Aging of supersaturated Al-Cu-Co decagonal phase

Aging of the supersaturated Al₆₈Cu₁₁Co₂₁ decagonal phase was studied by transmission and scanning electron microscopy. Changes in the twofold [12100] electron diffraction patterns are reported. The decagonal phase aged at 700–800°C exhibited a reduction of the intensities of the quasiperiodic odd-n reflections and the appearance of extra-reflections along quasiperiodic directions.     

Phase diagram of Cu-Au-type alloys

We investigate the ordering phase diagram of an binary alloy on a face centered cubic lattice. In Ising spin language the nearest-neighbor interactions are antiferromagnetic with strengthJ, the next-nearest-neighbor interactions are ferromagnetic with strength J, and the external magnetic field ish. For > 0 and allh, the ground state is only finitely degenerate, so Pirogov-Sinai theory gives the exact form of the phase diagram in the limit of vanishing temperature. For=0 and vhv 12J the ground state is infinitely degenerate, and indeed the zero temperature entropy is nonvanishing at the four super-degenerate pointsh=± 47 or ±1Z7. We investigate the finite temperature behavior of the model using Monte Carlo simulations and (for=0) low temperature expansions. The most interesting portions of the phase diagram are those near the superdegenerate points. We rigorously map these points onto certain hard constraint lattice gases, but can draw no firm conclusions concerning the phase diagram in their vicinity.Supported in part by the National Science Foundation through Grant No. DMR81-14726. The simulations were carried out at the Center for Materials Science at Los Alamos National Laboratory.     

A stochastic model for solidification

A 3-dimensional (2-space, 1-time) model relating the diffusion of heat and mass to the kinetic processes at the solid-liquid interface, using a stochastic approach is presented in this paper. This paper is divided in two parts. In the first part the basic set of equations describing solidification alongwith their analysis and solution are given. The process of solidification has a stochastic character and depends on the net probability of transfer of atoms from liquid to the solid phase. This has been modeled by a Markov process in which knowledge of the parameters at the initial time only is needed to evaluate the time evolution of the system. Solidification process is expressed in terms of four coupled equations, namely, the diffusion equations for heat and mass, the equations for concentration of the solid phase and for rate of growth of the solid-liquid interface. The position of the solid-liquid interface is represented with the help of a delta function and it is defined as the surface at which latent heat is evolved. A numerical method is used to solve the equations appearing in the model. In the second part the results i.e. the time evolution of the solid-liquid interface shape and its concentration, rate of growth and temperature are given.     

High pressure phase diagrams of binary lanthanide alloys between La,Ce and Pr

Abstract

In comparison with the “regular” binary alloy phase diagrams between “regular” trivalent lanthanide metals, binary intralanthanide alloys of Ce exhibit many irregularities typical for Ce under pressure due to its f electron delocalization. For comparison with the La-Ce and Ce-Pr high pressure phase diagrams also the more regular La-Pr data are presented for the pressure range up to 40GPa.     

Phase diagram and magnetocaloric effect of alloys

We propose the phase diagram of a new pseudo-ternary compound, CoMnGe_1-xSn_x, in the range x0.1. Our phase diagram is a result of magnetic and calorimetric measurements. We find that the separate magnetic and structural transitions in CoMnGe are tuned together as the level of Sn substitution is increased. We demonstrate the appearance of a hysteretic magnetostructural phase transition in the range x=0.04–0.055, similar to that observed in CoMnGe under hydrostatic pressure. From magnetisation measurements, we show that the isothermal entropy change associated with the magnetostructural transition can be as high as in a field of 1 T. However, the large thermal hysteresis in this transition () will limit its straightforward use in a magnetocaloric device.     

Dynamic properties of phase diagram in cylindrical ferroelectric nanotubes

The dynamic properties of phase diagrams in cylindrical ferroelectric nanotubes were investigated by utilizing the effective-field theory with correlations. Three different structure mechanisms are discussed by performing three different interaction functions. Effects of structure factors of the ferroelectric nanotubes are shown in phase diagrams. A linear relation between the crossover value of transverse field in inner-layer of nanotube and the exchange interaction are revealed. Moreover, for the first time, the phase transition characteristics are shown both in 2-D and 3-D phase diagrams by comparing the variations of the ferroelectric region and the morphology vividly. In comparison, results of the usual mean-field approximation are also presented in some phase diagrams, it suggest that the differential operator technique with correlations reduces some extent of the ferroelectric features of the ferroelectric nanotube.     

The effect of pressure on magnetization and magnetic phase diagram of monocristals of Tb-Dy alloys

Abstract

The effect of pressure on magnetic phase transitions in monocrystals of Tb-Dy alloys is studied. It is shown that the destruction of antiferromagnetic structure and the establishment of ferromagnetic ordering is due to magneto-elastic interactions.     

Quadrupolar susceptibility and magnetic phase diagram of PrNi2Cd20 with non-Kramers doublet ground state

ABSTRACT

In this study, ultrasonic measurements were performed on a single crystal of cubic PrNi₂Cd₂₀, down to a temperature of 0.02?K, to investigate the crystalline electric field ground state and search for possible phase transitions at low temperatures. The elastic constant (C₁₁?C₁₂)/2, which is related to the Γ₃-symmetry quadrupolar response, exhibits the Curie-type softening at temperatures below ～30?K, which indicates that the present system has a Γ₃ non-Kramers doublet ground state. A leveling-off of the elastic response appears below ～0.1?K toward the lowest temperatures, which implies the presence of level splitting owing to a long-range order in a finite-volume fraction associated with Γ₃-symmetry multipoles. A magnetic field–temperature phase diagram of the present compound is constructed up to 28?T for H || [110]. A clear acoustic de Haas–van Alphen signal and a possible magnetic-field-induced phase transition at H ～26?T are also detected by high-magnetic-field measurements.     

Ni-Zn合金物相分析的实验研究

利用X射线物相分析技术,辅以金相分析方法、差热分析技术研究了Ni-Zn体系的4个合金.实验结果表明,Ni-Zn合金系中存在中间化合物NiZn3,该化合物稳定地存在于650℃到室温之间.400℃下NiZn3(75%Zn)化合物的点阵常量为a=3.31919(4)nm,b=0.8866(2)nm,c=1.2518(2)nm.     

Boron nitride phase diagram. State of the art

Abstract

From recent experimental data on BN thermodynamic propeties, the equilibrium phase diagram for boron nitride has been plotted, which differs from the generally accepted Bundy-Wentorfs one. At atmospheric pressure cubic boron niride has been shown to be a thermodynamically stable modification up to temperatures of 1600 K, which drastically changes the established notions of BN polymorphism, based on assumed analogy of phase diagrams for carbon and boron nitride. These studies have shown that according to the proposed equilibrium phase diagram the threshold pressure of cBN crystallization can be reduced from 4 down to 2 GPa with the supercritical fluids present, which opens new fields for developing methods for cBN low-pressure synthesis.     

Electron diffraction studies of variable periodicity in decagonal quasicrystals in aluminium-cobalt alloys

The reciprocal space vectors of the decagonal phase can be generated by a slight distortion of the basis vectors of a regular icosahedron to induce commensuration in one dimension. In different alloys a multiple of this basic periodicity has been observed. In particular, in Al-Co alloys, apart from the basic periodicity, doubling, tripling and quadrupling of this basic underlying periodicity has been observed. Stereographic analysis, simulation and experimental observation of electron diffraction patterns have been used to explore the nature of this varying periodicity in detail.     

Pressure effect on the magnetic phase diagram of Hf1-xTaxFe2 alloys

Abstract

The lattice constants of Hf_1-xTa_xFe₂ with C14 structure were measured at room temperature under pressure up to 5 GPa. The magnetic phase diagram obtained as a function of pressure was compared with that obtained as a function of Ta concentration x. It is shown that changes in lattice constants by applying pressure and /or by substituting Ta for Hf affect remarkably the magnetic phase stability, which is consistent qualitatively with a recent theoretical prediction based on the local spin density approximation.     

Incommensurability and metastability problems in NH4HSeO4 and ND4DSeO4: A reinvestigation of phase diagrams

Abstract

X-ray and neutron diffraction results on NH₄HSeO₄ and ND₄DSeO₄ are reported. Direct evidence of an incommensurate phase sandwiched between the monoclinic high-temperature phase (space group B2) and the low-temperature ferroelectric lock-in phase k = ? (space group P1) has been obtained in NH₄HSeO₄. The phase situation is more complicated in ND₄DSeO₄, where an incommensurate phase is found sandwiched between the B2 phase and a lock-in phase k = ¼ and where a phase in which several modulations coexist has been discovered in between the two lock-in phases k = ¼ and k = ?. The non-equilibrium processes, also present in ND₄DSeO₄, have been identified. All these results have clarified the situation about the phase diagram.     

Sound propagation anomalies and phase diagram of chromium alloys

Calculations of the complex elastic moduli Ĉ(T) in dilute Cr alloys are compared to measurements of the velocity and damping of sound near T _N and at high temperatures T>T _N (T _N — Néel temperature). The thermodynamic calculation is based on the covalent bond model of 3d ions in a state with different numbers n of covalent electrons. The parameters A _ij ⁽ⁿ⁾ of indirect exchange between the ions of the i and j sublattices are expressed in terms of the covalent bond energy Γ _ij ⁽ⁿ⁾ . The stability of the charge and spin density waves (CDWs and SDWs) is found by a variational method and is determined by the dispersion of Γ _ij ⁽ⁿ⁾ and by the Coulomb parameters U _n. For a small structural vector Q the phase diagram contains a superantiferromagnetic phase (SAFM) at temperatures T _N<T≲2T _N. The peak of the defect |ΔE(T)| of the modulus and of the sound damping Δ_h(T _N) near the first-order SDW-SAFM transition is determined by the structure of the transitional domain. Measurements of the anomalous growth of E(T) at temperatures T>T _N make it possible to determine the magnetostriction constants λ(T) of Ce alloys in the SAFM phase on the basis of the SAFM theory. Fiz. Tverd. Tela (St. Petersburg) 41, 1467–1472 (August 1999)     

Theoretical justification of the magnetic phase diagram of CuMn alloys

Some important observations on the magnetic susceptibility of concentration Cumn alloys by Gibbs et al. are reinterpreted in the light of the mean-field theory of classical vector spin models with competing exchange interactions. The phase diagram proposed by Gibbs et al. is shown to be in qualitative agreement with the corresponding phase diagram predicted by mean-field theory.     

High pressure phase diagram and phase transition kinetics of PbSe

Abstract

The cubic to orthorhombic transition in PbSe is investigated by electrical resistivity measurements up to 7 GPa and for temperatures between 20 °C and 300 °C. The phase diagram is constructed and the kinetics of the transition is discussed.