoid alpha crystals formed during rapid quenching.

The CaO-SrO-CuO-O-2 system: Phase equilibria and thermodynamic properties at 1123 K

Phase relationships in the CaO-SrO-CuO system in pure oxygen at 1.01 x 10(5) Pa pressure were determined by equilibrating different compositions at 1123 K for similar to 120 h and analyzing the phases present in the quenched samples using X-ray diffraction (XRD), optical and scanning electron microscopy, and energy dispersive analysis of X-rays (EDAX). Four solid solution series were observed in the system, The CawSr1-wO monoxide solid solution with rock-salt structure was found to exhibit an asymmetric miscibility gap, The mixing properties of the monoxide system were deduced using a subregular solution model, For the (CaxSr1-x)(2)CuO3 series, a complete solid solution range with orthorhombic space group Immm was obtained. Calcium substituted for strontium up to 68 at. % in SrCuO2+delta and 51.5 at. % in Sr14Cu24O41-delta. The tie lines between the solid solutions were determined accurately, The activity-composition relations in (CaxSr1-x)(2)CuO3, CaySr1-yCuO2+delta, and (Ca2Sr1-z)(14)Cu24O41-delta solid solutions were determined from experimental tie lines. Activities in the (CaxSr1-x)(2)CuO3 and CaySr1-yCuO2+delta series were close to the predictions of the Temkin model, The behavior of the (CazSr1-(z))(14)Cu24O41-delta solid solution was more complex, with the activity of SrCu(24/14)O-(41-delta/14) exhibiting both positive and negative deviations from ideality. Gibbs energy of formation of the CaCuO2+delta metastable phase at 1123 K was deduced from an analysis of the phase diagram.

Tie lines and activities in the system NiO-MgO-SiO2 at 1373 K

The isothermal section of the phase diagram for the system NiO-MgO-SiO2 at 1373 K is established, The tie lines between (NiXMg1-X)O solid solution with rock salt structure and orthosilicate solid solution (NiYMg1-Y)Si0.5O2 and between orthosilicate and metasilicate (NiZMg1-Z)SiO3 crystalline solutions are determined using electron probe microanalysis (EPMA) and lattice parameter measurement on equilibrated samples, Although the monoxides and orthosilicates of Ni and Mg form a continuous range of solid solutions, the metasilicate phase exists only for 0 < Z < 0.096, The activity of NiO in the rock salt solid solution is determined as a function of composition and temperature in the range of 1023 to 1377 K using a solid state galvanic cell, The Gibbs energy of mixing of the monoxide solid solution can be expressed by a pseudo-subregular solution model: Delta G(ex) = X(1 - X)[(-2430 + 0.925T)X + (-5390 + 1.758T)(1 - X)] J/mol, The thermodynamic data for the rock salt phase are combined with information on interphase partitioning of Ni and Mg to generate the mixing properties for the orthosilicate and the metasilicate solid solutions, The regular solution model describes the orthosilicate and the metasilicate solid solutions at 1373 K within experimental uncertainties, The regular solution parameter Delta G(ex)/Y(1 - Y) is -820 (+/-70) J/mol for the orthosilicate solid solution, The corresponding value for the metasilicate solid solution is -220 (+/-150) J/mol, The derived activities for the orthosilicate solid solution are discussed in relation to the intracrystalline ion exchange equilibrium between M1 and M2 sites. The tie line information, in conjunction with the activity data for orthosilicate and metasilicate solid solutions, is used to calculate the Gibbs energy changes for the intercrystalline ion exchange reactions, Combining this with the known data for NiSi0.5O2, Gibbs energies of formation of MgSi0.5O2, MgSiO3, and metastable NiSiO3 are calculated, The Gibbs energy of formation of NiSiO3, from its component oxides, is equal to 7.67 (+/-0.6) kJ/mol at 1373 K.