Ordering and Disordering in Alloys
Abstract
Proceedings of the European Workshop on Ordering and Disordering held in Grenoble, France, 10-12 July 1991.
Chapters (47)
This paper offers a bird’s-eye view of the whole subject-matter of the Workshop. — The relation of the kinetics of the establishment of long-range order to the nature of thermal treatment and to the ordering energy is briefly reviewed, and some current issues highlighted. Some matters relating to the measurement of order and to the role of antiphase boundaries are discussed. — Disordering by means of intense cold-work, with special reference to ball-milling, is examined. Thermal disordering, by annealing above T
c
, is compared with the aforementioned processes; there is evidence that thermal disordering is preferentially catalysed at antiphase domain boundaries. It is not known whether this also true for the alternative processes. The question is finally addressed whether an alloy partially disordered by mechanical deformation is in a different state from one partially ordered, to the same value of the LRO parameter S, by annealing an initially thermally disordered alloy.
During rapid solidification of intermetallic compounds, the atoms may not have time to find the lowest-energy sites in the crystal, resulting in the growth of a solid with partially or completely suppressed chemical order. A kinetic model has been developed for “disorder trapping” during rapid solidification, which predicts the long range order parameter, composition and temperature at the interface of a chemically ordered phase as functions of interface velocity and liquid composition. The model predicts that as the solidification velocity increases, the long range order parameter decreases. Beyond a critical velocity the order parameter is zero. The predicted transition to solidification of a disordered solid is discontinuous for thermodynamically first-order cases, and continuous for thermodynamically second-order cases.
Ni2TiAl (L21) and Ni3AI (L12), which in equilibrium are ordered to their melting points, have been kinetically disordered by rapid solidification following pulsed laser melting. The order-disorder transitions are second-order and first-order, respectively. The interface velocity and order parameter can be followed with nanosecond resolution by monitoring the reflectivity and lateral resistance of a thin film sample during and immediately after solidification. X-ray diffraction and Transmission Electron Microscopy (TEM) indicate that metastable fcc Ni3Al was retained by quenching. In rapidly solidified bulk Ni2TiAl TEM reveals a fine array of antiphase boundaries, indicating that the material formed from the melt with the unstable B2 structure and subsequently transformed to the equilibrium Heusler structure during cooling to room temperature. In rapidly solidified Ni2TiAl thin films, electron diffraction indicates that the unstable bcc structure was formed directly from the melt and retained upon cooling.
Two concepts from the kinetic theory of disorder→order transformations are presented. Kinetics provides control over the path taken en route to equilibrium, permitting different types of microstructures to be synthesized. Variations in microstructural evolution can be parameterized by two or more order parameters. “Kinetic paths” through two or more order parameters are convenient for experimental study because they are independent of the vacancy concentration in the material. Saddle points in the free energy surface have little consequence to the thermodynamic state, but if an alloy approaches a saddle point its kinetic evolution tends to stall. This stalled state is an example of a “pseudostable” state.
The dependence of electrical resistivity on short-range order (SRO) and long-range order (LRO) is shown in a condensed and simplified way. Experimental observations of recent years on ordering and disordering are reviewed briefly, whereby a special emphasis is laid on the thermal treatment applied. It is shown that a lot of information on the ordering process can be gained concerning ordering relaxation, equilibrium of order and order parameter. For a more complicated behaviour (critical behaviour at T or parallel development of SRO and LRO in addition to resistivity) information by other methods would be advantageous.
Technologically interesting Ni3Al based alloys can be effectively disordered by ball milling. The compound disordered in this way shows, when continuously heated, a broad exothermic transformation after which the ordered Ll2 phase is again obtained. The total enthalpy released in the transformation grows with the milling time.
It is observed that other kinetic processes, apart from the ordering one, are involved in the broad exothermic transformation. This study is devoted to the separation and deeper comprehension of these processes and for that purpose, isothermal measurements coupled with continuous heating measurements were performed. Isothermal measurements give a continuous decay of the calorimetric signal. At intermediate temperatures this decay leads to a state where the sample is still able to transform, as it is evidenced on further continuous heating experiments. Three different processes have been identified, the more energetic corresponding to the reordering process. The kinetic parameters of the reordering process have been obtained and are discussed.
Ordered alloys which cannot be disordered by heat treatments and quenching can still be effectively disordered by irradiation or by ball-milling. The disordered state is achieved during this extreme form of cold-work by the creation of several kinds of defects; in particular vacancies and dislocations. The increase of anti-phase boundary density and the consequent reduction of ordered domain size are other aspects of the disordering process. The enhancement of the free energy can destabilize the crystalline structure and amorphous regions can appear.
We present some results concerning the characterization of ball-milled Ni3Al based intermetallic powders, in the state which follows the complete destruction of long range order, as verified by x-ray diffraction. We believe that the observed continued lattice expansion is due to the combined effect of lattice strain, increase of defect concentration, especially vacancies and dislocations, and elastic mismatch. The lattice parameter is strongly influenced by the local atomic configuration, so that another contribution can come from the reduction of short-range order.
A comparison with some other L12 disorderable alloys is also presented.
The investigation of ordering-ordering kinetics (LRO or SRO) in inter-metallic compounds yields informations on the activation energy of the ordering process that can be compared to the energy of the interdiffusion. A systematic study of aluminide phases built either on a fcc (L12) or bcc (DO3) lattice shows that quite different behaviours are observed in the two types of structure. In L12 phases, the role played by the ordering energy has been tested in the pseudo-binary Ni3Al1-xMx (M = Fe, Mn, Cr) system which displays a strong decrease of the L12 phase stability when the transition metal M is increasingly substituted to aluminium. The activation energy (EA) of the ordering process is found to increase with the phase stability, in agreement with theoretical predictions. In binary bcc Fe1-xAlx phases, the result is different since the activation energy strongly decreases correlatively to the formation of long range ordered DO3 structure. Such an unexpected behaviour can be attributed to a large decrease of the migration energy through a lattice dynamics effect, as recently proposed in models which explain the enhanced diffusion observed in some bcc transition metals and alloys. Such a comparative experimental study shows that, depending on basic lattices, either thermodynamical or vibrational properties are determinant factors of the activation energy for the ordering kinetics.
The decomposition of the supersaturated Al-7 at%-Li α phase was studied by high resolution microscopy. Investigating the spatial arrangement of the metastable δ’ precipitates a further hint of congruent ordering prior to decomposition is obtained. During examination of δ’ precipitates in an 120 kV TEM a strain contrast appeared which increased with irradiation time (typically 30 min). In 400 kV electron microscope these effects came up faster and could be better resolved. As the L12 particle disappeared Moiré fringes became visible and also new lattice fringes. From these and the diffraction pattern the new phase could be identified as the stable δ-Al-Li phase (B32 with a lattice parameter of a0 = 0.641 nm).
Electron microscopy combined with electron diffraction are highly qualified to provide microstructural data on phenomena occurring in the vicinity of phase transitions. Such pretransition effects occur for diffusion controled as well as for diffusionless transformations.
a) for diffusion controled transformations short range order (SRO) is formed when approaching the ordering temperature Tc from above while upon approaching Tc from below, interfaces associated with the ordering show wetting phenomena.
b) for diffusionless transformations the pre-transition modulations associated with the displacive austenite-martensite transformation are considered.
We have investigated the behaviour of antiphase boundaries as a function of temperature in A3B binary ordered alloys using transmission electron microscopy. The study shows a typical wetting phenomenon at the order-disorder transition. At low temperature, the domain walls are perfectly thin and micro-faceted. Complete interfacial wetting by the disordered phase, characterized by a logarithmical divergence of the width of the walls, is shown to occur when approaching the order-disorder transition from below.
According to theoretical models, the value of ordering energy determining the “order-disorder” transition temperature Tt should affect the kinetics of long-range ordering (LRO). The kinetics of L12 ordering was studied by means of in situ resistometry within the γ’-phase of a quasi-binary system Ni75Al25-xFex, where Tt drops drammatically with x.
Experimental evidence of an increase of the LRO activation energy with increasing Tt has been achieved. The result was discussed in terms of a classical model of diffusion in L12 superstructure proposed by Schoijet et al.
Low temperature resistometry and high resolution electron microscopy have been used to clarify some irreversibilities remarked in disordering kinetics in alloys with low Fe content.
The high sensitivity of residual electrical resistivity to structural modifications in metallic materials was used for studying the composition and temperature dependence of the equilibrium and kinetics of atomic order, in binary Ni3Al intermetallic compounds with 24.6 to 26.5 at% Al and in ternary materials with 4 at% Ti or Ta. In the binary alloys, the resistivity characterizing the equilibrium long-range order exhibits a positive temperature dependence, which is quantitatively consistent with changes of the LRO parameter calculated by the Cluster Variation Method. The analysis of the isothermal ordering (or disordering) kinetics shows that two first order processes are involved. The order change-rates are dependent on composition; they are minimum at stoichiometry and increase with off-stoichiometry. The presence of a third element (Ti or Ta) results in a slower diffusion.
The change of the degree of order with increasing rolling reduction was determined by X-ray diffraction. During a subsequent isochronal annealing treatment between 273 K and 1473 K of 40% rolled samples, hardness, yield-stress and the intensities of the superlattice reflections were measured and the microstructural evolution was investigated by TEM. The correlation between the measured property changes are discussed.
The change of atomic order and of defect concentration during plastic deformation was investigated in short-range ordered austenitic Fe-Cr-Ni alloys and in long-range ordered substoichiometric boron-doped Ni3Al alloys, by measuring electrical resistivity changes. Analytical relationships were used for representing the respective contributions of these two processes to the total resistivity variation. Disordering efficiency was found to be very different in short-range and long-range ordered materials.
Aluminides having B2 ordered structure can accomodate in their lattice very high concentrations of vacancies. In the present work we study the effect of the different defect concentrations on the recovery and reordering phenomena of some rapidly solidified iron aluminides ordered alloys.
Several series of melt spun ribbons of binary, FeAl, and ternary, Fe-Al-Cr, compositions were prepared.
The kinetics of the annealing out of the frozen-in defects was investigated by Differential Scanning Calorimetry, and the variations in the microstructural characteristics before and after the heat treatments were analysed by X-ray Diffraction
The observed kinetics show single or multi-step recombination processes, depending on the composition of the samples.
Changes of resistivity during ordering and disordering have been investigated for L11-ordered CuPt alloys. Whereas for stoichiometric Cu50Pt50 a drastic influence of LRO only is observed, two contributions to resistivity of opposite sign were detected for off-stoichiometric Cu65Pt35. These two processes are attributed to LRO and SRO, respectively, being in competition: Short annealing times support SRO and the formation of LRO is suppressed. Prolonged annealing times yield a corresponding increase of LRO. These findings are confirmed by X-ray diffraction at wide and small angles.
The relative stability of phases in driven systems such as alloys under irradiation cannot be assessed from thermodynamic potentials. Nevertheless, starting from a mesoscopic stochastic description, one can define a potential which governs the probability distribution of macrovariables (for instance the composition and order parameters). As examples, we recall here the results obtained, in the simplest mean-field approximation, for order-disorder transitions on the BCC lattice (B2 ordered phase) and on the FCC lattice (L12, L10, D1a, LiFeO2-type ordered phases). Inversion of phase stability, occurrence of tricritical lines, sensitivity of phase boundaries to “cascade size effect” are predicted.
We discuss the thermodynamical behaviour of APB’s on the FCC lattice. Wetting and layering mechanisms are discussed in relation with pinning effects. We show, in particular, that only high order mean field theories, such as the Cluster Variation Method in the tetrahedron-octahedron approximation, predict the correct thermodynamical properties of surfaces and interfaces on the FCC lattice.
In systems with a body centered cubic crystal structure the Cluster Variation Method (CVM) in the irregular tetrahedron approximation allows to take first and second nearest neighbour pair interactions into account. With these interactions it is possible to obtain the most frequently observed superstructures B2, DO3, B32. For binary systems it could be shown [1] that this approximation yields almost identical phase diagrams to the Monte Carlo simulation technique which is considered to come closest to the real situation.
The ordering transition of FCC systems is modelized with Ising Hamiltonians (pair interactions).The Monte Carlo method is used to calculate the free energy of the lattice.This permits a discussion of the relative stability of various “long period” (LP) structures.This discussion is carried with Hamiltonians close to the degeneracy, where the simplest mean-field theories cannot be used.It appears that the LP structures are not stable with simple Hamiltonians(antiferromagnetic nearest-neighbour interactions).The antiphase surface free energy (σa) can then be calculated.A study of the variations of σa is carried among variable degenerate Hamiltonians. One shows an increase of σa when the Hamiltonian leads to strongest fluctuations close to the ordering temperature.
To model the behaviour of an ordered alloy under irradiation, we propose a mean-field approximation of the B2-A2 order-disorder transition on a B.C.C. lattice with two atomic exchange mechanisms acting in parallel: thermally activated and forced jumps. By integration of deterministic evolution equations, we show that irradiation can induce the stabilization of a two-phase alloy, while the classical equilibrium phase diagram only displays single phase fields (the B2-A2 transition is second order). The steady-state diagram is computed: in the two-phase region, the lever rule is obeyed and surface tension effects are identified; a new mechanism for precipitate redistribution under irradiation and shearing is discovered.
Linear Muffin Tin Orbital calculations of equations of state were performed for observed and hypothetical BCC based ordered structures in the Ni-Al-Ti system. Total energies were parameterized in the Connolly-Williams and ε-G approximations. The resulting effective cluster interactions (ECI) were used to calculate: 1) BCC based ground-states; 2) The A2→B2 and B2→L21 order-disorder transition temperatures; 3) Quasibinary phase diagrams for the NiAl-NiTi join. The BCC octahedron approximation of the cluster variation method (CVM) was used to calculate Tc(L21→B2) and TC(B2→A2). The simple cubic “cube approximation” of the CVM was used to calculate quasibinary NiAl-NiTi phase diagrams.
A statistical thermodynamic approximation -for cooperative phenomena in model systems is presented, to improve the quasichemical constant coupling approximation by introduc in a second pairs quasichemical reaction after averaging atomic groups. By means o-P two suitable temperature -functions related to -Fluctuations, an analytical expression of the free energy function is obtained. The first parameter takes into account the change of the order parameter after averaging and the decreasing importance of the second reaction at increasing temperatures. The second one is a correction of the system temperature as a consequence of fluctuations’ influence on the mean thermodynamic behaviour. Relations to obtain the right critical exponents and well approximaterfthermodynamic critical quantities for ordering systems are given. The approximation is applied to real systems, such as beta brass, taking into account next nearest neighbour interaction.
We study systems that can be described in terms of two kind of degrees of freedom when the corresponding ordering modes couple one to the other. The primary ordering mode gives rise to a diffusionless first-order phase transition and the secondary mode, that we suppose can be externally controlled, is associated to a continuous phase transition. Our interest is to analyze how the thermodynamic properties of the primary phase transition change as a function of the secondary ordering-mode. This can be experimentally realized in metallic alloys undergoing, at low temperatures, a martensitic phase transition and an order-disorder transition at much higher temperatures. In this case we obtain that, because of the directional character of the martensitic transition, the entropy-change dependence on atomic order is very small.
This paper presents the results obtained from an isothermal calorimetric study of the ordering process, at different temperatures Ta, in a Cu-Zn-Al alloy after quenches from several temperatures Tq. In all cases the ordering process involves the formation of the L21 superlattice through a vacancy assisted mechanism. The dissipated energy in the process was measured and the ordering kinetics studied by calorimetric tests.
The dissipated energy increases with the quench temperature, Tq, up to an intermediate value Tmax, then decreasing for higher temperatures Tq. This behaviour is determined by reordering taking place during the quench, which is due to a large vacancy concentration frozen in from Tq > Tmax.
The relaxation process does not follow a single exponential decay, but instead shows an initial fast decay followed by a progressive reduction of the relaxation rate. The Arrhenius dependence of the characteristic relaxation time of the process from Tq and Tt gives activation energies of 0.76 ± 0.03 eV and 0.43 ± 0.03 eV for the vacancy migration and formation respectively.
A brief review is presented of the mechanisms of energy storage leading to irradiation induced amorphization, with special attention to the comparison between chemical disordering and point defects. It is shown that in addition to chemical disordering, there is considerable experimental evidence for the point defect mechanism. A summary is presented of experimental results of amorphization induced by neutron, electron and ion irradiation of Zr(Cr,Fe)2 precipitates in Zircaloy, and of their theoretical interpretation. In that system, there is evidence for forms of energy storage other than chemical disordering, notably amorphization by departure from stoichiometry under neutron irradiation.
The ordered intermetallic compound Zr3Al was irradiated with 3.8 MeV Zr3+ lons at various fluences up to 5 x 1012 ions/mm2 at a temperature of 250 °C and the irradiation-induced microstructures were investigated by transmission electron microscopy. Disordering began at the lowest dose, 0.0033 dpa, and complete loss of chemical long-range order occurred at a dose of 0.33 dpa. The onset of amorphization was also observed at this dose. Electron diffraction patterns from irradiated samples showed satellite reflections along <011> in thin foils in [100] orientation and streaking along <111> in foils oriented [011]. These diffraction effects are attributed to the presence of irradiation-induced microstructural defects that, when imaged in dark field, resemble rows of dislocation loops. A model is proposed for these arrays of loops, which are suggested to have Burgers vectors of the Frank type. The model accounts for the contrast effects observed in the images and the streaking and satellites seen in the diffraction patterns. At the highest dose, 1.6 dpa, a new phase, Zr5A13, appeared unexpectedly, most likely as a consequence of irradiation-induced solute segregation.
It is now well established that swift ions are able to induce disordering by electronic excitations in metallic compounds at low temperature. It is shown here that at least in Ni Zr2 this effect is also observed at room temperature.
The long-range atomic order of crystalline alloys can be destroyed by a variation of temperature resulting in regular melting or by non-equilibrium solid state processing resulting in glass formation. The destabilization of the crystalline lattice and evolution of the amorphous phase is triggered by a variety of methods including, for example, alloying beyond the equilibrium limit, mechanical deformation, irradiation, thermal annealing of thin film superlattices and application of high pressures. Under local equilibrium conditions, the crystal-to-glass transition exhibits similar features as equilibrium melting, whereas under non-equilibrium conditions catastrophic vitrification can occur at the To-temperature extended into the glass forming regime. It is shown that randomly frozen-in impurities or lattice defects cause rounding of the first-order melting transition to an isentropic transition resulting in a triple point between supersaturated crystal, undercooled liquid and glass at the crossover of the ideal glass transition and melting lines.
We have studied the phase transformations induced in the Ni-Ge binary system by mechanical alloying of Ni and Ge powder mixtures and by milling of NiGe melt-spun ribbons. We observed that mechanical alloying is preceded by significant x-ray line broadening, consistent with an acceleration of the mixing kinetics with grain refinement. During mechanical alloying, crystalline compounds form first and are subsequently amorphised at least partially. The prealloyed melt-spun samples also become nanocrystalline after deformation. Heavy Fe contamination from the milling medium occurs before amorphisation at long milling times.Although some unalloyed iron-chromium stainless steel fragments are present, most of the iron is in solution and can play an important role in the observed amorphisation. Some amorphisation occurs in nearly all our samples. Maximum amorphisation is obtained for the starting composition Ni60Ge40 which yields a nearly fully amorphous (NiFeCr)70Ge30 alloy. Calorimetric measurements indicate a heat of crystallisation of about 3 kJ/mole for this composition.The amorphous phase is paramagnetic while the crystalline alloy shows a Curie temperature T = 660 K.
Thermodynamic aspects of the formation of metastable alloys by ion bombardment, and phase transformations, are analysed using a model which combines classical elasticity theory and Miedema’s model of heats of formation of alloys. This model has recently been used successfully to predict the glass forming ranges of binary and ternary transition metal alloys.
A quantitative measure of the degree of order in amorphous or molten binaries is given by the chemical short range order parameters according to Warren and Cowley or according to Cargill and Spaepen. Both of them are based on partial coordination numbers which follow from partial structure factors via partial pair correlation functions. An overview on the chemical short range order parameters as obtained up to date is given. We restrict ourselves mainly on the discussion of the Warren Cowley short range order parameter and propose a refined short range order parameter which can be applied to alloys regardless to differences in the atomic diameters of the alloy components and which furthermore is convenient to calculate the contribution of the chemical short range order to the scattered intensity.
STM images of the free surface of electrodeposited amorphous CoP alloys have been obtained, in the scale of 50 Å to 1400 Å. From images showing atomic resolution the interatomic distances for first neighbours have been measured and the radial distribution function (extrapolated to three dimensions) has been calculated, showing two peaks which are in agreement with results obtained from scattering experiments for samples of similar composition. Calorimetric and magnetic characterization of the samples have also been carried out.
Although amorphous metals exhibit high degree of disorder, increasing evidence has been pointed out in recent years that they also yield a significant degree of order extending over distances as large as tens of angströms. The main features which characterize such a medium range order are most often reminiscent of both the local units observed in the parent crystal and of the topological connection scheme between units. In glass forming alloys, we relate the occurence of amorphization to an orderdisorder transition that takes place together with a lattice instability mechanism which is mainly due to an atomic size mismatch effect. A numerical simulation has been designed in two dimensions in order to explore into more detail the relevant parameters calculated as a function of physical parameters.
Hydrogen-induced internal friction may be a valuable tool for crystallization studies especially in binary transition metal glasses like Co33Zr67. The series of reorientation peaks observed in this alloy was used both phenomenologically to follow the transformation sequence and as a local probe to study the role of specific structural units. The latter is discussed especially for the initial transformation into a nanocrystalline state of a metastable CoZr2 phase with the cubic E93 structure. In this state, a discontinuity in the height and position of the internal friction peak is found at H concentrations near 5 at%, which is interpreted as a transition from the Zr4 tetrahedra in the grain boundaries to the Zr3Co tetrahedra within the grains as the dominant sites for the reorientation jump. The additional influence of hydrogen on the transformation sequence is correlated with the density of Zr4 tetrahedra, which mainly favours the equilibrium phase with the tetragonal C16 structure.
High energy ball milling induces order-disorder transitions on the Y1Ba2Cu3O7-x orthorhombic structure. A new metastable phase with a simple cubic perovskite structure (Y0.33Ba0.67)CuO3-y is formed. This new phase is created by a disorder on the Y and Ba cationic sites and on the oxygen sublattice. The evolution of the microstructure and superconducting properties during the milling process is presented. The thermal stability of the new metastable structure is also investigated.
Nanocrystalline Fe and Fe95Zr5 thin films with average grain sizes ranging from 5nm to 15nm made by e-beam codeposition have been investigated. The kinetics of grain growth during thermal annealing and 500keV Xe+ ion irradiation have been studied using dark-field transmission electron microscopy. In contrast to nanostructured pure elements produced by compaction of ultrafine powders, a considerable grain growth is observed at 673K in thin films of nanostructured Fe, but not in Fe95Zr5. The tracer diffusion of implanted gold have been investigated in these films and, for comparison, in crystallized, initially amorphous, Fe80Zr20. The broadening on annealing of the approximately Gaussian gold concentration profile was measured by Rutherford Backscattering Spectrometry. The diffusion constant corresponding to the time evolution of the tracer concentration can be expressed as
D* = 8.4·10−10 m2/s exp(− 150 kJ mole−1/RT)
for Fe95Zr5 in the temperature region around 800K. The diffusion constant is close to that observed for crystallized Fe80Zr20 and amorphous Ni65Zr35 thin films. This is in contrast to the reported much higher diffusion constants for nanostructured metals made of compacted powders.
The microscopic nature of mechanically alloyed powders has been investigated using the 57Fe and 119Sn Mössbauer spectroscopy. The following alloy systems have been treated by a low-energy ball mill; Al-Fe, Fe-Sn, Ag-Fe, Fe-C and Fe-B. To understand the elementary process of kneading, the repeated rolling method has been employed for the Al-Fe system and compared with the results from the ball-milling method. Pure Fe powders have also been ball-milled in order to understand the effects of heavy deformation using the same conditions and investigated by the 57Fe Mössbauer spectroscopy. Hyperfine interaction parameters obtained show that atomic dispersion and the formation of an amorphous, non-equilibrium solid solution and intermetallic compounds occur during mechanical alloying. From the experimental findings for each powder the characteristic features of the mechanical alloying process have been discussed.
We propose for the first time, a quantitative thermodynamic explanation for the formation of solid-solutions of immiscible elements by mechanical alloying. It is shown how codeformation of a hard and a soft phase results in the formation of disc or needle shape particles possessing large aspect ratios. As the particle size drops to about 10nm with continued deformation, the particle tips and edges generate nanometer size fragments upon subsequent necking. Using simple thermodynamic arguments we show how otherwise immiscible atoms of these fragments will enter in solution and that this process can lead to their full dissolution.
We report on the crystal to amorphous phase transition induced by ball — milling in pure Si and pure Ge powders. Based on X-ray diffraction patterns, scanning and transmission electron micrographs, EDX microanalyses, differential thermal analyses and differential scanning calorimetry, it seems that amorphization occurs during the continuous decrease of the particle grain size and the crystalline lattice parameter expansion which both lead to the destabilization of the diamond cubic structure. The analyses of the X-ray diffraction patterns reveal the coexistence of microcrystallites and of an amorphous phase. The transmission electron microscopy investigations reveal that some particles of about 100 to 200 nm exhibit an amorphous structure (bright field image and selected area diffraction patterns). Only some slight effect of the ball — milling onditions are noted on the critical values of the grain size and the lattice expansion of such a destabilization.
We have extended the previously developed expression for the effect of sharp concentration gradients on the driving force for crystallisation of binary amorphous interlayer to the case of ternary systems. We find that appropriately chosen thirs elements should further reduce this driving force. It was previously shown that the driving force for crystallisation is reestablished as the amorphous layer grows and the concentration gradient drops to a critical value. Appropriately chosen ternary elements should increase the critical thickness beyond which crystallisation may occur. These concepts were applied to Ni/Zr and (Ni-Cu)/Zr multilayers and the experimental results were found to support the theory.
Mechanichal alloying of Cu, Ti and TiH2 and grinding of -CuTi in H2 atmosphere lead to a mixture or a hydrogenated amorphous phase and TiH2. From the heat of transformation of metastable phases, their enthalpy of formation is determined along a section of the Cu-Ti-H phase diagram. The free energy of formation, estimated at 3 00 K, shows a driving force for amorphization at low hydrogen concentration. A phase separation is predicted for higher hydrogen contents. The effect of deformation of pure elements on amorphization is discussed.
Milling conditions are of fundamental importance in defining the energy transfer process and hence the nature of the final product when milling a given system starting from powders. In a planetary mill, the energy transfer is essentially governed by the hit of the ball against the vial wall. In order to analyse the hit mechanism, the velocities of the balls inside the mill have been estimated and reproduced in “free fall” experiments, in which the yield of the impact has been measured. From free fall experiments with bare balls and with powder coated balls taken out from the mill at different milling times, it follows that, at least in the early stages of milling, the hit is totally anelastic and that the energy of the ball is totally dissipated onto the powder. The outcome of the free fall experiments can be used as a guide in understanding the energy transfer per hit realized in a planetary mill.
Amorphous ultra-fine Fe-B binary and (Fe, Co, Ni)-B ternary powders were prepared by a chemical reduction of metal ions in an aqueous solution using KBH4. The powders have a spherical shape with diameters from 20 to 50 nm. The chemical composition of ultra-fine powders depends on the mixed molar ratio of KBH4 to metal ions in the Fe-B binary system. The Mossbauer results show that depending on the molar ratio, two kinds of distribution of internal magnetic field P(Bhf> exist. From Mb’ssbauer, X-ray and inductively coupled plasma (ICP) spectroscopy measurements we propose the following reaction process for the formation of ultra-fine Fe-B powders;
(1) 4Fe2+ + 2BH4- + 6OH- →2Fe2B + 6H2O + H2
(2) 4Fe2+ + 2BH4- + 7OH- →Fe3B + Fe + BO2- + 5H2O + 5/2
H2
The nearly free electron model has been applied to indagate the stability of the quasicrystalline state through the analysis of X-ray or neutron diffraction data. Several alloys, representative of the families of Aluminium-Transition Metal (Al-TM) and of PSD-type systems have been considered. The validity of the picture is discussed in the framework of recent measurements of electronic properties of the icosahedral state. Model predictions and experimental data agree for both classes of icosahedral systems. On such grounds a specific electron phase, characteristic of the quasicrystalline state of matter is defined.
Interpenetrating Pseudo — Icosahedral atomic Clusters (IPIC) are found in various \(m\dot{3}\dot{5} \) — approximant crystals such as α — AlMnSi, R — AlLiCu, presenting one orientational family of IPIC., c — Ti2Ni, ß — AlMnSi, μ, — Al4Mn, exhibiting several ones, and in many models of icosahedral or decagonal phases. We have developped a theory of G — approximant crystals based on IPIC., where G is a non — crystallographic point group (G = \(m\dot{3}\dot{5} \), Cn, D2n, with n = 5, > 6) allowing to describe what happens in reciprocal space for the intense Bragg peak positions and their intensities, which is easily generalizable to quasicrystals and provides a continuity of the perception of the symmetry group G in reciprocal space when small atomic rearrangements occur in real space during the phase transition quasicrystal <-> approximant crystal. This theory is new and completes the edifice of the theory of crystallographic groups by considering non — crystallographic point groups.
The concept of magic strain is reviewed and illustrated using a simple two dimensional lattice. Practical use of the concept for making theoretical predictions of new ordered structures is demonstrated; first, using the two dimensional example, and second, using realistic calculations which predict that cubic silicon can be transformed to a stable body-centered tetragonal structure with five-fold coordination.
Bimetallic nanoalloys combining magnetic and noble metals are promising for applications in magnetic sensors, catalysis, optical detection, and biomedical imaging. Their development relies on understanding morphology, electronic structure, and crystallography. This study explores iron-based magnetic nanoalloys using efficient synthesis and advanced characterization. Molecular dynamics (MD) simulations examined atomic-scale morphology and structural features, linking them to magnetic behavior. A spin-lattice dynamics algorithm simulated iron-copper (FeCu) nanoalloys of varying sizes and compositions. FeCu nanoalloys were synthesized via a one-step reduction reaction and analyzed using multiple techniques, yielding nanoparticles with high saturation magnetization and an 11 nm average size. Simulations and experiments confirmed core-shell (CS) and Janus morphologies, where copper shells an iron core. Findings suggest that composition, rather than morphology alone, predominantly influences magnetic properties, while the core-shell morphology enhances oxidation resistance due to the noble copper metal employed. This study is the first to integrate the spin-lattice algorithm with experimental analysis, providing consistent insights into design and accurate characterization. Thus, it confirms the practical and novel synthesis of low-size FeCu nanoparticles with exact ideal superparamagnetic properties—exhibiting no hysteresis—suitable for various research and industrial applications.
Recently-developed high-entropy alloys (HEAs) containing multiple principal metallic elements have extended the compositional space of solid solutions and the range of their mechanical properties. Here we show that the realm of possibilities can be further expanded through substituting the constituent metals with metalloids, which are desirable for tailoring strength/ductility because they have chemical interactions and atomic sizes distinctly different from the host metallic elements. Specifically, the metalloid substitution increases local lattice distortion and short-range chemical inhomogeneities to elevate strength, and in the meantime reduces the stacking fault energy to discourage dynamic recovery and encourage defect accumulation via partial-dislocation-mediated activities. These impart potent dislocation storage to improve the strain hardening capability, which is essential for sustaining large tensile elongation. As such, metalloid substitution into HEAs evades the normally expected strength-ductility trade-off, enabling an unusual synergy of high tensile strength and extraordinary ductility for these single-phase solid solutions.
Featured Application
This review article deals with the synthesis and applications of CuFe bimetallic nanoparti-cles. Although much research has been published on nanomaterials, overall review articles on CuFe nanoparticles are lacking and this article presents the latest data on the synthesis, prop-erties and possible applications of CuFe nanoparticles.
Abstract
Bimetal CuFe (copper-iron) nanoparticles, which are based on the earth-abundant and inexpensive metals, have generated a great deal of interest in recent years. The possible modification of the chemical and physical properties of these nanoparticles by changing their size, structure, and composition has contributed to the development of material science. At the same time, the strong tendency of these elements to oxidize under atmospheric conditions makes the synthesis of pure bimetallic CuFe nanoparticles still a great challenge. This review reports on different synthetic approaches to bimetallic CuFe nanoparticles and bimetallic CuFe nanoparticles supported on various materials (active carbide, carbide nanotubes, silica, graphite, cellulose, mesoporous carbide), their structure, physical, and chemical properties, as well as their utility as catalysts, including electrocatalysis and photocatalysis.
Using mechanical attrition (milling) which also transforms the material into nanocrystalline form, various L1 2 or γ-type Ni 3 A1-Fe alloys have been prepared in metastable-γ form and their magnetisations M(γ) and M(γ) and Curie temperatures T c (γ) and T c (γ) measured.
Using magnetic properties we determine isothermal ordering kinetics in the nanocrystalline γstate and find that the ordering reaction cannot propagate across the nanograin boundaries. Each grain must obtain its own γ nucleus thus resulting in a Johnnson-Mehl-Avrami exponent n =1.
We review recent results in new materials development by mechanical preparation techniques (mechanical alloying and milling) with an emphasis on research performed in Grenoble. Two general categories are considered: nanocrystalline and amorphous materials obtained by polymorphous (iso-compositional) transformations and mechanical alloying of highly supersaturated solid-solutions. We then consider nanostructures obtained on the path back towards equilibrium structures by nucleation and growth processes or by phase-separation. Design of new magnetic and mechanically attractive nanocomposites using amorphous precursors and metal-ceramic nanocomposites obtained by reactive milling are also considered.
Carbon-supported Pt–Co alloy catalysts with Co contents of 10–50 wt% have been synthesized by reducing aqueous Co2+ ions with borohydride or precipitating as hydroxide in the presence of a commercially available carbon-supported Pt catalyst followed by heat treatment in a 90% Ar–10% H2 mixture at 900 °C for 1 h. X-Ray diffraction indicates the Pt–Co alloys thus obtained to have the ordered Pt3Co or PtCo type structures or the disordered Pt type structure depending on the Co content. High resolution transmission electron microscopy (HRTEM) studies also confirm the formation of ordered structures. Structural analysis of the products after annealing at various temperatures 500 ≤
T
≤ 900 °C for 1 h suggests the ordering to be maximized for an annealing temperature of around 650 °C. Evaluation of the Pt–Co/C alloy catalysts for oxygen reduction in half cells employing KOH as the electrolyte and in proton exchange membrane fuel cells indicates that the alloys with the ordered Pt3Co or PtCo type structures have higher catalytic activity with lower polarization losses and higher power densities than Pt or disordered Pt–Co alloys. With the ordered phases, the activity increases with the extent of ordering. The enhanced catalytic activity is explained based on optimal structural and electronic features consisting of optimum number of Pt and Co nearest neighbors, Pt–Pt distance, and d-electron density in Pt.
The influence of boron on the structural stability of sub-microcrystalline Ni3Al intermetallic compounds was investigated by comparing a high-purity material with a boron-doped (0.1 wt%) compound. The nanocrystalline structure was obtained by severe shear deformation under quasi-hydrostatic pressure. Residual electrical resistivity, Vickers microhardness, X-ray diffraction and transmission electron microscopy were used to characterize the material evolution during thermal treatments in the temperature range 293–1313 K. After severe deformation the materials were disordered, with a small crystallite size of about 20 nm, similar in both materials. During isochronal anneals, the evolution of the microstructure, the long-range ordering and the recovery of the investigated properties took place at higher temperatures in the boron-doped compound, i.e. the thermal stability of the cold-worked structure was higher.
We used Mössbauer spectrometry, X-ray diffractometry, a novel imaging method of electron energy loss spectrometry, and small-angle
neutron scattering (SANS) to study early stage thermal instabilities of nanophase Fe-Cu alloys prepared by mechanical attrition.
Mössbauer spectrometry confirmed previous reports of an extended Cu solubility in the body-centered cubic (bcc) phase of the
as-milled material. Mössbauer spectrometry also provided evidence that in the compositional range of bcc-face-centered-cubic
(fcc) two-phase coexistence, the bcc phase had a Cu concentration nearly the same as the overall composition of the alloy.
After the as-milled powders were annealed at temperatures as low as 200 °C, however, Mössbauer spectrometry showed significant
chemical unmixing of the Cu and Fe atoms. In annealed bcc Fe-20 pet Cu alloys, SANS measurements indicated that Cu segregated
to grain boundaries. This segregation of Cu atoms to bcc grain boundaries did not alter significantly the tendency for grain
growth, however. X-ray diffractometry showed that grain growth during thermal annealing was similar for all alloys, although
grain growth was small at temperatures below 300 °C. The two-phase (bcc plus fcc) alloy of Fe-30 pc Cu was more unstable against
chemical segregation than were the single-phase (bcc or fcc) alloys. Energy-filtered imaging indicated that the Cu atoms segregated
to regions around the bcc grains, perhaps to the adjacent fcc crystallites.
We have calculated free energy and vibrational entropy differences in Ni3Al between its equilibrium ordered structure and a disordered fcc solid solution. The free energy and entropy differences were calculated using the method of adiabatic switching in a molecular-dynamics formalism. The path chosen for the free-energy calculations directly connects the disordered with the ordered state. The atomic interactions are described by embedded-atom-method potentials. We find that the vibrational entropy difference increases with temperature from 0.14kB/atom at 300 K to 0.22kB/atom at 1200 K. We have calculated the density of states (DOS) of the disordered phase from the Fourier transform of the velocity-velocity autocorrelation function. The disordered DOS looks more like a broadened version of the ordered DOS. Analysis of the partial density of states shows that the Al atoms vibrations are most affected by the compositional disorder. The phonon partial spectral intensities along the 〈100〉 direction show that the vibrational spectrum of the disordered phase contains intensities at optical mode frequencies of the ordered alloy. We find that the volume difference between the ordered and disordered phases plays the most crucial role in the magnitude of the vibrational entropy difference. If the lattice constant of the two phases is set to the same value, the vibrational entropy difference decreases to zero.
The solid state reaction of the normally immiscible copper and cobalt powders has been studied at the equiatomic composition with the mechanical alloying (MA) technique, supplemented by the milling of cobalt and copper elemental powders. The diffraction of unmilled pure cobalt powder shows the coexistence of the two face-centred-cubic (f.c.c.) and hexagonal-close-packed (h.c.p.) allotropes. After 1 h of milling the neutron diffraction pattern reveals only the highly distorted h.c.p. phase. Further, the degree of distortion in the h.c.p. phase is highly dependent on the crystallographic directions. Mechanical alloying the Cu-Co equiatomic mixture creates an almost entirely f.c.c. single phase after zh of treatment. The lattice parameter of the Cu(Co) extended solid solution decreases on increasing the milling time. Moreover, a thermal treatment at 700°C of the powders M A 16 h demixes the pure constituents. A comparison with previous data on the Cu-Co system prepared by rapid quenching, evaporation and magnetron sputtering methods confirms the MA method to be an efficient synthesis tool of new metastable non-equilibrium phases.
We report on the formation by mechanical alloying of fcc solid solution of Fe and Cu, which are immiscible. Intense codeformation of Cu with Fe and other high-melting bcc metals generates small fragments with tip radii of the order 1 nm such that capillary pressures force the atoms on these fragments to dissolve. The critical radius for dissolution is found to increase with solute content inside the spinodal thus leading to full dissolution of one of the components. The fcc-FeCu solid solutions have Curie temperatures similar to those of vapor-deposited alloys and a positive heat of mixing that is in accord with theoretical results.
ResearchGate has not been able to resolve any references for this publication.