Computer simulation of the structure of MZr2 liquid and amorphous alloys

In the present study, we compare the structure of NiZr₂ and CuZr₂ disordered (liquid and amorphous) alloys. While Cu and Ni have similar atomic radii, the formation heats of these alloys differ by more than a factor of two. Moreover, the most stable crystal phase in the CuZr₂ alloy has the C11b lattice while the most stable phase in the NiZr2 alloy has the C16 lattice. Comparing these two alloys allows us to directly explore how the chemistry and atomic size affect the disordered phase structure. We find that all differences in the structures of the disordered alloys are readily explained by the smaller Ni–Zr separation driven by the higher heat of mixing and the structure of the disordered alloys does not necessarily resemble the local packing structure of their crystalline counterparts. Comparison of the disordered alloys and their most stable crystal phase structures explains the better glass formability of the CuZr₂ alloy.     

The origins of phase stability in ordered and disordered Ni-Pt alloys

Using the atomic-sphere approximation formulation of the Korringa-Kohn-Rostoker coherent potential approximation (KKR CPA) method, we have studied the effects of relativity on the electronic structure of ordered and substitutionally disordered Ni-Pt alloys. The inclusion of mass-velocity and Darwin terms are found to be essential for describing the experimentally observed ground-state properties. For the stability of disordered Ni-Pt alloys we find that, in addition to relativity, the minimization of charge-transfer effects are important. We also find that the treatment of ordering tendencies based on the band energy term alone is not sufficiently accurate for alloys with charge-transfer effects. Further analysis, in terms of basis functions, densities of states and non-spherically averaged charge densities, indicate the importance ofs- andd-electrons of Pt for the stability of both ordered and disordered Ni-Pt alloys.     

Large negative magnetic contribution to the thermal expansion in iron-platinum alloys: quantitative theory of the Invar effect

We show that the large negative magnetic contribution to the thermal expansion in disordered Fe-Pt alloys can be understood within the disordered local moment (DLM) approach. On the basis of first principles calculations we quantitatively describe the spontaneous volume magnetostriction for various Pt concentrations. It is found that the Invar effect in these alloys is entirely related to the state of thermal magnetic disorder modeled by the DLM states. We also show that the experimentally observed anomaly in the temperature dependence of the magnetization is due to a spontaneous reduction of the local magnetic moments rather than to "hidden excitations."     

The influence of the metalloid species on the electronic structure of disordered transition-metal-metalloid alloys

The results of x-ray emission investigations of the electronic structure of Fe-Si and Fe-P disordered alloys are presented. Relying on the analysis of the spectrum parameters and data available in the literature, a qualitative model of the electronic-structure formation in disordered Fe-P alloys is proposed. The model allows one to account for the concentration dependence of the average magnetic moment per iron atom in the disordered systems investigated.     

Electronic structure calculations of Fe-rich ordered and disordered Fe-Al alloys

Tight Binding Linear Muffin-Tin Orbital (TB-LMTO) electronic calculations are presented for the magnetic and structural properties of ordered and disordered FeAl alloys. The total energy, bulk modulus, lattice parameter and magnetic moments of B2, D03 and B32 ordered structures and A2 disordered structure were calculated for different compositions. The different structures are obtained by varying the position of Fe and Al atoms in a BCC superstructure. In this way, we examine the order-disorder transition that takes place in these alloys. Disordered alloys present both larger Fe magnetic moment and lattice parameter than ordered ones. In this work comparison of the calculated quantities with available experimental results is provided and it can be concluded that the results are in quantitative agreement with the experimental trends. Received 7 May 2002 / Received in final form 20 September 2002 Published online 4 February 2003 RID="a" ID="a"e-mail: eaf@we.lc.ehu.es     

Single-site approximations for equilibrium and transport properties of random alloys

Introducing the configuration averaging as a projection operator, we propose a new formulation of the average t-matrix and the coherent potential approximations that treats on equal footing both the equilibrium and the residual transport properties of disordered alloys.     

Atomic ordering and ferromagnetism in binary alloys

Atomic and magnetic ordering in binary alloys are studied within the mean field approximation. The influence of magnetism on the spatial order-disorder critical temperature and, conversely, the influence of chemical order on the Curie temperature are analyzed in detail. Phase diagrams for various cases are presented. In particular we find that the interplay of the two order parameters yields in some cases unusual sequences of phases as a function of temperature. For example we obtain a possible sequence with increasing temperature of spatially ordered ferromagnet → spatially ordered paramagnet → spatially disordered ferromagnet → disordered paramagnet.     

Hydrogen Chemisorption on Disordered Binary Alloys

The one-dimensional tight-binding model and the Green's function method are used to study the chemisorption energy of an atomic H on disordered binary alloys within the average T-matrix approximation. The one-electron chemisorption theory is used to describe the chemisorption process. The chemisorption energy is calculated as a function of the concentration percentages of the two components of the disordered binary alloys. The effect of the surface segregation of the CuNi system is also investigated.     

Ab-initio calculations of the electronic structure of impurities and alloys of ferromagnetic transition metals

We review recent theoretical work on the electronic structure and the magnetic properties of ferromagnetic transition-metal alloys. All calculations are based on density-functional theory in the local-spin-density approximation. We report about calculations for dilute alloys using the KKR-Green's function method and for concentrated disordered alloys using the charge-self-consistent KKR-CPA method.     

Effect of short-range order on electronic and magnetic properties of disordered Co-based alloys

We here study electronic structure and magnetic properties of disordered CoPd and CoPt alloys using augmented space recursion technique coupled with the tight-binding linearized muffin tin orbital (TB-LMTO) method. Effect of short-range ordering present in disordered phase of alloys on electronic and magnetic properties has been discussed. We present results for magnetic moments, Curie temperatures and electronic band energies with varying degrees of short-range order for different concentrations of Co and try to understand and compare the magnetic properties and ordering phenomena in these systems.     

Ordering of ternary alloys in the self-consistent field approximation

In the self-consistent field approximation, without allowance for correlation, a static theory of the ordering of ternary alloys is constructed which makes it possible to take into account the long-range nature of the interatomic interaction forces in substitutional and interstitial alloys. The theory enables one to take into account the possible existence in the alloy of two subsets of points differing by the species of atoms that occupy them. The stability of the disordered state in such alloys is analyzed and an expression obtained for the temperature at which the disordered state loses stability.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 61–64, November, 1980.     

Magnetic phase diagram of the FexMn0.7-xAl0.3 alloys series obtained by Mössbauer spectroscopy

By Mössbauer spectroscopy the magnetic phase diagram of the Fe_xMn_0.7-xAl_0.3 spin glass alloy has been obtained. X-ray diffraction (XRD) has shown that all the alloys are in the BCC phase. The broad spectral lines observed in the Mössbauer spectra are indicative of the disordered character of these alloys. Depending on the composition and the temperature the alloys behave as paramagnetic (P), ferromagnetic (F) and reentrant spin glass (RSG). For the alloys rich in iron the only detected magnetic transition is from F to P. For medium iron content two transitions were observed, namely from RSG to F and from F to P. The RSG phase is obtained as a consequence of the disordered character of the alloys and the competitive exchanges due to iron and manganese atoms.     

A new first principles approach to calculate phonon spectra of disordered alloys

The lattice dynamics in substitutional disordered alloys with constituents having large size differences is driven by strong disorder in masses, inter-atomic force constants and local environments. In this paper, a new first principles approach based on special quasirandom structures and an itinerant coherent potential approximation to compute the phonon spectra of such alloys is proposed and applied to Ni?.?Pt?.? alloy. The agreement between our results and experiments is found to be much better than for previous models of disorder due to an accurate treatment of the interplay of inter-atomic forces among various pairs of chemical species. This new formalism serves as a potential solution to the longstanding problem of a proper microscopic understanding of lattice dynamical behavior of disordered alloys.     

Study of Disordered Fe2Cr(1−x)Mn x Al Alloys

Hyperfine Interactions - A series of ferromagnetic alloys Fe2Cr(1?x)Mn x Al were prepared by arc melting for x=0 to 0.05. The alloys are single phased, but disordered with B2 type structure....     

替代式合金系统的电阻率

本文将文献[1]的无序晶态合金电阻率理论推广到包含长程有序的系统,从而建立了适用于晶态金属,无序及有序替代式合金的Ziman型电阻率理论。根据这个理论我们详细讨论了这类系统电阻率的温度依赖性。文中着重指出:合金系统结构因子的超结构峰对电阻率有重要贡献。这个贡献在低温下是一个T²项,它比电子-电子散射引起的T²项大得多,因而合金系统电阻率在T<<Θ(Θ是德拜温度)时有ρ≈ρ₀+ρ_a(T/Θ)²+ρ_i(T/Θ)⁵的形式。据此,许多A-15化合物正常态电阻率在低温下的反常行为很容易解释。作为例子,我们将低温电阻率的理论表达式与V₃Si的测量值作了比较,符合得很好。     

57Fe Mössbauer spectroscopy on FePt invar alloys

From ⁵⁷Fe Mössbauer effect investigations on disordered FePt-Invar alloys we determine the concentration dependence of both the mean isomer shift and the mean magnetic hyperfine field. Evaluating these changes in terms of charge and spin densities at the nuclei and combining the results with X-ray investigations we find the electronic state of the iron atoms being 3d⁷4s¹ in these alloys. We present a “magnetic” phase diagram and discuss the influence of the martensitic transformation.     

The electronic structure of non-dilute alloys

Recent developments in the multiple scattering theory of disordered substitutional alloys are reviewed. In view of experimental advances in alloy spectroscopy, the approaches involving the most realistic single particle Hamiltonians are emphasized. Specifically, we focus on the application of the average t-matrix approximation to the muffin tin model of the alloy. It is shown that the electronic structures of α phase CuZn and paramagnetic CuNi alloys can be characterized in terms of their respective complex energy bands. The interpretations of several experiments, bearing on the real as well as the imaginary parts of the complex bands, are discussed.     

Hyperfine interactions in hydrogenated vanadium-iron alloys

The changes in the magnetic properties of disordered bcc V-Fe alloys induced by hydrogenation have been studied by Mössbauer spectroscopy. A shift in the critical composition for the onset of magnetic ordering to lower iron concentration was found in these alloys as a result of hydrogenation.     

Temperature magnetism of disordered iron–aluminum alloys in a model of two intra-atomic interactions

A system of band electrons with the two constants of intra-atomic exchange interaction simulates the different local environment of magnetic atoms in alloys. Temperature behavior of the magnetic characteristics of disordered Fe-Al alloys is investigated on the basis of an improved theory of dynamic fluctuations in electron spin density. The nature of the alloys’ different properties and the unusual temperature dependence of the magnetization and magnetic susceptibility is discussed.     

Mössbauer spectroscopy of disordered magnetic systems

In the given summarizing article, we present the most important aspects of the technique of studying disordered magnetic systems by the Mössbauer effect being developed by us for the broad circle of researchers in magnetism and Mössbauer spectroscopy. The potentialities of this technique in investigating local magnetic characteristics and spin structures of disordered magnetics are assessed by the interpretation of Mössbauer spectroscopy data of various classes of well-studied magnetics (Fe?Al, Fe-V). In the concluding part, we present original results of Mössbauer investigations of binary (Fe?Cr, Fe?Pd and Fe?Au) and quasi-binary (Fe?Pd?Au) alloys characterized by non-trivial macroscopic magnetic properties and complex magnetic phase diagrams.