Phase diagrams calculated for Fe-rich Fe-Si-Co and Fe-Si-Al ordering alloy systems

Theoretical analysis based on the calculation of phase diagrams was employed for Fe-Si-Co and Fe-Si-Al ordering systems to clarify the necessity for the occurrence of phase separation in Fe-base ternary ordering systems. The free energy of Fe-base ternary ordering alloys where B2 and D0₃ ordered structures are formed is evaluated statistically using a pairwise interaction approximation up to second nearest neighbours, taking into account not only the atomic interaction but also the magnetic interaction, based on the Bragg-Williams-Gorsky model. The calculated phase diagrams are consistent with the experimentally obtained ones. The phase diagram calculation in this work is useful to predict the equilibrium states of the ternary ordering systems. The phase separation in ordering alloys is caused by the contribution of excess free energies due to ordering. The influences of ferromagnetism on the two-phase regions are also demonstrated.     

Nuclear Spin-Ordering Due to Correlated Atomic Motion in bcc 3He

We propose a new way to treat nuclear magnetism of solid ³ He. We argue that the magnetic interaction arises indirectly as a consequence of correlated zero-point motion of the ions. This motion lowers the energy of the ground state, and results in a coherent state of oscillating electric dipoles. Distortion of the electronic wavefunctions leads to hyperfine magnetic interactions with the nuclear spin. Our model describes both the phonon spectra and the nuclear magnetic ordering of bcc ³ He using a single parameter, the dipolar interaction energy E ₀ . The model yields correctly both the u2d2 symmetry of the ordered phase and the volume dependence of the magnetic interaction.     

The phase field investigation of B2 (FeAl) phase antisite defect during homogenous transformation of Fe–24Al alloy

The B2 (FeAl) antisite defects and microstructure evolution during homogenous transformation of Fe–Al alloys are investigated by phase field kinetic model considering long-range elastic interaction energy. The results show that congruent ordering transformation dominates the aging initial stage, and two phase structures of B2 ordered phase and disorder phase are formed. The antisite atom occupation probability which indicates antisite defects decreases during congruent ordering transformation of aging initial stage. With increasing aging time, spinodal decomposition occurs within B2 structure ordered phases, and spinodal decomposition and ordering coexistence microstructures are formed at this stage. At the aging of final stage, the increase of antisite atom occupation probability is attributed to the presence of spinodal decomposition. For Fe–Al alloys with different composition, we find that with increasing Al atom concentration, the Al_Fe antisite defects increase, and the Fe_Al antisite defects decrease. In contrast, with the increase of Fe atom contents, the numbers of Fe_Al antisite defects increase and that of the Al_Fe antisite defects decrease.     

Interfaces in ordered intermetallics

In this paper, we have examined different types of interfaces that occur between orientational/translational variants generated during the ordering process. This has been illustrated citing examples of ordering of the FCC structure into DO₂₂, Dl_a and Pt₂Mo type structures in some nickel base alloys. Microstructures consisting of more than one ordered structure have also been investigated. Superlattice domains of DO₂₂ and Pt₂Mo type structures have also been found to coexist in a microscopic scale of mixing in Ni-V alloys while mixed domains of Dl_a and Pt₂Mo type structures on a much finer scale have been observed in Ni-Mo alloys. The formation of different variants (rotational and translational) of ordered structure(s) from the disordered lattice has been explained on the basis of group theoretical and symmetry considerations.     

New aspects of π–d interactions in magnetic molecular conductors

Abstract

The 2 : 1 cation radical salts of bent donor molecules of ethylenedithio-tetrathiafulvalenoquinone-1,3-dithiolemethide (EDT-TTFVO), ethylenedithio-diselenadithiafulvalenoquinone-1,3-dithiolemethide (EDT-DSDTFVO), ethylenedithio-diselenadithiafulvalenothioquinone-1,3-diselenolemethide (EDT-DSDTFVSDS), ethylenedioxy-tetrathiafulvalenoquinone-1,3-dithiolemethide (EDO-TTFVO) and ethylenedioxy-tetrathiafulvalenoquinone-1,3-diselenolemethide (EDO-TTFVODS) with FeX₄^? (X = Cl, Br) ions are prepared by electrocrystallization. The crystal structures of these salts are composed of alternately stacked donor molecule and magnetic anion layers. The band structures of the donor molecule layers are calculated using the overlap integrals between neighboring donor molecules and are compared with the observed electronic transport properties. The magnetic ordering of the Fe(III) d spins of FeX₄^? ions is determined from magnetization and heat capacity measurements. The magnetic ordering temperatures are estimated by considering a combination of a direct d–d interaction between the d spins and an indirect π–d interaction between the conduction π electron and the d spins, whose magnitudes are separately calculated from the crystal structures with an extended Hückel molecular orbital method. The occurrence of a π–d interaction is proved by the negative magnetoresistance, and the magnitude of magnetoresistance reflects the strength of the π–d interaction. The effect of pressure on the magnetoresistance is studied, and the result indicates that the magnitude of magnetoresistance increases, namely, the π–d interaction is enhanced with increasing pressure. From these experimental results it is shown that (EDT-TTFVO)₂?FeBr₄ is a ferromagnetic semiconductor, (EDT-DSDTFVO)₂?FeX₄ (X = Cl, Br) and (EDT-DSDTFVSDS)₂?FeBr₄ are metals exhibiting antiferromagnetic ordering of the d spins, and (EDO-TTFVO)₂?FeCl₄ and (EDO-TTFVODS)₂?FeBr₄?(DCE)_0.5 (DCE =-dichloroethane) are genuine antiferromagnetic metals. Among them, the (EDT-TTFVO)₂?FeBr₄ salt is the first π–d molecular system where the d spins of FeBr₄^? ions are ferromagnetically ordered through antiferromagnetic interaction with the conduction π electrons. Corresponding to this ferromagnetic ordering, an anomalous dielectric slow-down phenomenon toward the ordering temperature is observed. The π–d interaction in (EDT-DSDTFVSDS)₂?FeBr₄ is very large and comparable to that in λ-(BETS)₂?FeCl₄, which has the highest reported value so far, while the d–d interaction is fairly small. Concerning the ratio between the magnitudes of π–d and d–d interactions (J_πd/J_dd), this salt is currently the best π–d molecular system.     

Ordered structures in ternary hcp alloys

A study of ordered structures in ternary hcp alloys has been undertaken. For this, the hcp structure has been divided into several sublattices and used to generate ordered structures with three types of atoms. Nine ground-state ordered structures have been identified on the basis of maximum or minimum number ofAB, BC andCA bonds. Complete crystallographic details about these structures have been worked out. In an alternate approach, ordered ternary structures were obtained by populating 8 types of sublattices (which generate Ti₃Al structure) with three types of atoms. Thus, fortyeight ordered structures were found. For each structure, complete structural details have also been worked out, some of which are reported. Configurational energy of each structure has been calculated using pairwise interactions up to third neighbour distances. To gain insight regarding low-energy structures, some assumptions were utilized to reduce the number of independent parameters in the energy expressions and their consequences explored. Two types of degenerate situations have been observed. One type of degeneracy occurs for ideal hcp alloys where only first neighbour interactions are considered. Another degenerate situation occurs for non-ideal hcp alloys where interactions are considered up to third neighbour distances.     

Wetting Phenomena in bcc Binary Alloys

We study the influence of the surface orientation on the wetting behavior of bcc binary alloys, using a semiinfinite lattice model equivalent to a nearest-neighbor Ising antiferromagnet in an external magnetic field. This model describes alloys that exhibit a continuous B2–A2 order–disorder transition, such as FeAl or FeCo. For symmetry-breaking surfaces like (100), an effective ordering surface field g ₁0 emerges. Such a field not only crucially affects the surface critical behavior at bulk criticality, but also gives rise to wetting transitions below the critical temperature T _c. Starting from the mean-field theory for the lattice model and making a continuum approximation, a suitable Ginzburg–Landau model is derived. Explicit results for the dependence of its parameters (e.g., of g ₁) on the microscopic interaction constants are obtained. Utilizing these in conjunction with Landau theory, the wetting phase diagram is calculated.     

Magnetic Anisotropy of PrNi5 Single Crystal at Millikelvins

Magnetic ordering in the hyperfine enhanced nuclear ferromagnet PrNi₅ was investigated in external magnetic fields up to 72 mT at temperatures from below the ordering temperature up to about 20 mK. An anisotropy of the magnetization and ac susceptibility was observed for two single crystalline samples having a or c crystallographic axis parallel to the external field. The results show that the nuclear magnetic moments of Pr are ordered in the basal plane of the hexagonal crystal structure of PrNi₅.     

Direct Measurements of Magneto-caloric Effect of Gd5Si2Ge2 Alloys in Low Magnetic Field

Direct measurements of the magneto-caloric effect of Gd₅Si₂Ge₂ alloys under heat treatment conditions are investigated by measuring adiabatic temperature change (ΔT _ad) in a magnetic field of 1.5 T using a homemade magnetocaloric effect measuring apparatus. The crystal structure, microstructure as well as the chemical composition of the alloys are measured using X-ray diffraction (XRD), scanning electron microscope (SEM) with energy dispersive X-ray Detector (EDX). It is found that the microstructure of the alloys could be fully homogenized and the impurities in the alloys could be remarkably removed via appropriate heat treatment. As a result, the maximum adiabatic temperature change (ΔT _ad) of the alloy annealed at 1573 K increases by 105% from 1.9 to 3.9 K for a magnetic field change from 0 to 15 kOe when compared to the as arc-melted material, while the magnetic ordering temperature slightly reduced.     

Investigation of ordering phenomenon in Me–Pt (Me=Fe,Ni) liquid alloys

Abstract

The phase diagrams of Fe–Pt and Ni–Pt liquid alloy systems show the existence of FePt and NiPt intermetallic compounds, respectively, in their solid intermediate states, and the associative tendency between unlike atoms in these liquid alloys has been analysed using the self-association model. The concentration dependences of mixing properties such as the free energy of mixing, G_M; the concentration fluctuations, S_cc(0), in the long-wavelength limits; the chemical short-range order (CSRO) parameter, α₁; as well as the chemical diffusion, enthalpy and entropy of the mixing of Fe–Pt and Ni–Pt liquid alloys have been investigated to determine the nature of ordering in the liquid alloys. The results show that heterocoordination occurs in the alloys at all concentrations. The effect of CSRO on S_cc(0), chemical diffusion, D, and the order parameter, α₁, has been considered. The ordering phenomenon in the liquid alloys is also related to the effect of the atomic size mismatch volume on S_cc(0).     

A First‐Principles Investigation of the Compositional Dependent Properties of Magnetic Shape Memory Heusler Alloys

The interplay of structural and magnetic properties of magnetic shape memory alloys is closely related to their composition. In this study the influence of the valence electron concentration on the tetragonal transformation in Ni₂Mn_{1 + x}Z_{1 ? x} (Z = Ga, In, Sn, Sb) and Co₂Ni_{1 + x}Ga_{1 ? x} is investigated by means of ab initio calculations. While the type of magnetic interaction is different for the two series, the trends of the total energy changes under a tetragonal transformation are very similar. We find that tetragonal structures become energetically preferred with respect to the cubic one as the valence electron concentration e/a is increased regardless of the system under consideration. In particular, the energy difference between the austenite and martensite structures increases linearly with e/a, which is in part responsible for the linear increase of the matensite transformation temperature. The substitution of nickel by platinum increases even further the transformation temperature.     

Transition from ferro- to ferrimagnetic ordering via Mn-disorder in (Ni,Co)MnGa quaternary Heusler alloy

An inclusive investigation of electronic and magnetic properties of ordered and disordered (Ni,Co)MnGa quaternary Heusler alloy has been undertaken using the density functional theory-based full potential linearized augmented plane wave method within generalized gradient approximation as exchange–correlation potentials. We observe that the increasing Mn concentration in Ni_1?x CoMn_1+x Ga (x = 0, 0.25, 0.50 and 0.75) lowers the ferromagnetic (FM) character of the parent (Ni,Co)MnGa alloy. This change creates two inequivalent Mn sites aligning antiparallel to each other and generates the ferrimagnetic (FiM) ordering in all resultant disordered alloys. Further, this replacement sets off a structural change from Y- to X_a-structure. The highest magnetic moment has been found to be 4.95 μ_B for ordered (Ni,Co)MnGa alloy, whereas it decreases with increase in Mn-concentration for disordered systems. The variation of half-metallicity with increasing Mn concentration is analysed. The stability of FM and FiM state for the present ordered and disordered alloys, respectively, is also examined.     

Effect of boron on the ductility of ordered Ni-Ni4Mo alloys

Boron was identified to have a beneficial effect on the room temperature ductility of ordered Ni-Ni₄Mo alloys. An addition of 0.01 to 0.03 wt % boron to hypostoichiometric alloys was found to increase the tensile ductility from about 5% to 30% in the ordered state produced by exposure at 600 to 800 °C, of the annealed material. The boron effect was, however, found to diminish with exposure time at a given temperature and not to be maintained at elevated temperatures. The beneficial effect of boron on room temperature ductility was also found to be considerably less pronounced in stoichiometric Ni₄Mo alloy. Both electron energy loss spectroscopy and Auger electron spectroscopy techniques revealed no preferential segregation of boron to grain boundaries. Experimental results suggested that boron decelerates the kinetics of heterogeneous grain boundary ordering which leads to an improvement in ductility.     

Exchange coupled nanocomposite hard magnetic alloys

Abstract

The processing, structures and phase constitutions and the magnetic properties of nanocomposite hard magnetic alloys are reviewed. The emphasis is on rare earth (RE)–iron–boron alloys in which the hard magnetic phase RE₂Fe₁₄B is intermixed with one or more soft magnetic phases. Processing–structure–property relationships are the principal focus, in particular, the role of the hard and soft nanocrystallite dimensions in promoting intergrain ferromagnetic exchange coupling and the consequent enhancement of remanent magnetisation and the technologically important maximum energy density. The powder processing, chill block melt spinning, mechanical alloying and thin film deposition routes to develop nanocrystalline and nanocomposite structures are reviewed. The coercivity mechanism in ultrafine grained alloys and the influence of crystallite dimensions are discussed, as are the effects on intrinsic and extrinsic properties of RE substitutions, replacement of iron by other transition metals and enrichment of the boron content. Exchange enhancements in Sm–Co based nanocomposite bulk alloys and in nanoscale FePt/α-Fe composite thin films are briefly considered, together with thin film materials involving exchange coupling between ferromagnetic and antiferromagnetic phases, in core–shell type structures of transition metal compounds surrounded by oxides and in mechanically alloyed materials. The processing and magnetic properties of bonded magnets based on nanocrystalline/nanocomposite REFeB alloys are discussed. The possibility of producing anisotropic hard/soft composites with properties approaching the theoretical maximum is considered and the extent to which this goal has been realised for fully dense alloys identified.     

Effect of high-pressure high-temperature processing on the phase composition and magnetic state of Mn<Subscript>1 + <Emphasis Type="Italic">x</Emphasis></Subscript>Sb (0 ≤ <Emphasis Type="Italic">x</Emphasis> ≤ 1.0) alloys

The phase composition and magnetic state of Mn_{1 + x} Sb (0 ≤ x ≤ 1.0) alloys have been determined before and after high-pressure high-temperature processing (p = 8 GPa, T = 2300 K). Exposure to high pressure and temperature is shown to extend the homogeneity range of hexagonal Mn_{1 + x} Sb alloys (sp. gr. P6₃/mmc, no. 194) to x = 0.5. The alloys with x > 0.5 consist of two phases: hexagonal Mn_{1 + x} Sb and cubic Mn₃Sb (sp. gr. Pm3m, no. 221). The magnetic state of the alloys containing manganese in excess of the equiatomic composition can be understood in terms of a two sublattice model and ferrimagnetic ordering.     

Observations of an ordered Pd7Eu phase in Pd-Eu alloys

The Pd-Eu alloys containing 6.1 to 14.3 at% Eu have been studied by metallographic examinations, X-ray diffraction, electron microscopic observations and electrical resistance measurements. The results for the annealed alloys indicate the presence of an ordered phase in the form of a Pd₇Eu superlattice, which is similar to the order found in Pd₇R (R=cerium, gadolinium, dysprosium, yttrium or samarium) previously. The order-disorder transition of the ordered Pd₇Eu phase in the alloys between Pd-10 and 16.6 (Pd₅Eu) at% europium alloys is accompanied with a peritectoid reaction:-Pd + Pd₅Eu Pd₇Eu at 850 ± 10K. The ordering temperatures for Pd-6.1 and 7.5at % Eu alloys are lower than the peritectoid reaction temperature, and they correspond to the solid solubility limit of europium in the-Pd phase.     

Long-term ageing characteristics of some commercial nickel-chromium-molybdenum alloys

The long-term ageing characteristics of some commercial Ni-Mo-Cr alloys (the high-temperature HASTELLOY alloy S and the corrosion resistant HASTELLOY alloys C-4 and C-276) at 810 K were investigated. It was found that the three alloys undergo the following long-range ordering reaction: disordered f c c lattice ordered orthorhombic, Pt₂Mo-type superlattice. Ordering was found to cause considerable strengthening without severe loss of tensile elongation. Deformation in the ordered state occurred predominantly by twinning. The corrosion rates of alloys C-4 and C-276 in boiling sulphuric-ferric sulphate solution did not seem to be greatly affected by the long-range ordering reaction. In addition to ordering, the three alloys were also found to undergo grain boundary reactions. The resulting phase in alloys S and C-4 assumed a dispersed morphology and was identified as carbide, probably of the Type M₁₂C. In alloy C-276, however, which contains higher amounts of iron and tungsten, the boundary precipitate was in the form of a continuous layer consisting of M₁₂C and Mu-phase. This could account for the reduced tensile elongation of alloy C-276 relative to alloys S and C-4 and also to its high corrosion rate.     

The mechanism of crystallographic ordering in CuPt

The present paper is primarily a study of the ordering characteristics within the CuPt system, near the CuPt composition, using X-ray diffraction, optical microscopy (in conjunction with polarized light), high-voltage electron microscopy and dilatometry.In platinum-rich off-stoichiometric alloys, a wide two-phase region consisting of ordered + disordered platelets was established and the phase boundaries were accurately located. For isothermally ordered stoichiometric alloys, in general two categories of diffraction sequences were observed, depending on the annealing temperature. For anneals in the range 620°C<T<815°C (=T_c), a series of broad, asymmetric X-ray line profiles were obtained during the early part of the ordering cycle: this represents a continuous reaction. However, when samples were annealed a temperatures lower than 620°C, there was unmistakable micrographic evidence of the coexistence of both the ordered and disordered phases: this represents a discontinuous reaction. After making allowances for a number of side-effects which had broadened X-ray reflections at high temperatures, a nucleation-and-growth model is proposed for CuPt at all ordering temperatures.The parallel microscopic studies also exhibited quite contrasting morphologies above and below 620°C: a lamellar structure is the product at high temperatures, whereas a grain-boundary reaction, generating very coarse domains, is observed at lower temperatures. A modified microstructure was observed for samples annealed at T<475°C, when ordered spherulites were seen to grow within the grains. Samples cooled slowly through T _c order by a diffusion-controlled shear process.     

Electrical resistivity of Mn-doped SnTe crystals at low temperature

The temperature dependence of the resistivity over the range 1.8–20 K has been measured on the narrow-gap semiconductor SnTe with various Mn contents (<2.2at.%) and carrier concentrations [p = (1.2–8) × 10²⁰ cm^–3]. The resistivity shows an anomaly at some magnetic ordering temperature T _m, which depends sublinearly on the Mn content c, but not linearly. However, together with a negative magnetoresistance, we have confirmed that the carrier scattering in this crystal is due to the s-d interaction as in dilute magnetic alloys.