Order-disorder Transformation Research Articles

Microstructural Evolution and Mechanical Behaviour of Near-Eutectic High Entropy Alloy

In the current work, the thermal stability of the eutectic regions and precipitation and mechanical behaviour of near eutectic high-entropy alloy is studied. The alloy solidified as FCC and underwent sequential ordering to L12 with order–disorder transition temperature between 700°C and 1000°C. Cooling rate, variation in the concentration boundary layer, thermal gradient and interfacial energy were attributed to define the eutectic morphology (L12 and B2). Thermal exposure resulted in homogeneous precipitation of acicular Al-Ni-rich B2 phase within the proeutectic and irregular eutectic regions. The preferential B2 precipitation within the irregular eutectic regions was attributed to the lower stability and higher lattice strain associated with these regions. The FCC: B2 phase fraction after heat treatment was similar to that of the eutectic alloy. Furthermore, lamellar degradation, globularization and Ostwald ripening of the B2 phase were observed after heat treatment. Lamellar degradation occurred via mechanisms like cylinderization, edge spheroidization, termination migration and boundary splitting. The experimental observations on the phase evolution, stability and order–disorder transformation matched well with the CALPHAD predictions except for the σ phase formation. B2 precipitation during heat treatment enhanced the mechanical behaviour of the alloy. However, the work hardening rate was superior for as-cast alloy.

Thermal stability and oxidation resistance of non-equimolar ratio AlTiVCrNb refractory high-entropy alloys

In this work, we investigated the thermal stability, oxidation resistance and high temperature mechanical properties of refractory high entropy alloys Ti75Al10V5Cr5Nb5, Ti67Al12V7Cr7Nb7 and Ti55Al15V10Cr10Nb10(at.%). It is found that the second phase would be precipitated and occurs order-disorder transformation in refractory high entropy alloys at high temperature. The HCP phase produced orderly B2 phase and decreased the orderly B2 phase precipitation with the increasement of temperature in Ti67Al12V7Cr7Nb7 and Ti55Al15V10Cr10Nb10. According to the oxidation kinetics of high entropy alloys after 100h oxidation at 800 °C, the oxidation index factor n is between 1.1 and 1.2 and the degree of oxidation is between oxidized and inoxidized, all three refractory high entropy alloys formed oxides with rutile structure, the Ti67Al12V7Cr7Nb7 being less oxidation resistant due to the generation of Nb2O5 which is less stable at high temperature. Ti67Al12V7Cr7Nb7 has a yield strength of 522 MPa with 50% elongation at 650 °C. Ti75Al10V5Cr5Nb5 and Ti67Al12V7Cr7Nb7 have lower strength but 150% deformation without fracture when stretched at 750 °C. The multi-component intermetallic compound nanoparticles formed by the second phase and ordered phase together in the refractory high entropy alloys have an Orowan strengthening effect, to improve the mechanical properties at high temperature since it effectively prevents dislocation movement and stores dislocations.

Pressure-Induced Ferroelastic Transition Drives a Large Shape Change in a Ni(II) Complex Single Crystal.

Crystals with significant length reduction at an accessible low pressure are highly desirable for piezo-responsive devices. Here, we show a molecular crystal [Ni(en)3](ox) (en = ethylenediamine and ox = oxalate anion) that exhibits an abrupt shape change with a contraction rate of ∼4.7% along its c axis near the phase transition pressure of ∼0.2 GPa. High-pressure single-crystal X-ray diffraction and Raman spectroscopy measurements reveal that this material undergoes a first-order ferroelastic transition from high-symmetry trigonal P3̅1c to low-symmetry monoclinic P21/n at ∼0.2 GPa. The oxalate anions serve as unique components, and their disorder-order transformation and rotation of 90° through cooperative intermolecular hydrogen bonding triggered unconventional anisotropic microsize contraction under compression, which can be appreciated visually. Such a prominent directional deformation at a low pressure driven by molecular motors of oxalate anions provides insights for the design of novel molecular crystal-based piezo-responsive switches and actuators in deep-sea environments.

Soft, strong, tough, and durable bio-hydrogels via maximizing elastic entropy.

Load-bearing soft tissues are soft but strong, strong yet tough. These properties can only be replicated in synthetic hydrogels, which do not have the biocomplexity required by many biomedical applications. By contrast, natural hydrogels, although retaining the native complexity, are weak and fragile. Here we present a thermomechanical casting method to achieve the mechanical capabilities of synthetic materials in biopolymer hydrogels. The thermomechanical cast and chemically crosslinked biopolymer chains form a short-range disordered but long-range ordered structure in water. Upon stretch, the disordered structure transforms to a hierarchically ordered structure. This disorder-order transformation resembles the synergy of the disordered elastin and ordered collagen in load-bearing soft tissues. As entropy drives a reverse order-disorder transformation, the hydrogels can resist repeated cycles of loads without deterioration in mechanical properties. Gelatin hydrogels produced by this method combine tissue-like tunable mechanical properties that outperform the gelatin prepared by synthetic approaches, and in vivo biocomplexity beyond current natural systems. Unlike polymer engineering approaches, which rely on specific crosslinks provided by special polymers, this strategy utilizes the entropy of swollen chains and is generalizable to many other biopolymers. It could thus significantly accelerate translational success of biomaterials.

Simultaneously elevating the resistive switching level and ambient-air-stability of 3D perovskite (TAZ-H)PbBr3-based memory device by encapsulating into polyvinylpyrrolidone

The study about simultaneously enhancing the resistive switching level and ambient-air-stability of perovskite-based memorizers will promote its commercialization. Here, a new 3D perovskite (TAZ-H)PbBr3 (TAZ-H+ = protonated thiazole) has been fabricated as FTO/(TAZ-H)PbBr3/Ag device, which only exhibits binary memory performance with the high tolerant temperature of 170 °C. After encapsulating by polyvinylpyrrolidone (PVP), the (TAZ-H)PbBr3@PVP composite-based device can demonstrate ternary resistive switching behavior with considerable ON2/ON1/OFF ratio (105.9: 103.9:1) and high ternary yield (68 %). Specially, this device presents good ambient-air stability at RH 80 % and thermal tolerance of 100 °C. The binary resistive switching mechanism can be ascribed to the halogen ion migration induced by bromine defects in the (PbBr3)nn– framework. But the ternary resistive switching phenomenon in the (TAZ-H)PbBr3@PVP-based device could be depicted as the carrier transport from filled traps of PVP to (PbBr3)nn– framework (ON1 state) and then carriers flowing in the re-arranged (TAZ-H)nn+ chain in 3D channels (ON2 state). The PVP treatment can not only modify the grain boundary defects, but also facilitate the transport of injected carriers to the perovskite films via Pb-O coordinated bonds and inhibition of order-disorder transformation. This facial strategy for implementing ternary perovskite-based memorizers with good ambient-air-stability is quite meaningful for high-density memory in harsh environments.

A Systematical Rheological Study of Maize Kernel.

In this study, the rheological behavior of maize kernel was systematically investigated using a dynamic mechanical analyzer. The loss in toughness caused by drying resulted in a downward shift in the relaxation curve and an upward shift in the creep curve. The long relaxation behavior became obvious when the temperature was above 45 °C, resulting from the weakening of hydrogen bonds with temperature. The maize kernel relaxed more rapidly at high temperatures, caused by a reduction in the cell wall viscosity and polysaccharide tangles. The Deborah numbers were all much smaller than one, suggesting that the Maxwell elements showed viscous behavior. Maize kernel, as a viscoelastic material, showed a dominant viscous property at high temperatures. The decline in β with increasing drying temperature indicated an increase in the width of the relaxation spectrum. A Hookean spring elastic portion made up the majority of the maize kernel creep strain. The order-disorder transformation zone of maize kernel was about 50-60 °C. Due to the complexity of maize kernel, the William-Landel-Ferry constants differed from the universal values; these constants should be ascertained through experiments. Time-temperature superposition was successfully used to describe the rheological behavior. The results show that maize kernel is a thermorheologically simple material. The data acquired in this study can be used for maize processing and storage.

New approach to the compound energy formalism (NACEF) part I. Thermodynamic modeling based on the sublattice model

In the CALPHAD modeling, the Compound Energy Formalism (CEF) is widely used because it is a general formalism which allows to select the most appropriate model for different kind of phases (gas phase, solid and liquid solutions, interstitial solutions, intermetallic compounds, solid oxide phases, ionic melts, phases that show chemical order-disorder transformations …). It consists in dividing the phase into sublattices and the models differ by the nature of the constituents (atoms, ions, vacancies, etc.) present in the different considered sublattices where mixing may or may not occur. For many intermetallic compounds which present a crystal structure containing many crystallographic sites on which disorder occurs, the number of parameters to be evaluated which varies exponentially with the number of sublattices becomes a problem in multicomponent systems. This yields the necessity to simplify the description and reduce the number of sublattices by combining different sites in single sublattices and this leads to that different simplifications can be used to model a phase in different systems. It is quite known that the CEF approach does not allow continuous compatibility between the different simplifications which constitutes a problem for the development of multicomponent databases.In the present paper, we present a New Approach to the Compound Energy Formalism (NACEF) which is derived mathematically from the CEF and therefore retains its full potential while providing several important improvements. As example, considering the particular case of the complex σ phase, the present work shows that NACEF makes it possible to make compatible a model based on the full description (five-sublattice model) with simplifications which can be made of it. On the other hand, it is also shown that NACEF form combined to a simple Muggianu type extrapolation allows to obtain reliable estimates of the energies of ordered configurations in multicomponent systems using those for the binary configurations obtained from DFT calculations.

Partially Oxidized Purely Organic Zwitterionic Neutral Radical Conductor: Multi-step Phase Transitions and Crossover Caused by Intra- and Intermolecular Electronic Interactions.

Formation of a partially charge-transfer or partially oxidized/reduced state has been one of the most important requirements for the development of highly conducting molecular materials, such as organic metals and superconductors. This requirement has been fulfilled by combining appropriate electron-donor and acceptor molecules to construct multi-component molecular complexes/salts, such as (TTF+0.59)(TCNQ-0.59) and (BEDT-TTF+0.5)2X-, where TTF = tetrathiafulvalene, TCNQ = tetracyanoquinodimethane, BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene, and X = monovalent inorganic anion. Here, we propose a methodology to fulfill this requirement by a single neutral molecule; namely, we have connected two TTF+0.5-type partially oxidized π-skeletons through a boron anion to design a purely organic zwitterionic neutral radical {[(PDT-TTF-Cat)2]+B-}•. This molecule was successfully obtained as air-stable crystals containing solvent tetrahydrofuran (THF) molecules. Measurements of electrical resistivity, magnetic susceptibility, and X-ray diffraction reveal that the partially oxidized state is certainly formed, which enables realization of a 3/4-filled electron band. Furthermore, this system has intramolecular charge degrees of freedom, attributable to the two TTF+0.5 π-skeletons introduced into the molecule. The resulting interplay of intra- and intermolecular charge degrees of freedom (or simply, intra- and intermolecular electronic interactions) has led to multi-step phase transitions and crossover, providing unique strongly correlated electron properties, such as the formation of a three-dimensional charge-ordered dimer-Mott insulating state and its melting triggered by disorder-order transformation of the co-crystallized solvent THF molecules at low temperatures.

Influence of La Addition on Order–Disorder Transformation and Deformation Softening Behavior of Fe–6.5 wt % Si Alloy

Influence of La Addition on Order–Disorder Transformation and Deformation Softening Behavior of Fe–6.5 wt % Si Alloy

Study of solid state transformations and their effect on mechanical response in the stoichiometric Ni2Cr using in-situ synchrotron radiation diffraction experiments

A complete microstructural characterization of the stoichiometric Ni2Cr alloy after different ageing times in which different degrees of order are attained has been carried out. The study of order-disorder transformation has been done using synchrotron radiation diffraction monitoring eventual changes in high-energy X-ray diffraction (HEXRD) patterns every six seconds in the course of differential scanning calorimetry DSC experiments. According to acquired data, the onset of the order-disorder transformation occurs at 630 °C, temperature higher than that reported in previous works. In addition, the appearance of fine carbide precipitation has also been found inside the grains and at grain boundaries. Both ordering of the stoichiometric Ni2Cr and carbide precipitation seems to be responsible for the strengthening of the alloy.

The effects of discontinuous precipitation and ordering on the age-hardening in Au-20Ag-30Cu (wt.%) alloy

Discontinuous precipitation (DP) and ordering have been typically considered to influence the mechanical properties of many alloy systems during the aging process. In the present study, the microstructural evolution and the effect of discontinuous precipitation and ordering on the age-hardening behavior in Au-20Ag-30Cu (wt.%) ternary alloy were investigated. Au-20Ag-30Cu (wt.%) alloy is found to be typically age-hardened. Thermodynamic calculation forecasts the discontinuous precipitation forming Cu-rich α1 phase and Ag-rich α2 phase, companied by the disorder-order transformation forming AuCu3 phase with L12 structure for this Au-Ag-Cu alloy. The DP is experimentally observed to form at grain boundaries consisting of alternative α1 and α2 two-phase lamellae. And the L12-type AuCu3 ordered phase, which presents nanometric spherical particle, forms at grain interior and the lamellar α1 phase of DP. The DP and L12-type AuCu3 nanoprecipitates both grow with the elevated aging temperature. The DP at grain boundaries is found to deleteriously affect the overall age-hardening with different deformation mechanisms between these two lamellar phases of DP. High-density dislocations are observed in the lamellar α1 phase of DP around the hardness indentation, which indicates that the interior L12-type AuCu3 nanoprecipitates in α1 phase can effectively impede the movement of dislocations to boost the age-hardening. The overall age-hardening is determined by the competitive effect of the DP and L12-type AuCu3 nanoprecipitates. This study offers theoretical foundation for controlling the microstructures and optimizing the mechanical properties of Au-Ag-Cu alloys.

Activation energy of phase transformations at high-temperature synthesis of tungsten carbide by electrothermal explosion under pressure

The effect of activation energy on phase transformations (transitions) in the W–C system during the synthesis induced by an external heat source was investigated by electrothermal explosion (ETE) under pressure. The ETE technology combines self-propagating high-temperature synthesis (SHS) with additional sample heating by Joule heat – electric current passing through the synthesized mixture, and it makes it possible to determine the chemical reaction rate that is highly susceptible to external impacts such as pressure, concentration, sample shape, any film present on combustion products, etc. The chemical reaction rate, i.e. external source current, may be controlled by changing the activation energy. The study was conducted in the following conditions: temperature Т = 293÷3700 K; carbon concentration of 49.8–50.2 at.%; quasi-static compression at P = 96 MPa; external source voltage and current density V = 10 V, I = 20 МА/m2, respectively; samples 8 mm in diameter weighing 6 g. The Т–τ thermogram of the W–C system was used to determine the following parameters: four stages of the synthesis process, temperatures of special points of phase transformations, temperature boundaries of phases and process activation energy. Thermograms of intermediate states are presented as isothermal plateaus of phase transformations. The analysis of experimental results and the physical representation of the process make it possible to assert that temperature plateau parameters are the effective value of activation energy for synthesis mode maintenance. Each of the 4 W–C mixture synthesis stages is described. Pre-explosion stage I – sample heating in the temperature range of Т = 293÷563К, endothermic reaction, effective activation energy for synthesis mode maintenance Q = 2.96 kJ, and taking into account 1-mole mass Еа = 111.6 kJ/mol. Low-temperature (563–1190 К) stage II – ignition, Q = 5.46 kJ, Еа = 109.2 kJ/mol. High-temperature stage (III) in the range of Т = 1190÷2695К, order–disorder transformation, Q = 14.25 kJ, Еа = 424 kJ/mol. Finally, Stage IV occurs in the range of Т = 2695÷3695К, Q = 14.31 kJ, Еа = 143.2 kJ/mol. It was shown that the limiting stage with the highest activation energy is the melting process.

MnO2 Polymorphs as Cathode Materials for Rechargeable Ca‐Ion Batteries

AbstractCalcium‐ion batteries (CIBs) have attracted extensive interest for energy storage due to the high operating potentials and the abundant resources of calcium. However, the application of CIBs has been limited by the absence of high‐voltage and high‐performance cathode materials. Herein, for the first time, four kinds of MnO2 polymorphs (α, β, γ, and δ‐phase) as cathode materials for CIBs are reported, and the δ‐MnO2 exhibits the highest electrochemical performance, which is mainly due to the higher Ca‐ion diffusivity of δ‐MnO2. The excellent rate capability obtained herein outperforms those of previously reported for all oxides as cathode materials for CIBs. Furthermore, the Ca‐ion storage mechanism of δ‐MnO2 based on the reversible order–disorder transformation is demonstrated by the in situ/ex situ characterizations. In addition, the capacity decay mechanism of δ‐MnO2 during the cycling process is confirmed as the dissolution of Mn and the optimization strategy is proposed. These results demonstrate that the δ‐MnO2 is a promising cathode material for CIBs, providing new opportunity for the development of high‐performance CIBs.

Biotropic liquid crystal phase transformations in cellulose-producing bacterial communities

Biological functionality is often enabled by a fascinating variety of physical phenomena that emerge from orientational order of building blocks, a defining property of nematic liquid crystals that is also pervasive in nature. Out-of-equilibrium, "living" analogs of these technological materials are found in biological embodiments ranging from myelin sheath of neurons to extracellular matrices of bacterial biofilms and cuticles of beetles. However, physical underpinnings behind manifestations of orientational order in biological systems often remain unexplored. For example, while nematiclike birefringent domains of biofilms are found in many bacterial systems, the physics behind their formation is rarely known. Here, using cellulose-synthesizing Acetobacter xylinum bacteria, we reveal how biological activity leads to orientational ordering in fluid and gel analogs of these soft matter systems, both in water and on solid agar, with a topological defect found between the domains. Furthermore, the nutrient feeding direction plays a role like that of rubbing of confining surfaces in conventional liquid crystals, turning polydomain organization within the biofilms into a birefringent monocrystal-like order of both the extracellular matrix and the rod-like bacteria within it. We probe evolution of scalar orientational order parameters of cellulose nanofibers and bacteria associated with fluid-gel and isotropic-nematic transformations, showing how highly ordered active nematic fluids and gels evolve with time during biological-activity-driven, disorder-order transformation. With fluid and soft-gel nematics observed in a certain range of biological activity, this mesophase-exhibiting system is dubbed "biotropic," analogously to thermotropic nematics that exhibit solely orientational order within a temperature range, promising technological and fundamental-science applications.

Suppressing disorder-order phase transition of Gd2Zr2O7 pyrochlore by Dy3+ doping and their impact on luminescence

The functionality of the Gd2Zr2O7 matrix is dictated by ordering at the cationic lattice (pyrochlore or defective fluorite) and associated local manifestations. In this work, we have synthesized nano and bulk undoped Gd2Zr2O7 (GZO) and Gd2Zr2O7:Dy3+ (GZOD) by molten salt synthesis adopting a crystal engineering approach and investigated their impact on photoluminescence properties. DFT calculations along with the higher intensity of yellow emission in the bulk GZOD confirm that the presence of Dy3+ is energetically favorable in defect fluorite structure compared to ordered pyrochlore counterpart. Increase/adjustments in the Zr–O bond distances in Dy3+ surroundings facilitate the release of local strain/stress in defective fluorite structure, which is not feasible in the perfectly ordered pyrochlore structure. Overall, the preference of defective fluorite structure by the GZO NCs, GZOD NCs, and bulk GZOD has been attributed to inherent flexibility in local structure with varying coordination numbers and polyhedral arrangements that assist in reducing the strain arising due to smaller particle size and/or by Dy3+ doping. It is believed that the present work provides simplistic pathways to control the order-disorder transformation, and in turn, the functionality of the Gd2Zr2O7 matrix for better hosting of nuclear waste and phosphor centers.

Intermetallic compounds of the Cr–Mn system investigated using in situ powder neutron diffraction: The reported order-disorder transformation of the σ phase elucidated

Cr–Mn system is very important for many applications. In the present work, the reported intermetallic compounds have been studied using in situ high temperature neutron powder diffraction. For the first time, site occupancies in a σ phase are obtained as a function of temperature by Rietveld analysis. They are compared with calculated data obtained from DFT. The previously reported order-disorder transition in the σ phase around 1000 °C is shown to be in fact a congruent phase transformation to the bcc Cr–Mn solid solution. This solid solution is probably continuous between pure Cr and δ-Mn. The second reported intermetallic α’ phase (Cr2Mn3) phase is shown to be in fact a ternary nitride of the Cr–Mn–N system that should not appear in the binary phase diagram. The present work clarifies and explains all the features of the previous literature on this system and brings major revisions to the phase diagram. General conclusions are drawn concerning mistakes that can be done in the evaluations of phase diagrams.

Breaking with the Principles of Coreduction to Form Stoichiometric Intermetallic PdCu Nanoparticles.

Intermetallic nanoparticles (NPs) have shown enhanced catalytic properties as compared to their disordered alloy counterparts. To advance their use in green energy, it is crucial to understand what controls the formation of intermetallic NPs over alloy structures. By carefully selecting the additives used in NP synthesis, it is here shown that monodisperse, intermetallic PdCu NPs can be synthesized in a controllable manner. Introducing the additives iron(III) chloride and ascorbic acid, both morphological and structural control can be achieved. Combined, these additives provide a synergetic effect resulting in precursor reduction and defect-free growth; ultimately leading to monodisperse, single-crystalline, intermetallic PdCu NPs. Using in situ X-ray total scattering, a hitherto unknown transformation pathway is reported that diverges from the commonly reported coreduction disorder-order transformation. A Cu-rich structure initially forms, which upon the incorporation of Pd(0) and atomic ordering forms intermetallic PdCu NPs. These findings underpin that formation of stoichiometric intermetallic NPs is not limited by standard reduction potential matching and coreduction mechanisms, but is instead driven by changes in the local chemistry. Ultimately, using the local chemistry as a handle to tune the NP structure might open new opportunities to expand the library of intermetallic NPs by exploiting synthesis by design.

Volume effects on microstructures and magnetic properties of MnGa thin films during order-disorder transformation

In this study, L10 MnGa thin films were grown on an amorphous glass substrate, and the relationship between the volume effect and the magnetic properties of thin films as a function of the film thickness was investigated. An amorphous thin film and weak magnetic properties were observed at the thickness of 10 nm. With the increase in thickness, the magnetization (M10 kOe) and coercivity (Hc⊥) also increased. The maximum M10 kOe and Hc⊥ values were 106.3 ± 5.7 emu/cm3 and 8.54 kOe in a 200 nm-thick sample, respectively. X-ray diffraction indicated that the order-disorder transformation occurred when the thickness was greater than 25 nm. Plan-view transition electron microscopy (TEM) images indicated the emergence of larger grains and blurred grain boundaries when the thickness was increased beyond 25 nm, while cross-sectional TEM images suggested that the appearance of nuclei promote the growth of columnar MnGa grain structures. Energy dispersion spectroscopy analysis indicated that the composition of MnGa was Mn-rich (or Ga-poor) in the nuclei at the initial growth state and then gradually changed to Mn50Ga50 as the thickness increased. In addition, the effects of the base pressure of the chamber on the phase and magnetic properties of the MnGa films were also investigated.

Probing the Short-Range Ordering of Ion Irradiated Gd2Ti2-yZryO7 (0.0 ≤ y ≤ 2.0) Pyrochlore under Electronic Stopping Regime

A series of Gd2Ti2-yZryO7 (0.0 ≤ y ≤ 2.0) solid solutions has been synthesized by a solid-state process using a three-stage heating protocol. Raman spectroscopy was used to investigate the structural modification with an increasing Zr content in pristine samples. Short-range ordering or local disorder around cationic sites was investigated in irradiated samples with 120 MeV Au+9 ions by Raman spectroscopy. Phase transformation was observed in Zr-rich pyrochlore compositions due to the anion disordering around the cationic sites in pristine samples. Moreover, SHI irradiation-driven phase transformation was observed dominantly in Zr-rich compositions due to cation disordering. Like XRD analysis in the previous study, the present study provides information about the higher efficacy of Zr-rich composition towards SHI-induced disordered transformation with very little amorphization. Since Raman spectroscopy is able to observe the polyhedral structure of AO8 and BO6 even at nanoscale also, therefore, this study shows relatively smaller SHI induced disordered ion tracks as compared to XRD analysis. Thus, the short-range ordering becomes important to investigate the local order-disorder transformation on SHI irradiation.

A study of order-disorder phase transformation in Ag50Pd50 alloy

In this study, the structure of Ag50Pd50 alloy was investigated with temperature dependent in-situ X-ray diffraction, residual resistometry, and differential scanning calorimetry (DSC) as well as differential thermal analysis (DTA) techniques. Isochronal annealing experiment was performed to determine the impact of ordering on the residual electrical resistivity. The residual resistivity curve shows a sharp minimum near 250 °C indicating the on-set of ordering. The in-situ X-ray diffraction data taken for a well annealed sample shows no super lattice reflections over the investigated temperature range but the lattice parameter determined from this data shows an abrupt decrease in its value at 250 °C showing an order-disorder phase transformation (ODPT). The diffraction pattern of a sample annealed at 230 °C shows splitting of the high angle 511 fundamental reflection due to the tetragonal distortion. The peak intensity and c/a ratios are found to be 1:1.94 and 1.0028, respectively. A similar phase change was also observed during a DSC experiment around 250 °C on this alloy. The existence of these anomalies may be attributed to L1o - FCC type order-disorder phase transformation at this temperature.