H. Bakker

Vacancies in CoGa

Abstract Thermal and constitutional vacancies are studied for five compositions of the intermetallic compound CoGa. Macroscopic density measurements are used to determine the structural and frozen-in thermal vacancy content. The formation of thermal vacancies is derived from thermal expansion measurements of the macroscopic length and the lattice parameter up to 900°C. The results of these vacancy concentration measurements are interpreted in terms of triple-defect formation. The temperature dependence of the creation of triple defects cannot be described by a simple Arrhenius behaviour. However a simple nearest-neighbour interaction model, with a Bragg-Williams approximation gives a very good description of all results both as a function of temperature and composition. From a simultaneous least-squares fit of all results with this model two energy parameters and a composition dependent triple-defect formation entropy are derived. In this way some insight into the interaction energies in CoGa is obtained.

A calculation of vacancy concentration and thermodynamic properties of VIII-IIIA intermetallic compounds with the B2-structure

Abstract VIII-IIIA intermetallic compounds with B2-structure exhibit a special type of disorder: at higher temperatures large numbers of group VIII anti-structure atoms and of vacancies may be formed. Calculations were carried out on the vacancy, and anti-structure atom concentration and on other thermodynamic properties of these compounds as a function of composition and temperature. Starting point of these calculations was a nearest neighbour interaction model, while the Bragg-Williams approximation was applied. A formalism developed by Schapink was used. As a result three basic equations were obtained from which defect concentrations and thermodynamic activities can be computed. A comparison was made with previous calculations by Libowitz and Lightstone, and Neumann et al . It was shown under which simplifications our results transform into theirs. As an application a fit was made of the vacancy concentrations in CoGa and CoAl. Moreover for CoGa a fit of the thermodynamic activity of gallium was made. This gave some insight in the interaction energies.

Production and magnetic properties of nanocrystalline Fe and Ni

Abstract Magnetic properties of pure nanocrystalline iron produced by ball-milling and heavy cold deformation are investigated, using low field magnetization measurements at low temperatures and Barkhausen-noise technique at room temperature. It was observed that the saturation magnetization was practically constant up to the grain size of D = 7 nm. On the other hand, the coercive force, H, determined from the Barkhausen-noise curves, was very sensitive to the grain size. Furthermore, the Barkhausen-noise values showed a systematic, but different behavior with the grain size for the ball-milled powders and for the cold-worked wires. Similar measurements were carried out for cold-worked nickel wires.

Kinetics of surface segregation in alloys

Abstract The kinetics of surface segregation in ordering alloys are studied with the help of a multilayer model in slabs of different thicknesses. The time evolution of the concentrations of atomic layers perpendicular to the (100) planes of an AB-type BCC ordering alloy (CoFe) are calculated. As a result of the competition between the surface segregation of A atoms and ordering in the bulk, a metastable configuration with two anti-phase boundaries (APB) inside the slab, with odd numbers of layers, was obtained if we started from an ordered initial state with B atoms on the free surfaces (B-termination). The effect of the temperature was also investigated for a slab with 41 layers and from the calculation of the free energies of the metastable and stable states it was shown that the metastable state can exist below T 1 = 0.95 T c , where T c is the critical temperature of the order-disorder transition in bulk. It was obtained that T 1 decreases with decreasing thickness of the slab, while the difference of free energies of the metastable and stable states slightly increases. The effect of the slab thickness is a typical size effect: the time necessary to reach the steady state decreases with decreasing number of layers.

On the elastic mismatch in the order-disorder transformation and solid state amorphization of intermetallic compounds. II, Criteria for the solid-amorphous transformation in intermetallic compounds

Abstract Using simple thermodynamic considerations for the Gibbs-free energies of the crystalline and amorphous states a general condition for the solid-state amorphization of intermetallic compounds by irradiation or mechanical impact was derived. It was obtained that the amorphization is possible if the maximum elastic energy, ΔH el m , stored during the (chemical) order-disorder transition is larger than ΔH top a , the enthalpy of the topological change of amorphization. This condition can be rewritten into the form: the amorphization is not possible if the order-disorder temperature is less than the melting temperature. T c T m , and if T c > T m —in contrast to Johnsons condition—the possibility of the amorphization depends on the relative magnitude of the elastic mismatch energy and the chemical energy of ordering. If their ratio is large enough the amorphization is possible. Detailed calculations, carried out for a large number of compounds, led to a satisfactory agreement with the experimental observations.

Atomic disorder and phase transformation in L12-structure Ni3Si by ball milling

Abstract The structural evolution of the L12-compound Ni3Si upon ball milling was followed by X-ray diffraction, high-field magnetization measurement and differential scanning calorimetry (DSC). Anti-site disorder was generated in the early stages of milling and a phase transformation from the disordered L12-compound to the nanocrystalline f.c.c. solid solution of silicon in nickel was observed after milling for long periods. The long-range-order parameter decreases monotonously with time in the early stages of milling and reaches zero after long periods of milling. The lattice parameter and unit-cell volume also decrease gradually in the early stages of milling. A discontinuous decrease of the lattice parameter is observed during the phase transformation. The magnetic behaviour at 4.2 K of ball-milled samples differs from that of the starting compound. Exothermic peaks resulting from atomic reordering and phase restoration are evident in the DSC scans of ball-milled samples. The variation of the transition heat and temperature with milling time in the intermediate stages gives more insight into the process of phase transformation.

Nucleation controlled transformation in ball milled FeB

Abstract X-ray diffraction reveals the loss of sharp crystalline peaks, and differential scanning calorimetry shows nucleation controlled transformation besides grain growth in mechanically ground FeB intermetallic compound. On the other hand, the Mossbauer hyperfine parameters are close to the values of the disordered intermetallic compound (α-FeB) and different from those of the amorphous FeB produced by sputtering or evaporation. This controversy is attributed to the disordered interfacial phase existing at the grain boundaries.

On the elastic mismatch in the order-disorder transformation and solid state amorphization of intermetallic compounds—I. Estimation of the elastic mismatch energy during order-disorder transition

Abstract Using a method similar to the thermodynamic theory of almost completely ordered alloys an expression for the elastic mismatch energy of an ordered alloy as a function of the long-range order parameter, η, is derived from the continuum elastic theory. It is shown that, similarly to the chemical ordering energy in Bragg-Williams approximation, this has a simple (1 − η2)-type dependence. It is illustrated by detailed calculations that the relation obtained can also be derived requiring that the slope of the elastic mismatch energy at η = 0 should be equal to zero. Is is also shown that the expression proposed for the elastic mismatch energy of a (disordered) solid solution in the Miedema-model contains the interactions between the image elastic fields as well and can be considered to be on the same level of approximation as our result. The maximum elastic energy which can be stored during a disordering process is compared to the elastic mismatch energy of the solid solution (at the same atomic fraction). It is concluded that the differences between the above values are comparable to the differences obtained by using uncorrected volumes for the atoms and holes or using corrected values of them according to the Miedema-model.

Phase transformations in the NbAu system by ball milling and the study of the metastable phases

Abstract A detailed study on the NbAu binary alloy system upon ball milling is reported. An analysis of the energy levels of the NbAu alloy reveals that the solid solution has a lower enthalpy than the amorphous state for all compositions. The ball milling experiments were performed on five compositions of the NbAu alloys. All five alloys transformed to solid solutions after 40h of grinding. The structures of these solid solutions are different depending on the composition of the starting material. In further experiments, the properties of the metastable phases obtained by ball milling were studied. The lattice parameter of the solid solutions turns out to obey Vegards law. The exothermic heat of the phase restoration appears to be approximately proportional to the enthalpy as predicted by Miedemas semi-empirical model. The magnitude of the peak temperatures and the activation energies follows the same trend as the melting temperaures. Examination of particle size and shape for two composition gives more insight in the milling process. Finally, the results for prolonged milling are presented.