We shall describe the change of magnetic order occurring in the pseudo-ternary system Tb1-xYxNiAl, which was investigated in the range of 0 less than or equal to x less than or equal to 0.8 by ac- and de-susceptibility measurements and neutron diffraction experiments. An anomalous transition from antiferromagnetic to ferromagnetic order was observed. Characteristic features of the transition are (a) it occurs in a narrow range of the Y content between x=0.01 and x=0.05, (b) both types of magnetic order coexist in different domains of the same compound, and (c) the preferred type of order changes with temperature. [S0163-1829(99)01113-3].

Abstract The chemical compositions of γ and γ′ phases in the alloy SC16 were determined by means of energy-dispersive X-ray spectroscopy in the transmission electron microscope (TEM/EDS) after heat treatment at different temperatures. According to these measurements, single-phase alloys with compositions identical to those of the γ and γ′ phases in SC16 were produced. The temperature dependence of the lattice parameters of these alloys was deduced (i) from the measured values of the lattice parameters at room temperature and (ii) from the measured unidirectional thermal expansion up to 1323 K. These measurements show that the lattice misfit between the γ and γ′ phases in unconstrained condition at the different elevated temperatures at which their equilibrium composition in the alloy SC16 was determined, is positive. The results are compared with the misfit values in the alloy SC16 reported in literature for constrained and unconstrained condition and also with the predictions from models on rafting.

The chemical compositions of gamma and gamma' phases in the alloy SC16 were determined by means of energy-dispersive Xray spectroscopy in the transmission electron microscope (TEM/EDS) after heat treatment at different temperatures. According to these measurements, single-phase alloys with compositions identical to those of the gamma and gamma' phases in SC16 were produced. The temperature dependence of the lattice parameters of these alloys was deduced (i) from the measured values of the lattice parameters at room temperature and (ii) from the measured unidirectional thermal expansion up to 1323 K. These measurements show that the lattice misfit between the gamma and gamma' phases in unconstrained condition at the different elevated temperatures at which their equilibrium composition in the alloy SC16 was determined, is positive. The results are compared with the misfit values in the alloy SC16 reported in literature for constrained and unconstrained condition and also with the predictions from models on rafting.

Neutron diffraction and susceptibility measurements have been performed to investigate the anomalous transition from antiferro- to ferromagnetic order which takes place in the ternary intermetallic compound TbNiAl when Ni is substituted for Cu within the same crystal structure. The transition is anomalous because 1) a few per cent of Cu already suppresses antiferromagnetism, 2) both ferro- and antiferromagnetic types of order coexist in different "domains" in the transition range 0.01 ⩽ x ⩽ 0.10, and 3) the balance between both types of domains further depends on temperature.

The glass transition has been studied in the ternary systems Gex/2Asx/2Se(1−x), in which the properties of the glass can be tuned from fragile to strong by changing x. In the present work we report on neutron inelastic scattering measurements of Gex/2Asx/2Se(1−x) for x = 0.066. This glass former falls into the category of the so-called fragile glasses. The characteristic dynamic behavior, also observed in other fragile glasses, is primarily characterised by the onset of marked anharmonicity in the low energy spectrum above the glass transition temperature Tg. These anharmonic processes have been identified with the fast β-relaxation processes, predicted by the mode-coupling theory (MCT). The Q and temperature dependence of the elastic and quasielastic neutron scattering provide information about special features of the glass transition and intermediate range order in this system.

In TbNiAl, which adopts the hexagonal ZrNiAl-structure, the Tb-site forms a triangular lattice in the a-b plane. The coupling between nearest neighbours was shown to be antiferromagnetic, giving rise to the formation of frustrated moments of Tb. TbNiAl has two antiferromagnetic phases which differ in the ordering of the frustrated spins. On the other hand, TbCuAl is a ferromagnet. It is shown by means of neutron diffraction that the stoichiometric substitution of Ni by a few per cent of Cu leads to an instant suppression of antiferromagnetism. The ordering temperature increases from TN = 47 K for TbNiAl to Tc = 55 K for TbNi0.90Cu0.10Al since the number of frustrated bonds is strongly reduced. In the range 0.01 ⩽ x ⩽0.10, surprsingly, both ferro- and antiferromagnetic types of order coexist in different domains. The volume fraction with antiferromagnetic order decreases with both increasing x in different samples and with increasing temperature below Tc in one sample. Compounds in the range 0.1 < x ⩽1.0 are ferromagnets but at intermediate concentrations 0.60 ⩽ x ⩽0.80, a dominat short-range magnetic order is observed. The results are discussed with a local magnetic exchange coupling between Tb spins.

In Ti-Mo the low-temperature instability of the bcc lattice leads to precipitation of metastable omega particles. Their coarsening has been found to be controlled by Mo diffusion, because of the law solubility of Mo in the omega phase. Irrespective of the annealing temperature, the coarsening of omega particles was found to stop when their average diameter reached congruent-to 160 angstrom. Such a stagnation of the microstructure can be understood as an effect of elastic inhomogeneity of the two-phase system. The shape of the omega particles was determined as a prolate ellipsoid of revolution. The aspect ratio of these coherent particles was constant at 2:1 not only during their coarsening but also in the state of stable particle size. Upon further annealing, the stabilization of the two-phase microstructure was destroyed by the nucleation of platelike alpha-Ti particles that grew at the expense of omega phase. The rapid growth of alpha particles during the dissolution of omega particles is explained by a structural transition omega --> alpha. After the dissolution of the omega phase, the following growth of the semicoherent alpha-Ti particles mainly took place on the incoherent rim of the plates. This led, in contrast to the self-similar coarsening of coherent omega, precipitates, to continuously increasing aspect ratios of alpha-Ti particles. Their shape evolution was found consistent with predictions from literature, based on the elastic anisotropy of the crystal lattice.

Production of amorphous alloys by solid state reactions (SSR) has attracted much interest during the last few years. One of the methods to induce such a reaction is the irradiation of suitable crystalline alloys by fast particles. Examination of this kind of SSR in M[sub 3]P type of brazing alloys (M: Metal) is attractive because of the following reason: In brazed joints of candidate structural materials like 316L stainless steel for applications in fusion reactors, crystalline intermetallic phases have been detected which are unstable relative to the amorphous state when irradiated at moderate temperatures with fast particles. It is expected that the transition to the amorphous state is accompanied by changes of the mechanical properties, which are of fundamental interest in this context. Until now, only a few studies on the evolution of mechanical properties during amorphization have been performed. Measurements of microhardness of the crystalline and the corresponding amorphous phase do not exist to the authors knowledge. In this communication, the authors present results on changes of microhardness, due to amorphization by fast ions. The measurements have been performed on a model alloy Ni[sub 3]P and on the brazed joint of stainless steel 316L, containing M[sub 3]P (M: Ni,more » Fe, Cr) as one of the phases. Though microhardness is not a fundamental property of materials, it is a manifestation of several related properties, such as yield stress, ductility, work-hardening, elastic modulus and residual stress states. It represents a resistance for indentation and is, therefore, appropriate for comparative purposes.« less

The direction dependence of the diffusion coefficient of 113 Sn has been measured in single crystals of Ni 61 Sn 39 by the layer grinding technique. The activation energies for diffusion parallel and perpendicular to the hexagonal c-axis are 2.93 and 3.07 eV, respectively, the D 0 -values 4.6 and 9.9 cm 2 /s. The experimental values of Φ=D ⊥/D ∥ are between 0.56 and 0.75 depending on the temperature. The experimental values are compared with theoretical values calculated for the different possible diffusion mechanisms. The main mechanism should be a normal vacancy mechanism superimposed by minority mechanisms the increase of which with rising temperature explains the temperature dependence of Φ

The temperature dependence of the lattice parameters in the beta -phase (BCC structure) of pure Zr and Zr-2 at.%Co was determined between 900 and 1550 degrees C by neutron diffraction. All diffraction patterns show a high diffuse background, which is identified as thermal diffuse scattering and originates from particular low-energy phonons in BCC Zr and its alloys. The lattice parameter of pure BCC Zr is given by aZr=3.14(17)*10-5T+3.579(5) in AA (T in K). Alloying with Co results in a contraction of BCC Zr, from which it is concluded that Co is dissolved substitutionally. The consequences of these results for the fast diffusion of Co in BCC Zr are discussed.

The direction dependence of the diffusion coefficient of Sn-113 has been measured in single crystals of Ni6Sn39 by the layer grinding technique. The activation energies for diffusion parallel and perpendicular to the hexagonal c-axis are 2.93 and 3.07 eV, respectively, the D0-values 4.6 and 9.9 cm2/s. The experimental values of PHI = D(perpendicular-to)/D(parallel-to) are between 0.56 and 0.75 depending on the temperature. The experimental values are compared with theoretical values calculated for the different possible diffusion mechanisms. The main mechanism should be a normal vacancy mechanism superimposed by minority mechanisms the increase of which with rising temperature explains the temperature dependence of PHI.

The direction dependence of the diffusion coefficient of 113 Sn has been measured in single crystals of Ni 61 Sn 39 by the layer grinding technique. The activation energies for diffusion parallel and perpendicular to the hexagonal c-axis are 2.93 and 3.07 eV, respectively, the D 0 -values 4.6 and 9.9 cm 2 /s. The experimental values of Φ=D ⊥/D ∥ are between 0.56 and 0.75 depending on the temperature. The experimental values are compared with theoretical values calculated for the different possible diffusion mechanisms. The main mechanism should be a normal vacancy mechanism superimposed by minority mechanisms the increase of which with rising temperature explains the temperature dependence of Φ

The phonon dispersion of the high-temperature bcc phase of Hf has been measured. The dispersion resembles very much those of the other group-IV metals: bcc Ti and bcc Zr. It is dominated by the low-energy L 2/3(1,1,1) mode and by the low-energy T1 [xi-xi-0] phonon branch with transverse [110BAR] polarization. These large-amplitude vibrations displace the lattice toward the stacking sequence of the high-pressure omega-structure and the low-temperature hcp structure, respectively, and are interpreted as dynamical precursor fluctuations toward these low-symmetry phases within the high-symmetry bcc phase.

The growth of omega-inclusions during the annealing of Ti-20 at.% Mo at temperatures below 500-degrees-C was studied by small-angle X-ray scattering (SAXS). A time-scaling behavior of the structure function was observed, but the scaling function F(x) appeared to be different from the "universal" shape found for many decomposing binary systems. To explain this effect, a trapping of molybdenum at the surface of the omega-inclusions is proposed, which might be due to the strong lattice misfit between the omega-inclusion and the cubic matrix. Furthermore, the inclusion size R(t) was found to increase, in a first step, as R proportional t1/3 with annealing time t, but then to stabilize at a constant value R almost-equal-to 75 angstrom. This is interpreted as a coarsening process, progressively hindered by attractive elastic interactions between omega-precipitates.

Lors de la transformation,l'alliage se decompose en deux phases cristallines non cubiques.Identification de ces phases.