Stoichiometric Intermetallic Compound Research Articles

High Temperature Resistant Intermetallic Nial-Based Alloys with Refractory Metals Cr, Mo, Re - Structures - Properties - Applications -

ABSTRACTThe stoichiometric intermetallic compound NiAl with B2 superlattice structure exhibits superior physical and high-temperature mechanical properties, and excellent oxidation resistance. The main disadvantages of polycrystalline NiAl are the lack in plasticity and fracture toughness below the brittle-to-ductile-transition temperature of about 550°C. Insufficient high-temperature strength and creep resistance occur at temperatures above 800°C. Despite these facts NiAl-based alloys are still considered as promising structural materials for high-temperature applications. The refractory metals Cr, Mo, and Re with b.c.c. and h.c.p. lattice structures form with NiAl quasi-binary eutectic systems, showing high melting temperatures and thermally stable microstructures. Elasticity, solid solution hardening, fibre reinforcement, and creep properties were investigated in view of the constitutional defect structure and microstructural features. Especially the fibre reinforced NiAl matrix composites possess optimum high-temperature strength up to 1200 °C, and improved creep resistance as well.

The vacancy wind effect for chemical diffusion in intermetallic compounds

In this paper we examine the behaviour of the vacancy wind factors in the intrinsic diffusivities and the chemical or interdiffusivity in highly ordered stoichiometric AB intermetallic compounds. We show that the six-jump cycle gives quite different limits for these factors from those given by the well-known Manning expressions. Remarkably, there is non-trivial behaviour of these factors even when the vacancy flux is zero. The findings are all verified by computer simulations of a four-frequency model in the square planar lattice and an Ising model in the B2 structure. The consequences of the findings for the interpretation of experimental data are commented on.

Relating tracer and chemical diffusion coefficients in intermetallic compounds taking the DO 3 and A15 structures

In this article we employ computer simulation to explore the validity of the Darken/Manning relation between the chemical diffusion coefficient and the tracer diffusion coefficients of the components in stoichiometric intermetallic compounds A 3B taking the DO 3 and A15 structures at vanishingly small vacancy contents. The analysis centres on the validity of Manning’s random alloy expression for the vacancy wind factor. The models for both the DO 3 and A15 structures use eight atom-vacancy exchange frequencies. For the DO 3 structure it is found that the actual vacancy wind factor is usually somewhat larger than that predicted by Manning but overall the agreement is good. At worst the use of Manning’s expression would underestimate this factor by about 30 or 40%. For the A15 structure a similar result is found except when diffusion along the chains in the structure is rapid. Then Manning’s expression fails badly when a constant geometrical tracer correlation factor is employed. In both the structures if the geometrical correlation factor is varied to reflect the structure actually explored by the atoms (mainly the majority atoms A) the agreement is improved very dramatically.

The synthesis of NbSi2 by mechanical alloying

The stoichiometric intermetallic compound NbSi2 has been synthesized by mechanical alloying (MA) elemental Nb and Si powders. The alloying process has been investigated by means of x-ray diffraction (XRD) and differential scanning calorimetry (DSC). It was found that the formation of the Nb2Si intermetallic compound occurs abruptly after 65 min of milling without any interruptions during the alloying process. However, short interruptions at a 5 min interval during ball milling result in a gradual reaction for the formation of the NbSi2 compound as well as a new metastable bcc structured solid solution. We conclude that the temperature rise during mechanical alloying plays an important role in initiating the abrupt reaction after an incubation milling time.

Magnetism, disorder and frustration in heavy-fermion compounds

We examine the magnetic properties of four strongly correlated electron compounds based upon the presence or absence of crystallographic disorder and geometric frustration. All four materials (URu2Si2, CePd2Al3, UNi4B and URh2Ge2) exhibit unusual and not fully understood behavior in their magnetic ordering that are surprising for stoichiometric intermetallic compounds. We summarize these behaviors and relate them to the appearance of disorder and frustration

Films of the intermetallic compound CuTi2 and their applications in Cu/CuTi2/Si contact structures

AbstractTo prepare constituent materials applicable to a Cu metallization technology of Si‐LSI, the preparation conditions of Cu‐Ti intermetallic compound films have been examined and the thermal stability of the Cu/CuTi2/Si contact structure has been studied using CuTi2 compound films as a diffusion barrier. As a result, a stoichiometric intermetallic compound film of CuTi2 was prepared successfully at a substrate temperature of 300°C by dc sputtering using a composite target with an area ratio of Cu : Ti = 51 : 49.From the study on the CuTi2/Si contact structure, it was revealed that the CuTi2 phase was stable up to 500°C and a thin interface layer of Ti silicide was formed at the interface of CuTi2/Si but the excess diffusion and/or reaction have not been observed. Ti silicide layer formed at the interface is believed to be effective for low contact resistance. To examine whether or not CuTi2 films are suitable for Cu metallization technology, the thermal stability of Cu/CuTi2/Si contact structure was examined. It was found that the structure is successfully stable up to 500°C and is useful for Cu metallization technology. The reason is believed to be as follows: the interface of Cu/CuTi2 is in the chemical equilibrium state and CuTi2 itself is a stable compound phase at high temperatures in the Cu‐Ti alloy system.

Atomic disorder induced by mechanical milling in the intermetallic compound CoAl

The stoichiometric intermetallic compound CoAl with the CsCl-structure was subjected to ball milling. Structural changes during ball milling were studied by measurements of the magnetization and the lattice parameter. The behaviour of Coal upon milling turns out to be quite similar to that of CoGa. The increase of the magnetization and the decrease of the lattice parameter with the milling time show that, similar to CoGa, triple-defect disorder is generated by ball milling. By comparing the measured magnetization to the magnetization of Co x Al 100− x compounds with excess Co, the concentration of defects is derived for milled samples. In order to interpret our results, the existing experimental data of lattice parameters, X-ray densities calculated from the lattice parameters and macroscopic densities for Co x Al 100− x compounds were analyzed according to Edelin's equations. The defect volumes for vacancies and anti-site atoms obtained by fitting these experimental data have reasonable values. By means of the defect concentrations obtained from our magnetization measurements and the defect volumes by fitting the experimental data, the change of the lattice parameter was calculated by means of Edelin's equation. The agreement between the calculated lattice parameter and the measured lattice parameter is quite satisfactory.

A thermodynamic evaluation of the iron-zinc system

Experimental thermodynamic properties and phase equilibrium data have been used in deriving a thermodynamically consistent analytical description of the thermochemical properties and phase diagram of the Fe-Zn system. A set of coefficients is used to describe the conventional Gibbs free energy of the pure components and stoichiometric intermetallic compounds and the excess Gibbs energies of the solution phases are given. Thermodynamic and phase boundary values calculated with these coefficients agree well with experimental data within experimental error.

Theory of antiferromagnetic ordering in Pt3Fe alloys. I. Self-consistent band-structure calculation

The band structure of the paramagnetic phase of the stoichiometric intermetallic compound Pt3Fe was calculated self-consistently using the linearised hybridised KKR scheme in the framework of the local density approximation of the exchange-correlation potential. The results of the band-structure calculations are discussed in connection with the unusual antiferromagnetic ordering in this compound near stoichiometric composition. Nesting-type features are found at the Fermi surface of the paramagnetic phase. The calculation of the static susceptibility function chi (q) demonstrates the existence of peaks for the q vectors corresponding to antiferromagnetic structures.

Characterization of the surface net and electronic structure of AuGa2(001)

The (001) face of the stoichiometric intermetallic compound AuGa2 has been studied for the first time using Auger electron spectroscopy, low energy electron diffraction, and electron energy loss spectroscopy. The argon ion bombarded and annealed surface is primarily Ga terminated, and structurally is comprised of two perpendicular domains which are based on a ((2)1/2×(18)1/2 )R45 unit cell. Bulk and surface plasmon loss features were observed at 15.6 and 10.4 eV, respectively. Loss features were also observed for the valence band and Ga 3d quasiparticle transitions.

The antiferromagnetic and ferromagnetic properties of Fe2MnSi

Abstract The results of extensive magnetization, susceptibility, X-ray and neutron powder diffraction measurements on the stoichiometric intermetallic compound Fe2MnSi are reported. At room temperature the compound is paramagnetic enabling the crystallographic structure to be established as L21 with some degree of preferential DO3 disorder. Between 69 and 214 K the compound is ferromagnetic but below 69 K the manganese atoms become antiferromagnetically ordered whilst the iron atoms remained ferromagnetically aligned. This coincides with a change in the configurational magnetic symmetry to rhombohedral, in which the antiferromagnetically aligned moments are directed along the [111] direction.

High density of states and strong exchange interactions in Ni 3Ga

The magnitude and temperature dependence of the magnetic susceptibility of the stoichiometric intermetallic compound Ni 3Ga can qualitatively be accounted for by a density of states curve varying appreciably over a few hundredths of an electronvolt in the Fermi region and a large exchange enhancement factor. These conclusions are confirmed by the non-linear behaviour of the magnetization in high magnetic fields and by giant moment inducement when dissolving small amounts of iron.