Diffusion mechanisms in the Fe3Si alloys

In this paper,the possible reascon for the high thermal vacancy concentration and the low migration barriers for the Fe atorn diffusion in the stoichiometric DO3 structure Fe3Si have been discussed.The high thermal vacancy conenrrarion was attributed to the compression of Fe-Fe atcmic pairs and the tension of Fe-Si atomic pairs in Fe75Si25.The deformations (compression or tensicn)of the atompairs incrcase the interatomic potentials and thus dercrease the enthalpies of vacancy formation.The low minration bamiers for the Fe atom diffusion in Fe75Si25 were related to the symmetric property of the triangualar bamiers.Additionally.it was considered that the Si atoms in Fe3Si could probably migrate via nearest-neighbour jumps without disturbing the long-range order of atomic arrangements,provided that during the diffusion process the residence time on the antistructure sites is very short.     

Extended scaling in the magnetic critical phenomenology of the σ-phase Fe0.53-Cr0.47 and Fe0.52-V0.48 alloys

The magnetization of the sigma-phase Fe(0.53)Cr(0.47) and Fe(0.52)V(0.48) alloys was studied as a function of temperature and field. The experiments show that both materials behave magnetically as re-entrant spin glass systems. Field versus temperature diagrams were obtained where the locations of the paramagnetic phase, the intermediate ferromagnetic-like phase and the spin glass fundamental state were displayed. These diagrams are in qualitative agreement with the predictions of the mean field theory for the interplay between the ferromagnetic and spin glass orderings. The critical phenomenology near the para-ferromagnetic transition could be investigated. It was found that the paramagnetic susceptibility is quite well described by the extended scaling scheme, where the reduced temperature is written as τ = (T - T(c))/T. The value obtained for the susceptibility critical exponent γ is intermediate between the prediction of the 3D Heisenberg universality class and the large values observed in spin glasses, as previously found in other re-entrant systems. The data do not confirm the validity of the extended scaling in the ferromagnetic-like phase. Using either the conventional or extended scaling protocols, the exponents β and δ were found to have values close to those reported for spin glass transitions. Despite the relevance of disorder and the anomalous values determined for β, γ and δ, the Widom scaling relation holds as an equality.     

Kinetics and products of crystallization of Fe84−xVxB16 amorphous alloys

Isothermic crystallization of a series of Fe_84–xV_xB₁₆ (x = 0 to 8 at. % V) amorphous alloys was studied by the electrical resistivity, coercivity and⁵⁷Fe Mössbauer spectroscopy methods. Two stages of the process were observed corresponding to the primary -Fe(-V) formation by the growth-of-nuclei mechanism and the eutectic mixed borides — metal crystallization. Concentration dependences of kinetics parameters (Avrami exponent, activation energy) and the changes of electrical and magnetic properties were found and the influence of vanadium on the crystallization process is discussed.Dedicated to Dr. Svatopluk Krupika on the occasion of his 65th birthday.Thanks are due to Prof. L. Potocký, afárik University, Koice, for his kind providing the amorphous ribbons, and to Dr. J. Krejí from the author's Institute for the EDAX analyses.     

Order–disorder transformation of the O phase in Ti2AlNb alloys

The free energy of the O₂ (Ti₂AlNb) phase, based on the Gorsky–Bragg–Williams (G–B–W) approximation considering nearest-neighbour interactions, has been utilized to derive expressions for the transition and instability temperatures. The relations among the long-range order parameter, transformation temperatures and free energy as a function of the concentrations of Al and Nb have been established. The results are compared with those of earlier experimental and theoretical investigations.     

Electronic band structures of the alkali metals and of the noble metals and their α-phase alloys

In the monovalent metals the electronic band structure is strongly affected by the size of the band gap E _s-E _p at the Brillouin zone faces, a large gap implying a large distortion of the Fermi surface. Here E _s and E _p are the energies of the purely s-like and p-like states on the zone faces. We have made crude estimates of E _s-E _p for the alkali and noble metals, in terms of the s-p excitation energy Δ_sp of the free atoms. These suggest a single model which correlates most of the experimental information about the band structures of these metals. In particular the Fermi surface of lithium appears to make considerable contact with the zone faces. In the α-phase alloys of the noble metals, the solute always has a larger value of Δ_sp than the solvent, which raises the energy E _p relative to E _s. The Fermi surface becomes more nearly spherical in copper alloys than in copper, since E _p<E _s, whereas it distorts further in the gold alloys (E _p>E _s). This accounts for many Knight shift, electronic specific heat, magnetic susceptibility and other data on these alloys. Furthermore it provides the extension of Jones' explanation of the Hume-Rothery rule demanded by the non-spherical Fermi surface in pure copper and gold.     

Kinetics of the β→α transformation and the hierarchical nature of structure defects in the two-phase state in the Pd-H system

Possible types of time dependences p(t) describing the kinetics of the β→α a transformation in the Pd-H system are presented based on the theory of hierarchical structures. It is shown that one factor influencing the change in the time dependence p(t) is the degree of regularity in the distribution of defects in the hierarchical complexes and the strength of the hierarchical coupling determining the number of hierarchically coupled levels. Analysis of the relations obtained and the experimental data made it possible to distinguish the most likely physical causes for the retardation of the β→α a transformation. Fiz. Tverd. Tela (St. Petersburg) 49, 1621–1626 (September 1999)     

Low-temperature specific heat of icosahedral and amorphous Pd−U−Si alloys

Icosahedral (I) Pd_0.588U_0.206Si_0.206 can be obtained from melt-spun amorphous (A) ribbons by annealing. The specific heatC (measured betweenT=0.1 K and 20 K) shows very similar behavior for both phases. The main features ofC are as follows. (i) The vibrational heat capacityC _ph dominatesC at highT.C _ph is almost identical in both phases, in agreement with recent inelastic neutron scattering data. (ii) Shallow maxima in (C–C _ph)/T vs.T are found at 5.4 and 4.3 K forI andA phases, respectively, associated with magnetic order. These maxima are suppressed by 20% in an applied magnetic field of 6 T. (iii) A large quasi-linear contribution is observed with a low-T coefficient =165 mJ/mole U K² for theI phase and =120 mJ/mole U K² for theA phase. In the low-T region,C is hardly affected by a field of 6 T. This hints at the formation of a narrow 5f band with a comparable density of states for bothI andA phases.     

Variation in the structural state of the α-phase in the Pd-In-H system during the β → α transformation

Increasing the concentration of indium atoms and hydrogen was found in the α-phase of the foil of a Pd-In alloy (with 5.3 and 5.0 at % In) after its electrolytic hydrogenation and 500-h relaxation. It is shown that the structural variations in the foil during the β → α transformation are nonmonotonic.     

First-principles study of the effect of iron on the crystal structure,stability and chemical bonding in the β-based AlCu ordered η2-phase and the pretransition state of a solid solution

First-principles calculations showed that the thermodynamic stability of β-based ordered η₂-AlCu phase doped with Fe is due to iron substitution in the copper sublattice (Fe_Cu), which corresponds to the maximum number of Fe–Al bonds in the first cubic coordination polyhedron. This iron localisation leads to stable ω-like atomic displacements and pentagonal Al-nets in the (010) plane of η₂-AlCu(Fe). This phase with iron substituting copper (e/a?=?1.925) is an energetically preferred η-based non-canonical approximant of the icosahedral phase (e/a?=?1.86). The energy gain for the Fe_Cu position is determined by strong covalent Fe3d–Al3p bonding, while there is a weak Fe3d–Cu4s3d hybridisation for the Fe_Al substitution. Using a composite cluster model, we demonstrate that short-range order in the pretransition state of the β-Al–Cu–Fe solid solution observed prior to the precipitation of η-phase is stabilised due to formation of Fe–Al bonds in the first cubic coordination polyhedron of the composite cluster.     

Nucleation of the Al6(Fe,Mn)-to-α-Al–(Fe,Mn)–Si transformation in 3XXX aluminium alloys. II. Transformation in cast aluminium alloys

The nucleation behaviour of the homogenization-induced Al₆(Fe,?Mn)-to-α-Al–(Fe,?Mn)–Si transformation is investigated in a companion paper to part I (a study with roll-bonded diffusion couples). Diffusion experiments using silicon-coated Al–0.53?wt%?Fe–1.02?wt%?Mn alloy blocks allow control of the thermodynamic driving force for transformation within a microstructure typical of a cast ingot. As expected, this microstructure appears to give ready and yet stochastic nucleation as silicon diffuses into the alloy sections. In addition, transmission electron microscopy is used to analyse partially transformed particles in heat-treated alloy samples of fixed silicon content. This confirms the suggestion made in part I that the transformation preferentially nucleates at matrix grain/cell boundaries. Nucleation theory suggests this results from the ability of the boundaries to relieve volume changes associated with the nucleation event.     

Kinetics of A1 ↔ B2 phase transformations in Cu-Pd alloys

The effect of different initial states on the kinetics of formation of an ordered structure in the Cu-Pd alloys with palladium contents of 47, 49, 50, and 55 at % is studied. The activation energy of atomic ordering in an equiatomic CuPd alloy after severe plastic deformation has been estimated. A nearly single-phase ordered B2-type state in the Cu-49 at % Pd and Cu-50 at % Pd alloy has been achieved. For this phase state of the alloys, the lowest reported room temperature resistivity has been achieved. The temperature boundaries of phase transformations in the investigated Cu-Pd alloys are found to be in contradiction with the generally accepted phase diagram.     

Kinetics and Mechanism of Nanostructures in Oxidation of Si1—xGex Alloys

We investigate the oxidation behaviour of Si1-xGex alloys(x=0.05,0.15,and 0.25),The oxidation of SiGe films with different compositions was carried out in O2(dry)atmosphere at 800,900 and 1000℃,respectively,for various lengths of time,The thickness and property of the nanoparticle and nanolayer in oxide films and germanium segregation in oxidation of SiGe alloys are measured by using a high precision ellipsometer.The results are in good agreement with the Rutherford backscattering spectrometry,profile dektak instrument and high-resolution scanning transmission electron microscopy.We found that the Ge content in the oxide layer increases with the Ge content in SiGe alloys,and that the Ge content in the oxide film decreases with the increasing oxidation temperature and time,Rejection of Ge results in Piling up of Ge at the interface etween the growing SiO2 and the remaining SiGe,which forms a nanometre Ge-rich layer.Substantial interdiffusion of Si and Ge takes place in the remaining SiGe,which leads to the complicated distribution of Ge segregation.We find a nanometre cap layer over the oxide film after fast oxidation,in which there are many Ge nanoparticles,We analyse the kinetics and mechanism of the nanostructure of the oxide and Ge segregation in oxidation of Si1-xGex alloys.     

On the control of microstructure in rapidly solidified Nd–Fe–B alloys through melt treatment

Rapidly solidified (RS) Nd₂Fe₁₄B alloys were prepared by melt-spinning under different melt treatment conditions i.e., the melt temperature was varied prior to ejection onto the quenching wheel. X-ray diffraction, transmission electron microscopy, thermal analysis and magnetic measurement were conducted on the as-quenched alloys to investigate their structural and magnetic characteristics. RS Nd₂Fe₁₄B alloys may display a variety of microstructures depending upon the thermal history of the melt before ejection: it was possible to synthesize an entirely amorphous structure, a partially amorphous structure containing nuclei and/or nanophases and a nanocrystalline structure. The relationship between the formation of crystalline nuclei or nanophases and the thermal history of the melt was studied. A lower melt ejection temperature produced a nanocrystalline microstructure, while higher melt ejection temperatures (T>1723 K) largely eliminated the presence of nuclei and associated nanophases and produced an amorphous product. The experimental results indicated that optimization of the melt treatment conditions will produce rapidly solidified Nd–Fe–B alloys with a more uniform microstructure.     

Magnetic properties and specific heat of Tm1−xLuxCu2Si2

The tetragonal TmCu₂Si₂ compound is the only magnetically ordered material of the RECu₂Si₂ group for which crystal field parameters were determined. Quadrupole splitting measured by means of Mössbauer spectroscopy showed that two lowest lying states are nonmagnetic singlets. Therefore, this material is likely to have an induced magnetic moment, mainly due to mixing of the two lowest states. We performed specific heat and magnetic susceptibility measurements of Tm_1−xLu_xCu₂Si₂ (x = 0, 0.025, 0.050, 0.10, 0.25, 0.50 and 1) alloys to determine the crystal field level scheme and compare it with the Mössbauer data. The saturation magnetization of the antiferromagnetic phase was calculated to be 3.2 μ_B and the moment is directed along the tetragonal c axis. No direct experimental evidence is known to support this prediction.     

Effects of a Cu-contained compound on the microstructures and thermoelectric properties of Zn–Sb based alloys

Zn–Sb based alloys with Cu₂Sb addition were prepared using spark plasma sintering technique and the effects of a Cu-contained intermetallic phase on the microstructures and thermoelectric properties were examined. Rietveld refinement reveals that there are many phases in the alloys, which involve β-Zn₄Sb₃, a major phase ZnSb, a small amount of an intermetallic compound Cu₅Zn₈ and unidentified impurity phases, the quantities of ZnSb and Cu₅Zn₈ increase from 67.3 wt.% to 91.8 wt.% and 0–4.3 wt.% with Cu₂Sb additive increasing, respectively. The ZnSb plays a fundamental role in controlling the thermoelectric properties, and Cu₅Zn₈ is of great significance to optimize the transport properties. The maximum thermoelectric figure of merit ZT of 0.72 is obtained for the alloy (Cu₂Sb)_0.05–(Zn₄Sb₃)_0.95 at 654 K, which is 0.25 higher than that of undoped β-Zn₄Sb₃ at the same conditions. Therefore, we conclude that a proper addition of Cu₂Sb can contribute to the improvement of thermoelectric properties of Zn–Sb based alloys.     

The heat treatment response of semisolid formed Al–5Fe–4Cu alloy

The effect of heat treatment on both microstructures and performances of semisolid Al–5Fe–4Cu alloy was investigated with scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) and tensile test. The results showed that the semi-solid Al–5Fe–4Cu alloy’s response to the solution heat treatment (SHT) is abnormal compared with the conventional semi-solid Al alloys, i.e. the tensile properties of the semi-solid Al–5Fe–4Cu alloy decreased significantly after SHT. Nevertheless, the semi-solid Al–5Fe–4Cu alloy subject to direct ageing without SHT could exhibit conspicuous improvement of the tensile properties. And, the reason of the abnormal results has been analysed and discussed in detail.