Materials Science and Engineering最新文献

The elastic and plastic strain sensitivity coefficients of resistance of rhodium, iridium and the Pd₂₀Ag₈₀ alloy are measured under various annealing and cold- or hot-working conditions. The experimental results are consistent with the theoretical predictions. This work has verified the theory presented in a previous paper. The three-dimensional effective electronic number should be used in the calculation of the elastic strain sensitive coefficient of resistance. In case plastic deformation occurs, the effects on the strain sensitivity coefficient of resistance by elastic deformation diminish and a different formula for plastic deformation should be employed.

We report results of ⁵⁷Fe Mössbauer measurements on the magnetically concentrated amorphous Fe₉₀Zr₁₀ alloy which give the first evidence of a microscopic origin for the low temperature magnetization anomaly at T_f = 35 K. This is shown by an anomalous increase in the average magnetic hyperfine field $\text{B}{}_{\mathrm{<mtext>hf</mtext>}}\text{(T)}$ below T_f, similar to that observed in crystalline and amorphous re-entrant spin-glass alloys in the concentration range near the percolation threshold.

Detailed experimental studies of quasi-crystalline Al_100−xMn_x(Si) alloys have shown that the magnitude of electrical resistivity as well as the excess specific heat term δγ are proportional to the percentage of manganese in the samples, as is the case in well-known very dilute AlMn alloys. We argue that the excess specific heat term and the high resistivity (about 150–200 μω cm for the quasi-crystalline phase) can be accounted for within the extended Friedel-Anderson s-d model, in which the dominant scattering is due to the hybridization of d electrons with the sp conduction band. Therefore, in our physical picture, the high resistivity in AlMn quasi-crystals is mainly due to the contributions from two effects: the existence of narrow resonant states of manganese at the Fermi level and the existence of correlation lengths of the order of 100–300 Å which cause the incoherence of transport electrons.

High pressure X-ray diffraction studies have been performed on rapidly solidified AlMn alloys (35 and 40 wt.% manganese) containing the quasicrystalline phase. Pressure experiments were undertaken using a diamond anvil cell, and X-ray diffraction analysis in the energy dispersive mode was performed using the synchrotron radiation facility at HASYLAB-DESY in Hamburg. Both the quasicrystalline phase and the AlMn α phase were found to have stable structures for pressures up to 47 GPa and they showed reversible behaviour for decreasing pressures. Using a Murnaghan-type equation of state, the best values of the bulk modulus and its pressure derivative for the icosahedral phase were found to be B₀ = 146 GPa ∓ 10 GPa and B′₀ = 1.7 ∓ 0.5 respectively. The bulk modulus and the pressure derivative for the α phase are B₀ = 85 GPa ∓ 2GPa and B₀^′ = 2.9 ∓ 0.1 respectively.

A series of AlMn ribbons were prepared by planar flow casting in the range of compositions between 4 and 15 at.% Mn. Their thermal stability was investigated by X-ray diffraction, transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). X-ray diffraction measurements on the as-quenched ribbons showed the presence of a supersaturated Al (Mn) solid solution and the icosahedral phase. The morphology depends on the composition and on the local quenching rate. Several exothermic effects (at least two) were observed on the DSC curves. The temperature and intensity of these effects varied when the manganese content was changed.

Identification of the individual stages along the DSC curve was carefully followed by stopping the DSC experiment at various points during the exothermic evolution, rapidly cooling the specimen and carrying out the X-ray diffraction measurements and the observations by TEM on the same specimen. It is shown that the thermal stability of the icosahedral phase is greatly influenced by its initial morphology.

The configurational entropy of two-dimensional random tilings of Penrose rhombi has been calculated by two methods. Estimating the configurational degeneracy by determining the number of available configurations of each tile during physical model building gave an entropy of 0.495k per tile for a compositionally unconstrained tiling. Geometrical models based on the number of configurations around each vertex without correlations between them, gave higher values.

The magnetic and structural properties of microcrystalline CoFeSiB films prepared by sputtering were studied. This new soft-magnetic material exhibits a high rotational permeability of 1200 combined with a high saturation magnetization of 1.6T. The soft-magnetic properties of this material can be explained by the fact that the thin film consists of f.c.c. crystallites with a perfect <111> texture normal to the film plane. Because of this textyre and the multiple twinning of the crystallites a six-fold symmetry and thus a small inplane anisotropy are obtained.

An attempt has been made to investigate the microstructure and soft magnetic properties of FeAl alloys produced by a rapid solidification technique, with the aim of estimating their aptitude as core elements in rotating machinery. Examinations by optical and transmission electron microscopy revealed that the specimen containing aluminium up to about 23 at.% consists of ductile ferrite, whereas further addition of aluminium gives rise to embrittlement by the formation of an ordered b.c.c. compound. Furthermore, the rapidly solidified alloys exhibited high saturation induction values, more than 1.88 T in the alloys with aluminium content below 10 at.%. For the coercivity behaviour and core loss at 50 Hz, notable improvement was achieved after annealing at elevated temperatures. It was thus concluded that the present alloys have potential for practical use as magnetic core materials in rotating machinery.

The investigation of the bulk electronic structure, interface and surface properties of metallic glasses by photoelectron spectroscopy is illustrated by an example of each of these.

We have studied the Hall effect in amorphous paramagnetic Zr_xCo_100−x alloys, with 48 ⩽ x ⩽ 80, between 1.5 and 300 K. In these alloys R_H shows a very small temperature dependence but a large variation with concentration, being negative in the high cobalt region and positive on the zirconium-rich side. This behaviour, which has also been found in other zirconium-based glasses, cannot be explained classically. By comparison with the ZrFe system we propose that a significant contribution to R_H via the side-jump mechanism, as proposed by Berger, has to be considered when trying to explain the behaviour of R_H in these ZrCo metallic glasses.