First-principles calculation of phase equilibria and phase separation of the Fe-Ni alloy system

Theoretical investigation of the phase equilibria of the Fe-Ni alloy has been performed by combining the FLAPW total energy calculations and the Cluster Variation Method through the Cluster Expansion Method. The calculations have proved the stabilization of the LIE phase at 1：3 stoichiometry, which is in agreement with the experimental result, and predicted the existence of L1 0 as a stable phase below 550 K; this L1 0 phase has been missing in the conventional phase diagram. The calculations are extended to the Fe-rich region that is characterized by a wide range phase separation and has drawn considerable attention because of the intriguing Invar property associated with a Fe concentration of 65%. To reveal the origin of the phase separation, a P-V curve in an entire concentration range is derived by the second derivative of free energy functional of the disordered phase with respect to the volume. The calculation confirmed that the phase separation is caused by the breakdown of the mechanical-stability criterion. The newly calculated phase separation line combined with the L1 0 and L12Eorder-disordered phase boundaries provides phase equilibria in the wider concentration range of the system. Furthermore, a coefficient of thermal expansion （CTE） is attempted by incorporating the thermal vibration effect through harmonic approximation of the Debye-Gruneisen model. The Invar behavior has been reproduced, and the origin of this anomalous volume change has been discussed.     

Thermodynamic database of the phase diagrams in the Mg-Al-Zn-Y-Ce system

The Mg-Al-Zn-Y-Ce system is one of the key systems for designing high-strength Mg alloys. The purpose of the present article is to develop a thermodynamic database for the Mg-Al-Zn-Y-Ce multicomponent system to design Mg alloys using the calculation of phase diagrams （CALPHAD） method, where the Gibbs energies of solution phases such as liquid, fcc, bcc, and hcp phases were described by the subregular solution model, whereas those of all the compounds were described by the sublattice model. The thermodynamic parameters describing Gibbs energies of the different phases in this database were evaluated by fitting the experimental data for phase equilibria and thermodynamic properties. On the basis of this database, a lot of information concerning stable and metastable phase equilibria of isothermal and vertical sections, molar fractions of constituent phases, the liquidus projection, etc., can be predicted. This database is expected to play an important role in the design of Mg alloys.     

Glide mechanism of ordinary dislocations in the γ phase of TiAl

Glide mechanism of ordinary dislocations in the γ phase of TiAl alloys is studied by in situ straining experiments. A polar source and several dynamic sequences showing moving dislocations are presented. It is shown that the deformation is due to rectilinear screw dislocations anchored at many pinning points. The origin of the frictional force opposing to the dislocation movements and that of these pinning points are analysed and discussed. Experimental observations show also that an increase of temperature leads to a fast deformation by bursts of dislocations. This temperature effect is interpreted by dynamic strain ageing.     

Determination of phase diagrams using the diffusion couple technique

1. Introduction It is commonly recognized that phase diagrams act as important guidelines in the development and production of new materials. The experimental de- termination of the phase diagrams is generally very time-consuming, especially for complex systems. As a summary on our previous work dedicated to measurement of phase diagrams, the present paper will demonstrate that flexible combination of the diffusion couple technique with equilibrated alloys is powerful and efficient in the dete…     

Effect of Ce on the thermal stability of the Ω phase in an Al-Cu-Mg-Ag alloy

This paper studies the effect of Ce on the thermal stability of the Ω phase in an Al-Cu-Mg-Ag alloy by TEM and tensile testing. It has been shown that Ce substantially increases the nucleation density of the Ω phase by acting as the heterogeneous nucleation center. Most impor-tantly,Ce improves the thermal stability of the Ω phase by decreasing the diffusion velocity of Cu atoms and increasing the energy barrier of the thickening ledge nucleation,thus improving the strength of the Al-Cu-Mg-Ag alloy at both ...     

Influences of solid particles on the formation of the third phase crud during solvent extraction

1. Introduction During solvent extraction, the solvent extraction rate depends on the chemical reaction rate and mass transfer speed on phase interface. To improve the area of contact interface, both phases must be ade-quately mixed to form an emulsion, that is, one phase uniformly disperses into another phase in the form of micro drops with the diameter usually more than 0.1 μm. However, this emulsion could be un-stable, when the mixing ceases. The dispersed phase (i.e., micro drop) must be …     

Mechanism and kinetics of discontinuous precipitation of the η phase in the 36NKhTYu alloy

A new mechanism of nucleation and growth of cells of the η phase in the deformed 36NKhTYu austenitic alloy has been found. The recrystallization nuclei are formed at the η-phase particles, which were precipitated according to the continuous mechanism. Simultaneously, they are nuclei of cells of discontinuous decomposition. Unlike the known growth mechanism, the η-phase cell grows in all directions until it collides with other cells. The Peterman-Hornbogen theory of discontinuous precipitation has been modified to describe the kinetics of the complex reaction of discontinuous precipitation of the η phase and recrystallization and also the kinetics of complex reaction of discontinuous coalescence of grains and phases. The theoretical dependences of the cell size on the aging time for these reactions are proportional to t ^1/3 and t ^1/4, respectively. A comparison of the theoretical and experimental results has been performed.     

The contribution of solid-state chemistry in the determination of multicomponent phase diagrams

For a long period of time, the determination of phase diagrams was only supported by experiments related to thermal effects or thermodynamic measurements： thermal analysis, calorimetric measurements, vapor pressures, and EMF measurements. As a matter of fact, solid-solid transformations were not so accurately determined and could not be taken into account in the system＇s analysis. First, X-ray diffraction methods were used as a support for the thermal analysis. Second, the implementation of novel tools in structural analysis （for example, the Rietveld method） has permitted to increase the knowledge of phase stability. Finally, modeling the phases using a Calphad method needed increasingly more structural results to determine and better understand the phase diagrams. On the other hand, the Calphad method has been widely developed for metallic systems, for oxide systems, and in the past 10 years, for some semi-conductor systems, for example, gallium arsenide, cadmium telluride, and lead telluride systems. In such applications, it is very important to bring point defects in the modeling of the phases to map the defects as a function of the chemical composition. Owing to its complexity, this characteristic, the knowledge of which is crucial for the understanding and the control of potential physical applications, was ignored in the previous assessment of semi-conductor systems.     

Estimate of the applicability of Pd–Pt nanoalloy for data recording by the method of phase change

Based on the computer simulation, the applicability of using individual nanoclusters of Pt, Pd, and particles of the Pd–Pt nanoalloy as unites of storage of data bits in nonvolatile memory devices, store capability of which is based on the principle of the phase change of the state of the carrier of information, has been estimated. To this end, the temperature and size limits of stability of different internal structures of nanoparticles in the course of the heating (to melting) and subsequent solidification (crystallization) with different rates of heat removal have been established. The results of the computer simulation of the nanoparticles of chemically pure platinum, palladium, and their alloy with different content of Pt atoms have been compared. It has been concluded that the best material for the memory cells the store capability of which is based on the occurrence of phase transitions is the nanoclusters of the Pd–Pt alloy with 10% platinum with a diameter D ≥ 3.5 nm.     

The phase diagram of the terbium–gold alloy system

The terbium-gold phase diagram has been investigated in the 0–100 at% Au field by differential thermal analysis (DTA), X-ray diffractometry (XRD), optical microscopy (LOM), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Eight intermetallic phases were found, namely: Tb₂Au orthorhombic oP12–Co₂Si, peritectic decomposition at 1000°C, TbAu, L.T. form, orthorhombic oC8–CrB type and H.T. form, cubic cP2–CsCl type, congruent melting at 1590°C, Tb₃Au₄ trigonal hR42–Pu₃Pd₄ type, peritectic decomposition at 1340°C, Tb₇Au₁₀ tetragonal tI136–Gd₇Au₁₀ type, peritectic decomposition at 1210°C, TbAu₂ tetragonal tI6–MoSi₂ type, congruent melting at 1265°C, TbAu₃ orthorhombic oP8–TiCu₃ type, congruent melting at 1215°C, Tb₁₄Au₅₁ hexagonal hP65–Gd₁₄Ag₅₁ type, peritectic decomposition at 1175°C, and TbAu₆ tetragonal tP56–SmAu₆ type, peritectic decomposition at 855°C. Four eutectic reactions were found to occur at 880°C and 20.0 at% Au, at 1195°C and 62.5 at% Au, at 1160°C and 71.0 at% Au and finally at 805°C and 89.0 at% Au. A catatectic reaction occurs in the Tb-rich region. The experimental results are discussed and compared with the general behaviour of the other R–Au systems.     

Effects of grain boundary η phase on the mechanical properties of Fe-Ni-Cr alloy

通常认为Fe-Ni—Cr合金在过时效后晶界上析出的η相为裂纹源，恶化合金性能．通过对一种Fe-Ni—Cr合金进行过时效后晶界上析出η相的研究发现，由于晶界η相与合金基体保持共格关系，η相对合金的室温性能影响不大；并且在高温变形条件下η相形成凸起，阻碍晶界滑动，减小在晶角处的应力集中，从而提高合金的高温塑性．断口扫描电镜观察表明，晶界η相的析出使合金高温断裂方式由原来的沿晶脆性断裂转变为穿晶韧窝状断裂．通过在合金中添加Nb，可以在晶界上析出更为弥散的η相，从而使得合金在高温下获得相当高的塑性．     

Evolution of pattern complexity in the Cahn–Hilliard theory of phase separation

Phase separation processes in compound materials can produce intriguing and complicated patterns. Yet, characterizing the geometry of these patterns quantitatively can be quite challenging. In this paper we propose the use of computational algebraic topology to obtain such a characterization. Our method is illustrated for the complex microstructures observed during spinodal decomposition and early coarsening in both the deterministic Cahn–Hilliard theory, as well as in the stochastic Cahn–Hilliard–Cook model. While both models produce microstructures that are qualitatively similar to the ones observed experimentally, our topological characterization points to significant differences. One particular aspect of our method is its ability to quantify boundary effects in finite size systems.     

Determination of phase equilibria in the Ni-Ta-Zr system at 1223 K

The phase equilibrium relations of the ternary Ni-Ta-Zr system at 1223 K were investigated by means of diffusion triple and electron probe microanalysis (EPMA) techniques. Two ternary compounds were found and a series of tie lines and tie triangles have been determined. The maximum solubility of Ta in NiZr2 is about 13.5% (atom fraction). The corresponding tentative isothermal section at this temperature was constructed based on current experimental information.     

Enthalpy of formation of quasicrystalline phase and ternary solid solutions in the Al-Fe-Cu system

1. Introduction A novel intermetallic-phase quasicrystal was discovered in 1984. At first, the quasicrystalline phase was observed in rapidly quenched alloys of the Al-Mn system [1]. Till date quasicrystalline phases have been obtained in more than 100 systems; mainly Al, Cu, Ti, and Zr-based systems. The pecu- liarity of the quasicrystalline phases is that the exis- tence of symmetry axes for the fifth and tenth orders are forbidden by classic crystallography. The quasi- crystalline phases…     

Influence of the structure and phase composition of the bond interface on aluminium–copper lap welds strength

The structure and phase composition of the bond interface of aluminium–copper lap welds produced by friction stir welding and tool-assisted friction welding were analysed. Microstructural analysis proved that no through-interface material flow took place in tool-assisted friction welding and that aluminium–copper joining resulted from the formation of a thin and continuous intermetallic layer at the lap interface. For the welds produced by friction stir welding, evidences of through-interface material flow were found, promoting mechanical interlocking of both base materials, at the lap interface, and formation of discontinuous intermetallic layers. Mechanical testing showed that the tool-assisted friction welds, with excellent surface finishing, had low strength, contrary to the friction stir welds, which displayed excellent bond strength. The comparison of the mechanical and microstructural results, for both weld types, pointed to the ineffectiveness of the continuous intermetallic layer in providing high strength bonding.     

Relationship between the types of binary alloy phase diagrams of Ⅷ and IB group elements and the Mendeleev numbers

The relationship between the types of binary alloy phase diagrams of VIII and IB group elements and the Mendeleev numbers was discussed for the first time using the VIII and IB group elements as solvent metals (A) and the other elements as solute metals (B), basesd on their alloy phase diagram types. The Mendeleev numbers of the solvent metals and the solute metals were expressed as MA and MB, respectively. A two-dimension map of MA/MB was drawn. It is indi-cated that there is an oblique line in the map, which divides the binary alloy phase diagram types of solvent metals intotwo symmetry parts, the phase diagram types of the other elements with solvent metals located at the above or down ofthe line respectively, while on the line, ΔM= 0. The phase diagrams between the solvent metals basically are simple systems, mainly belong to the types of continues solid solution and the peritectic (about 40% for each type). The solvent metals can be divided into three groups: Co, lr, Rh, Ni, Pt, and Pd as the first group; Ag, Au, and Cu as the second group;and Fe, Os, and Ru as the third group. The characteristics of the phase diagrams formed between the elements in each group were discussed. About 80% phase diagrams belong to complex systems and less than 20% belong to the simple systems. The regular variation of the chemical scale, the metallic radii of the atoms, the number of valence electrons, and the first ionization energy with the Mendeleev numbers and the crystal structure were introduced as well.     

Elasticity-based model of the variant selection observed in the β to α phase transformation of a Zircalloy-4 sample

The β→α texture inheritance of a Zircalloy-4 sample has been investigated after an α→β→α transformation cycle. The final inherited α texture has been determined from a crystal orientation map determined by electron back-scattering diffraction, whereas the texture of the high temperature β phase has been reconstructed by a method analysing the orientations and misorientations of α variants. The comparison of the α texture calculated from the parent β texture without variant selection with the experimental sharp α texture shows differences due to a strong variant selection mechanism occurring during the phase transformation at cooling.A model of a variant selection mechanism based on the elastic anisotropy of the parent β phase leads to a simulated inherited α texture with the main characteristics of the experimental texture.     

Effects of aging temperature on the precipitation behavior of Ω phase in an Al-Cu-Mg-Ag alloy

Microstructures and mechanical properties of an Al-Cu-Mg-Ag alloy aged for 1 h at temperatures in a range 25 °C to 450 °C were characterized in the present work by means of hardness tests, electrical conductivity measurements, and transmission electron microscopy (TEM). In-situ X-ray diffraction (XRD) was also employed to examine the precipitation behavior of Ω phase in a temperature range of 25 °C to 400 °C The in-situ Xray diffraction peak at 2θ = 26°–28° detected at elevated aging temperatures between 165 °C and 400 °C was attributed to the formation of Ω phase. TEM observations demonstrated the existence of Ω phase in the alloy when aged for 1 h at temperatures in a range 145 °C to 450 °C.