Short-range Order State Research Articles

Short-range order (SRO) and its effect on the primary creep behavior of a Ti–6wt.%Al alloy

Titanium alloys have been shown to exhibit significant levels of creep at room temperature at stresses well below the macroscopic yield strength. In this work, the effect of short-range order (SRO) on the room temperature creep behavior of Ti–6wt.%Al was studied. It was observed that SRO provides significant strengthening and that the creep transients are dramatically altered due to changes in the SRO state. Slip was observed to be planar in samples that were treated to have significant SRO and homogeneous in samples in which SRO was decreased by heat treatments.

Short-range-oxygen order and superconducting phase separation in La 2CuO 4+x

Structural features and physical properties in La 2CuO 4+x materials have been systematically investigated. A short-range-order (SRO) state has been observed at loww temperatures for 0.05<x<0.1. Anomalies in physical properties, in particular a superconducting phase segregation, are found to be in connection with the presence of this SRO state. We herein explain this SRO state in terms of intercalated-oxygen ordering, which could yield a local excess-oxygen content of x≊1/16 and a local hole concentration of n≊1/8.

The effect of microstructure on the temperature dependence of the interlayer coupling in Co/Cu multilayers

Three classes of giant magnetoresistance Co(1 nm)/Cu(2.1 nm) multilayers were sputter grown with different microstructures in respect to grain size and interface roughness, depending on deposition conditions. Magnetization and current in-plane giant-magnetoresistance (GMR) isothermal loops reveal an unusually high increase of coercivity from 280 down to 5 K. In addition, a systematic variation was observed in the temperature dependence of the indirect exchange coupling as the Co–Cu layering is modified in the three classes of Co/Cu multilayers. Specifically, the temperature dependence of the saturation (switching) field in the GMR-loops, and the indirect coupling strength, vary as (T/T0)/sinh(T/T0) whereas the spin-blocking temperature T0 is found equal to 84(4), 96(11), and 105(10) K for class A, B, and C multilayers, respectively. These results indicate that the desirable low hysteresis appears in the GMR loops at room temperature because the spin structure becomes unstable above the obtained T0 due to domain wall fluctuations. Such magnetic fluctuations define a short–range order state above T0 that depends on Co–Cu intermixing and geometric factors of the grains.

Ordering process of Cu3Pt from short-range order state studied by in-situ TEM observation

The state of short-range order (SRO) and the ordering process in Cu3Pt have been studied by the in-situ transmission electron microscope observation method. In order to detect weak diffraction-patterns and weak image-contrasts, an imaging plate system and/or digital-process of video-recorded signals were applied. It is confirmed that above the critical temperature for ordering, Tc, the alloy shows diffuse scattering in its electron diffraction pattern. The appearance of the diffuse scattering indicates that the “disordered” Cu3Pt (designated as A1) has an SRO-state in thermoequilibrium. Dark field images taken by using the diffuse scattering shows a fine dot-contrast, which indicates the presence of “microdomains”. The dot-contrast is flickering at higher temperatures, but the rate of flickering decreases with falling temperature and finally stops before reaching Tc. On further decreasing temperature, some of the microdomains grow up to large domains of the long period superstructure (L12-s) phase. This indicates that the transformation to L12-s via SRO proceeds without a process of nucleation even though the transformation is of a first-order.

In situ TEM observation of long range ordering via short range order in Cu3Pt

The thermoequilibrium state of short range order (SRO) and the ordering process to the long range order (LRO) phase in Cu3Pt have been studied by thein situ transmission electron microscope (TEM) observation method. It is confirmed that the alloy has the SRO state in thermoequilibrium within the temperature region for the disordered fcc phase (designated as A1). The SRO state can be interpreted in the ‘microdomain model’. The degree of order fluctuates temporally as well as spatially at higher temperature, but freezes at a lower temperature in the A1 region. In the temperature region of (A1+LRO), some of the microdomains in the SRO grow up to large domains of LRO. This indicates that the transformation process to LRO via SRO proceeds without a process of nucleation even though the transformation is of a first order. Division of A1 into subregions and physical meaning of the phase boundary between (A1+LRO) and A1 are discussed.

Short range order in Ni 4Mo and its high resolution electron microscope images

The short range ordered (SRO) structure in Ni 4Mo has been investigated by means of high resolution transmission electron microscopy with digital image processing and Monte Carlo simulation. High resolution transmission electron microscope (HRTEM) images were recorded with an Imaging Plate (IP) system. Conventionally printed images of the SRO state exhibit locally dot patterns corresponding to N 2M 2-type (chalcopyrite-like) structure. However, the dot patterns transform into those of subunit cell clusters of D1 a, D0 22 and Pt 2Mo structures, when the image is output with a different gray scale. The Monte Carlo simulation with appropriate interaction parameters of atoms has explained successfully the HRTEM images. It is concluded that the SRO state in Ni 4Mo involves subunit cell clusters of D1 a, D0 22 and Pt 2Mo structures in the atomistic level of microstructure. The concept of 〈1 1/2 0〉* static concentration waves corresponding to the N 2M 2 pattern is rationalized as a mesoscopically averaging view of the SRO state.

Novel ordering transitions in off-stoichiometric γ-TiAl intermetallics and the similarities between these and the special point ordering processes in Ni-Mo alloys

Ordering on Ti-rich (002) planes of Al-rich γ-TiAl (Ll0) phase can be described in terms of evolution of superlattice structures consisting of space filling arrangements of two-dimensional (2D) motifs of different shapes. These 2D structural units (tiles) comprising unimolecular clusters of Ti2Al, Ti3Al and Ti4Al stoichiometries and having the shapes of lean and fat rhombi and squares respectively, either singly or in combinations, constitute the building blocks of this variety of superlattice structures. The Al5Ti3 and the Al11Ti7 derivatives of the off-stoichiometric γ-TiAl phase, for instance, can be viewed as identical periodic arrangements of lean rhombi and squares and fat rhombi and squares, respectively. Previous investigations, involving projections from higher dimensional lattices, have shown that these two rather simple structures can be regarded as “unmodulated” periodic approximants of a hypothetical quasiperiodic superlattice (QPSL) structure and that these can transform continuously into the QPSL through an infinitum of long period antiphase boundary (LPAPB) modulated structures. Structures, which identify themselves with some of the intermediate stages of the periodic to quasiperiodic transformation, have been observed in some investigations including the present one. Periodic arrangements of the three types of tiles have also been used to describe the nature of Short Range Order (SRO) and various Long Range Ordered (LRO) phases in Ni-Mo and other f.c.c. alloys which show SRO maxima at 1120 and equivalent points in the reciprocal space. A remarkably similar state of SRO, albeit, with SRO maxima at incommensurate positions, has been observed in this transmission electron microscopy investigation on rapidly solidified Al60Ti40 and Al65Ti35 alloys. Occurrence of curved streaks of diffuse intensity during the early stages of the evolution of some of the Long Range Ordered (LRO) phases is another point which underscores the similarity between the two ordering systems. The results of the present investigation lend additional support to the hypothesis of continuous periodic to quasiperiodic superlattice transformations.

Short range order and its transformation to long range order in Ni 4Mo

The short range ordered (SRO) state and the time-evolution of the long range ordered (LRO) structure of D1 a in a Ni 4Mo alloy were investigated by Monte Carlo simulation based on an f.c.c. Ising model and by transmission electron microscopy (TEM). The simulation using appropriate values for pairwise atomic interactions up to the fifth nearest neighbors bears diffuse intensity maxima at {1 1/2 0} positions in the Fourier power spectrum for the early stage of SRO, and then the maxima shift to {4/5 2/5 0} for the stable D1 a structure as the long range ordering proceeds. The present results are in good agreement with the temporal change in electron diffraction due to the SRO–LRO transition in a quenched Ni 4Mo. The SRO state obtained in the simulation contains microclusters of subunit cells of D1 a, D0 22 and Pt 2Mo structures. If such a mixed state of microclusters is projected onto a two-dimensional {100} plane in a similar way to high resolution TEM, the mixed state exhibits locally a dot-pattern analogous to the projected N 2M 2-type (chalcopyrite-like) structure, which gives rise to superlattice reflections at {1 1/2 0}. This suggests that a statistically averaging view of the mixture of microclusters leads to the concept of static concentration waves of k=1 1/2 0, which has been often employed to describe the SRO state. The formation of D0 22 and Pt 2Mo-type subunit cells in addition to stable D1 a is explained in terms of their structural relationship. The transformation from SRO to LRO is attributed to continuous growth of D1 a segments into ordered domains.

Short-range-order state in the Sr2Nd1−xCaxCu2O5+yF1+δ (0⩽x⩽1) superconducting system

The structural features of the Sr2Nd1−xCaxCu2O5+yF1+δ (0⩽x⩽1) oxyfluoride system have been investigated by electron diffraction and high resolution electron microscopy. The short-range-order (SRO) state has been observed in the Sr2Nd0.5Ca0.5Cu2O5+yF1+δ oxyfluoride (Tc≈70 K). This kind of SRO structure can be understood in terms of the ordering of insertion of fluorine and/or oxygen atoms located between the SrO(F) double layers. The most probable ordering scheme is proposed. The effects of Ca content on this kind of SRO state as well as other locally ordered states have also been investigated.

Anisotropic Ferromagnet on a Triangular Lattice with Antiferromagnetic Next-Nearest-Neighbor Interactions

We study the magnetic ordering of an anisotropic Heisenberg ferromagnet on a triangular lattice with antiferromagnetic next-nearest-neighbor interactions by means of the mean field theory and a computer simulation. We find that, as the temperature is decreased, an incommensurate short-range order state and a ferromagnetic long-range phase appear successively. The phase transition between them is of the first order and the relaxation time of the transition is very long. Hence, we suggest that the low temperature phase realized in nature is a domain state in which ferromagnetic domains with various sizes are tangled with each other. We also suggest that the model would be applicable to the compound LiNiO 2 .

Short-range order in spin density wave antiferromagnets

The model of the short-range order state for itinerant antiferromagnets with SDW is developed. The thermodynamic transverse spin density fluctuation are shown to influence essentially on the formation of the long-range magnetic order. In the wide temperature range these occurs the shortprange, magnetic order regim. The properties of the low-frequency transverse excitations of the spin density are analized. The existence of a fully diffusive mode at small wave vectors is predicted.

On the interpretation of HREM images of partially ordered alloys

The ordering for 1 1 2 0 alloys has been studied by high-resolution electron microscopy (HREM). The distribution of the intensity maxima in the HREM image have been statistically examined, which provides a profound basis for the image interpretation. Processing of the HREM images allows “dark-field” images to be obtained, exhibiting a two-dimensional distribution of those columns which contain the most information in order to interpret the short-range order correlations. Pair correlations and higher cluster correlations between projected columns can be visualised, providing unique information about the ordering as retrieved from an experimental result without any other assumption. The method has been applied to Au 4Cr and to Au 4Mn to interpret the quenched short-range order state and the transition to long-range order.

Kinetics of short-range ordering in αCu-Al alloys

The ordering behaviour in quenched αCu-Al was investigated by differential scanning calorimetry (DSC) under rising temperature conditions and by electron diffraction. It was found that the ordering processes can be better explained in terms of a homogeneous short-range order (SRO) model rather than a heterogeneous disperse order (DO) model as previously interpreted for the same type of experiments. The DSC traces indicate that the ordering takes place in two stages: the stage 1 ordering at lower temperatures is associated with the migration of excess vacancies and the stage 2 ordering at higher temperatures is associated with the migration of equilibrium vacancies. At higher temperatures, a marked surge of energy absorption occurs (stage 3) which is attributed to the destruction of order. For furnace-cooled alloys, only stage 3 appears. The relative dominance of stages 1 and 2 is influenced by the quenching temperature, the quench rate, the density of vacancy sinks and the sample shape before quenching. A relationship describing the overall SRO kinetics for both stages together, in terms of either the reacted fraction or the first SRO parameter, is proposed. A method for evaluating boundary values for this parameter is developed, making use of the features displayed by the DSC thermograms. The mobility of the vacancies, which controls their life-time and the short-range ordering rate, was evaluated by computing frequency factors and activation energies. Values for activation energies controlling the short-range ordering rate are somewhat smaller than the effective values obtained for stage 1 from the DSC traces, suggesting that the presence of solute-vacancy complexes may be important mainly as the aluminium concentration increases. From estimations of solute-vacancy and divacancy binding energies, it is inferred that divacancy formation is unlikely in the alloys under study. Process frequency factor values are in very good agreement with those calculated for a short-range order state developed during two-stage ordering by a vacancy mechanism. A flow diagram of the factors controlling the fractional increase of short-range order during anisothermal experiments is proposed.

Electron microscopy and diffraction of ordering in a Ni-25wt.%Mo alloy

Electron microscopy and diffraction were employed to study the disorder-order change in a Ni-25wt.%Mo alloy in the temperature range 680–880 °C. In addition to confirmation about the presence of microstructure and clusters in the state of short-range order and the occurrence of metastable Ni 2Mo and Ni 4Mo phase, the present study has enabled the construction of a time-temperature-transformation curve. It is also shown that the change from f.c.c. solid solution to ordered h.c.p. Ni 3Mo proceeds through nucleation and growth and not by shear.

Short-range ordering by excess and thermal vacancies during linear heating experiments in α-CuAl alloys

The ordering behaviour in quenched α-CuAl was investigated by differential scanning calorimetry (DSC) under increasing temperature conditions. It was found that the ordering processes can be better explained in terms of an homogeneous short-range order (SRO) model rather than a heterogeneous disperse order model as previously interpreted for the same types of experiment. The DSC traces indicate that the ordering takes place in two stages; stage 1 ordering at low temperatures is associated with the migration of excess vacancies and stage 2 ordering at high temperatures is by the migration of equilibrium vacancies. The relative dominance of each stage is influenced by the quenching temperature, the quench rate, the density of vacancy sinks and the sample shape before quenching. A relationship describing the overall SRO kinetics for both stages together in terms of either the reacted fraction, or the first SRO parameter was proposed. A method for evaluating boundary values for this parameter was developed, making use of the features displayed by the DSC thermograms. The mobility of vacancies, which controls their lifetime and short-range ordering rate, was evaluated by computing frequency factors and activation energies. The resulting values for activation energies controlling the short-range ordering rate were somewhat smaller than the effective values obtained for stage 1 from the DSC traces, suggesting that the presence of vacancy-solute complexes may be important especially as the aluminium concentration increases. From estimations of vacancy-solute and divacancy binding energies, it was inferred that divacancy formation is unlikely in the alloys under study. The process frequency factor values were in very good agreement with those calculated for an SRO state developed during two-stage ordering by a vacancy mechanism.

Equilibrium and kinetics of local ordering in Ni(Al) solid solutions

Short-range order has been investigated in Ni(Al) solid solutions (with 6 and 10 at.% Al) by measurements of the residual electrical resistivity during thermal treatments. Local ordering results in an increase of resistivity, much stronger for the higher Al concentration. During isothermal annealing, the asymptotic residual resistivity, which characterises the equilibrium state of short-range order, was found to be a linear function of reciprocal temperature. In conditions of constant vacancy concentration, the approach to equilibrium could not be consistently represented by first-order kinetics, even with a distribution of relaxation times or by a sum of two exponentials. Indeed, for the 10 at.% Al alloy, the results clearly proved that the time constant was amplitude-dependent. Although a so-called cross-over was observed in that alloy, the quantitative evaluation of the relaxation of short-range order was carried out by application of a power law. The activation enthalpies of self-diffusion obtained lie in the same range as published diffusion data and they appear to increase when Al concentration goes from 6 to 10 at.%.

Electronic Structure of Cu-Pd Alloys

Cu-Pd alloys containing 12.6-60.6 at% Pd, when quenched from the fee phase field are known to exhibit diffuse scattering at the 100 and the 110 positions in reciprocal lattice and these diffraction effects are believed to be characteristic of the short range order(SRO) present in these alloys[1]. Moreover, quenched fcc alloys containing more than 13 at% Pd actually exist in a state of modulated SRO, i.e., the diffuse intensity maxima at 100 and 110 reciprocal lattice positions are associated with two-fold and four-fold splitting respectively [1,2]. These effects arise as a result of accidental minima in the ordering potential, V(k) in reciprocal space located away from the special point of the fee structure [2]. Another explanation for the above diffraction effects is from the theory due to Krivoglaz [3] and Moss [4] according to which the long range oscillatory part of V(k) plays a significant role in these alloys and Pd additions increase the flat portions of the Fermi surface of Cu in the &lt;110&gt; directions and this is manifested in the form of strong Kohn like anomaly surfaces in reciprocal space [1,4].

On the role of the electronic structure in order–disorder transformation of metallic alloys

Abstract Diffuse scattering of X-rays, electrons and neutrons from binary alloys provide detailed information about the state of compositional short-range order. Angle-resolved photoemission spectroscopy may be used to probe the corresponding electronic structure. Thus in principle these experiments together can shed light on the electronic causes of ordering. In order to facilitate the interpretation of both kinds of experiment from this point of view we develop a first-principles theory for each. Both theories are based on the self-consistent Korringa-Kohn-Rostoker coherent-potential approximation description of the electronic structure and are fully relativistic. To illustrate the power of such an approach we present explicit calculations for the Cu0·75Au0·25 system and comment on some recent experiments on this classic example of ordering alloys.

Ordering in rapidly solidified Ni 2Mo

Ordering processes in the Ni-Mo system have been a subject of several investigations. Although the ordering behaviour of the Ni/sub 4/Mo and the Ni/sub 3/Mo has been examined in detail, no such study has been reported in the case of the Ni/sub 2/Mo alloy. The lack of experimental work on ordering transformations in Ni/sub 2/Mo is presumably due to the difficulty in obtaining a single phase fcc alloy of this composition. Enhanced solid solubility of Mo in Ni, which accompanies rapid solidification processing (RSP) makes the formation of such a phase possible. The ordering processes in Ni-Mo based alloys show several remarkable features. Firstly, the alloy (15 - 28 at % Mo) quenched from the ..cap alpha.. -phase filed exhibit a short range order (SRO) characterized by the presence of intensity maxima at /1 1/2 0/ fcc positions of the reciprocal space. This state of SRO has been attributed to the occurrence of 1 1/2 O spinodal ordering in the system. Secondly, the transformation from the state of SRO to the equilibrium/metastable coherent long range ordered (LRO) structures appears to take place in a continuous manner at relatively low temperatures of aging. Three different coherent LRO structures, namely: the equilibriummore » Ni/sub 4/Mo (prototype structure D1/sub a/) and the metastable Ni/sub 3/Mo (DO/sub 22/) and Ni/sub 2/Mo (Pt/sub 2/Mo) structures have reported to evolve from the SRO alloy, depending upon the aging treatment and the composition of the alloy.« less

Ordering in Ni4Mo by TEM and APFIM

The state of long-range order (LRO) and short-range order (SRO) in Ni4Mo has been a topic of interest for a considerable time (see Brooks et al.). The SRO is often referred to as 1½0 order from the apparent position of the diffuse maxima in diffraction patterns, which differs from the positions of the LRO (D1a) structure. Various studies have shown that a fully disordered state cannot be retained by quenching, as the atomic arrangements responsible for the 1½0 maxima are present at temperatures above the critical ordering temperature for LRO. Over 20 studies have attempted to identify the atomic arrangements associated with this state of order. A variety of models have been proposed, but no consensus has been reached. It has also been shown that 1 MeV electron irradiation at low temperatures (∼100 K) can produce the disordered phase in Ni4Mo. Transmission electron microscopy (TEM), atom probe field ion microscopy (APFIM), and electron irradiation disordering have been applied in the current study to further the understanding of the ordering processes in Ni4Mo.