Ionic Bond ⇄ Covalent Bond Electron Transitions in Alloys

The ordering–separation phase transition, which has been earlier detected in many alloys and causes microstructural changes, is shown to begin with changes in the electronic structure of an alloy. In contrast to pure metals, where all valence electrons take part in the formation of a metallic bond, some pairs of valence electrons in an alloy turn out to be localized at two neighboring unlike atoms, which leads to the formation of common orbitals between them. This behavior is characteristic of the appearance of an ionic chemical bond. During the ordering–separation phase transition in an alloy, this localization of valence electrons is violated and some pairs of valence electrons become involved in the formation of hybridized orbitals between the pairs of neighboring like atoms. As a result, a covalent bond component appears instead of an ionic component in the alloy. It is concluded that the ionic bond covalent bond ? electron transition precedes the microstructural ordering–separation phase transition.     

Ordering–separation phase transition: The fate of the existing phase diagrams

Co₂V and Co₃V alloys are used as examples to discuss the principles of creating modern phase diagrams to take into account the discovery of an ordering–separation phase transition in alloys. The results of electron-microscopic studies of the Co₂V alloy after heat treatment at various temperatures are presented. A comparison of these results with the conclusions based on the existing Co–V phase diagram demonstrates their disagreement. This disagreement is explained in this work. It is concluded that the ordering–separation phase transition should be taken into account in plotting modern phase diagrams. The Fe–Cr and Ni–Cr phase diagrams are considered with allowance for the ordering–separation phase transition in them. A technique is described to determine the ordering–separation phase transition temperature by transmission electron microscopy. The introduction of the concept of the ordering–separation phase transition is shown to change the principles used by researchers to construct phase diagrams.     

Correlation between the structure and internal friction of metallic glass Cu45Ti55

The structural change of metallic glass Cu₄₅Ti₅₅ from room temperature to 800 K is investigated by combining internal friction using the torsion penulum method with X-ray diffraction (XRD), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). The internal friction curve exhibits a point of inflection at about 640 K and an internal friction peak between 640 and 780 K, which corresponds to the precipitation of CuTi₂ (tetragonal) and a metastable phase I (tetragonal) and the formation of CuTi (tetragonal), respectively. The result shows that internal friction is closely related to the phase transition during crystallization, and there is much correspondence between internal friction and DSC in describing the crystallization behavior of metallic glasses.     

The nature of the atom species involved in the reversible relaxation of metallic glasses

We have studied the reversible relaxation which appears in a number of metallic glasses, when cycled thermally. More precisely, we have analysed the variations of their electrical resistance, measured at 4.2 K, after sequential isochronal treatments conducted between ambiant and crystallisation temperatures. Among the 15 alloys studied, it occurs that all the alloys with several metallic elements exhibit a distinct reversible effect. In the alloys made of a single metal and one or several metalloids, a reversible component exists only for certain compositions. The kinetics and structural aspects of these results are discussed in terms of compositional short range ordering.     

Microscopic master equation approach to diffusional transformations in inhomogeneous systems—single-site approximation and direct exchange mechanism

A computer simulation technique based on microscopic master equations is developed for modeling the dynamics of morphological evolution during diffusional phase transformations in binary solid solutions including barrierless nucleation of ordered domains, subsequent domain growth and coalescence, coarsening of antiphase domains, compositional phase separation, Ostwald ripening, and kinetics of simultaneous ordering and phase separation. Assuming a direct exchange mechanism for atomic diffusion and using the single-site approximation, the kinetic equations produce equilibrium states closer to the Bethe approximation than the Bragg-Williams approximation. Computer simulation examples of microstructural evolution during ordering, spinodal decomposition, and simultaneous ordering and phase separation in a binary solid solution are presented using a second-neighbor interaction model.     

Effect of Oxygen on Microstructure Evolution and Glass Formation of Zr-based Metallic Glasses

The effect of oxygen on the microstructure evolution and glass formation of Zr-based bulk metallic glasses(BMGs)was studied in detail.It was found that oxygen did not form oxides or dissolve in glass matrix,but induced the precipitation ofα-Zr which has the high affinity and solubility of oxygen in the Zr-based bulk metallic glass(ZrBMG).With the precipitation ofα-Zr,the remaining melts contain much lower oxygen content and have strong glass formation,resulting in the formation ofα-Zr/BMG composite.The findings provide an important insight into the mechanism of the oxygen on glass formation,and give us a useful guideline to avoid the oxygen detrimental for designing new BMGs.     

Structural Relaxation, Glass Transition, Viscous Formability, and Crystallization of Zr-Cu–Based Bulk Metallic Glasses on Heating

In this article, we summarize our recent findings on relaxation, glass transition, viscous flow, and crystallization of Zr-Cu?Cbased metallic glasses on heating. At least two processes related to the diffusivities of different alloying elements take place in the glass-transition region of a Zr-Cu-Al-Ni glassy alloy. Also, we report an unusual solidification behavior of the bulk glassy alloy produced using low-purity Zr in which both primary and eutectic-type structural constituents were formed simultaneously during solidification of the melt. In addition, viscous flow and structure changes in the Zr-Cu-Al-Ag glassy alloy are examined in both the glass-transition and supercooled liquid regions. This alloy is found to exhibit localized phase separation leading to nanocrystallization before massive crystallization of the entire sample.     

On the iron-carbon phase diagram

Experimental results that are obtained by electron microscopy, X-ray diffraction, and carbide analysis and indicate the precipitation of carbon atoms clusters in a hypereutectoid steel during its annealing above the eutectoid temperature are presented. These results are compared to the reported data in order to construct a new Fe-C phase diagram, where cementite forms below the eutectoid temperature due to the tendency of the Fe-C system toward ordering and carbon unbound to iron precipitates above this temperature in the form of clusters or graphite particles due to the tendency of this system toward phase separation.     

Motion of Crystal Particles by the Electromagnetic Vibrations in Mg-Cu-Y Bulk Metallic Glasses

The method for producing Mg-Cu-Y and Fe-Co-B-Si-Nb bulk metallic glasses using electromagnetic vibrations is effective in forming the metallic glass phase. Disappearance or decrement of clusters by the electromagnetic vibrations applied to the liquid state is considered to cause suppression of crystal nucleation, because the electromagnetic vibrations vibrate the clusters vigorously in the melt. The purpose of this study was to investigate motion of the crystal particles by the electromagnetic vibrations in Mg-Cu-Y bulk metallic glasses. The electromagnetic vibration force vibrated the crystal particles or the clusters that become crystal nuclei in the melt, because the electric current for the electromagnetic vibrations concentrates in those. Thus, the electromagnetic vibrations were found to select vibration particles from the melt. Moreover, it was considered that composites for which second phases or other compounds are dispersed into the metallic glass phase or a nanostructure phase can be produced by the electromagnetic vibration process. This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred February 25–March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee.     

Computer simulation of structural transformations during precipitation of an ordered intermetallic phase

A computer the simulation of kinetics of precipitationof an ordered intermetallic phase from a disordered solid solution in a model binar two-dimensional alloy is considered. All diffusion processes, the concentrational delamination (clustering), ordering, antiphase domain boundary movement and coarsening, are described by the microscopic kinetic equations. The ordering transition of the first and second kinds are considered. It is shown that the conventional precipitation mechanism through nucleation and growth of an equilibrium ordered phase occurs only in a very narrow “strip” of the two-phase field in the phase diagram. In the remaining part, the decomposition always starts from a congruent ordering, which produces a transient nonstoichiometric ordered single-phase state with the same composition as the parent disordered phase and the same symmetry as the product intermetallic phase. Decomposition of the transient ordered phase occurs predominantly at the antiphase domain boundaries (APBs), which results in a two-phase morphology with layers of disordered films separating antiphase domains of the ordered phase. This decomposition is a result of a concentration instability on APBs, which is more substantial than the conventional homogeneous spinodal instability. Further morphological evolution after decomposition is controlled by coarsening, which reduces the order/disorder interfacial area. The predicted precipitation mechanism through a formation of a single-phase transient ordered state is general. It is expected in the most part of a two-phase field of a phase diagram of any alloy systems with intermetallic precipitates related to the parent one by ordering.     

Decomposition and dissolution kinetics ofδ′ precipitation in Al-Li binary alloys

Aside from its technological importance, the Al-Li alloy system also exhibits interesting phase transformations involving both equilibrium and metastable states. Recent theoretical studies have shown that a supersaturated solid solution could take different transformation paths when it is quenched into theα +γ′ field. Suggestions were made that a rapidly quenched solution phase should first undergo a congruent ordering transformation before it decomposes into a two-phase mixture by either a secondary spinodal decomposition or the classical nucleation and growth process. Moreover, a metastable miscibility gap was predicted at lower temperatures. The objective of this research is to study the transformation paths and dynamics in Al-Li binary alloys of three compositions (5.2, 7.0, and 12.0 at. pct Li). This investigation emphasizes thein situ small-angle X-ray scattering (SAXS) observations on specimens subjected to various aging conditions. Special attention is paid to the early stages of the transformation in an attempt to characterize the various possible modes of phase separation on one hand and to study the dynamics of the precipitation process on the other. The following results are obtained: the congruent ordering precedes decomposition at low temperatures; the metastableγ′ solvus curve is reconfirmed; but the predicted metastable miscibility gap is not found. Guinier radii measurements of the particles showed Ostwald ripening is quickly reached upon heating to the aging temperatures. Slowing down behavior is seen at aging temperatures close to the solvus boundary. Activation energies for Li diffusion were obtained using the modified Lifshitz, Slyozov, and Wagner (MLSW) model. A test of dynamical scaling behavior is carried out for the Al-12.0 at. pct Li alloy. Formerly Visiting Scientists at the University of Illinois     

The diffuse-scattering method for investigating locally ordered binary solid solutions

Diffuse-scattering investigations comprise a series of maturing methods for detailed characterization of the local-order structure and atomic displacements of binary alloy systems. The distribution of coherent diffuse scattering is determined by the local atomic ordering, and analytical techniques are available for extracting the relevant structural information. An extension of such structural investigations, for locally ordered alloys at equilibrium, allows one to obtain pairwise interaction energies. Having experimental pairwise interaction energies for the various coordination shells offers one the potential for more realistic kinetic Ising modeling of alloy systems as they relax toward equilibrium. Although the modeling of atomic displacements in conjunction with more conventional studies of chemical ordering is in its infancy, the method appears to offer considerable promise for revealing additional information about the strain fields in locally ordered and clustered alloys. The diffuse-scattering methods for structural characterization and for the recovery of interaction energies are reviewed, and some preliminary results are used to demonstrate the potential of the kinetic Ising modeling technique to follow the evolution of ordering or phase separation in an alloy system. With the Materials Science Division, Argonne     

Decomposition and dissolution kinetics ofδ′ precipitation in Al-Li binary alloys

Aside from its technological importance, the Al-Li alloy system also exhibits interesting phase transformations involving both equilibrium and metastable states. Recent theoretical studies have shown that a supersaturated solid solution could take different transformation paths when it is quenched into theα +γ′ field. Suggestions were made that a rapidly quenched solution phase should first undergo a congruent ordering transformation before it decomposes into a two-phase mixture by either a secondary spinodal decomposition or the classical nucleation and growth process. Moreover, a metastable miscibility gap was predicted at lower temperatures. The objective of this research is to study the transformation paths and dynamics in Al-Li binary alloys of three compositions (5.2, 7.0, and 12.0 at. pct Li). This investigation emphasizes thein situ small-angle X-ray scattering (SAXS) observations on specimens subjected to various aging conditions. Special attention is paid to the early stages of the transformation in an attempt to characterize the various possible modes of phase separation on one hand and to study the dynamics of the precipitation process on the other. The following results are obtained: the congruent ordering precedes decomposition at low temperatures; the metastableγ′ solvus curve is reconfirmed; but the predicted metastable miscibility gap is not found. Guinier radii measurements of the particles showed Ostwald ripening is quickly reached upon heating to the aging temperatures. Slowing down behavior is seen at aging temperatures close to the solvus boundary. Activation energies for Li diffusion were obtained using the modified Lifshitz, Slyozov, and Wagner (MLSW) model. A test of dynamical scaling behavior is carried out for the Al-12.0 at. pct Li alloy. Formerly Visiting Scientists at the University of Illinois     

Small angle X-ray scattering from an embrittling metallic glass

The metallic glass Fe_79.3B_16.4Si_4.0C_0.3 was examined by small angle X-ray scattering (SAXS) in the as-quenched condition and after low temperature annealing. In all cases, the annealing temperature was well below the crystallization temperature of the material. There was no significant change in SAXS intensity as the sample transformed from the ductile to the brittle state. Partially crystallized specimens prepared by up-quenching were used as calibration samples to estimate an upper limit of 0.9 vol% of a possible second phase with the electron density of amorphous Fe₃B in the most embrittled samples. It was concluded that phase separation is not the cause of annealing embrittlement in this system. The weak SAXS intensity suggests that the phase separation and chemical inhomogeneity in both the as-quenched and the annealed state are limited.     

块体金属玻璃的研究现状

介绍了块体金属玻璃的研究和进展。阐述了常见的制备块体金属玻璃的主要方法，如熔体水淬法、铜模吸铸法、熔体喷铸与真空吸铸相结合的制备技术、压铸法、定向凝固铸造法、磁悬浮熔炼铜模冷却法、以金属玻璃为基复合材料的制备方法以及金属玻璃合金丝和管的制备，简要说明了块体金属玻璃的性能特征及应用，并提出了研究过程中值得重视的三个问题。     

The early stages of solute distribution below a transition temperature

We now recognize that there are many possibilities for decomposition: ordering in the matrix followed by precipitation, decomposition followed by ordering, transition phases,etc. Both energetic and kinetic factors can cause such sequences. The formation of coherent phases in some Al, Cu, and Au systems is reviewed. A simpler understanding of Guinier-Preston (GP) zone development in these alloys has emerged. Also, it is not clear that homogeneous nucleation occurs at all in most systems, either in precipitation or on ordering, because fluctuating regions, like the low-temperature phase, are often present in the solid solution about the solvus. Further-more, coherency strain may play an important role in the beginnings of these phase transfor-mations, controlling whether precipitation or ordering or even displacive transformation occurs, and may strongly affect the coherent phase diagram. The Institute of Metals Lecture was established in 1921, at which time the Institute of Metals Division was the only professional division within the American Institute of Mining and Metallurgical Engineers. It has been given annually since 1922 by distinguished people from this country and abroad. Beginning in 1973 and thereafter, the person selected to deliver the lecture will be known as the “Institute of Metals Division Lecturer and R.F. Mehl Medalist” for that year.     

Ordering–separation phase transitions in a Co<Subscript>3</Subscript>V alloy

The microstructure of the Co₃V alloy formed by heat treatment at various temperatures is studied by transmission electron microscopy. Two ordering–separation phase transitions are revealed at temperatures of 400–450 and 800°C. At the high-temperature phase separation, the microstructure consists of bcc vanadium particles and an fcc solid solution; at the low-temperature phase separation, the microstructure is cellular. In the ordering range, the microstructure consists of chemical compound Co₃V particles chaotically arranged in the solid solution. The structure of the Co₃V alloy is shown not to correspond to the structures indicated in the Co–V phase diagram at any temperatures.     

The phase separations of Fe-Al-Co ordering alloys

The phase diagram for the Fe-rich Fe-Al-Co ordering system has been determined by observing phase separation by means of electron microscopy and magnetization-measurements. Two types of phase separation, into B2 and B2¹ (Al-rich B2 and Co-rich B2¹) and into α-Fe (A2) and B2, exist. The A2 + B2 phase separation occurs in a band shaped region connecting Fe-22–25 at.% Al with Fe-25–30 at.% Co. The B2 + B2¹ separation is localized in a wide island shaped region, which centres approximately at 25 at.% Al and 25 at.% Co. This island has a steep slope with temperature on the Co side. The microstructure of a B2 + B2¹ mixture is finely periodic along the 〈100〉 directions, i.e. a so-called [100] modulated structure. The B2 + B2¹ phase separation has been calculated thermodynamically by taking into account the ferro-magnetic and ordering excess free energies. The magnetic effect accounts for the shape of the B2 + B2¹ phase separation island.     

高磷矿氢气还原过程的原位研究

通过原位观察的方法,研究了H₂气氛、不同温度下高磷矿还原过程的矿相结构演变规律和温度对金属铁析出形态的影响.高磷矿的鲕状结构在还原过程中未被破坏掉;随着温度的提高,矿相结构的演变加快,温度较高时Fe发生明显偏聚,金属铁从赤铁矿相中加快析出.还原温度对高磷矿中金属铁的析出形态影响显著:在700℃时金属铁析出在矿石颗粒表面形成致密铁层;800℃时析出的金属铁形貌复杂化,铁层致密度下降,出现孔洞,同时有少量的短小铁晶须;900℃时铁晶须数量明显增多,同时伴随着大量多孔海绵铁的析出.因此,可以通过调节还原温度和时间,使得气基还原过程避免黏结失流,同时高效还原高磷矿.     

Structural and phase transformations in high-nitrogen austenitic steel 05Kh20AG10N3MF during thermal action

The structural and phase transformations in high-nitrogen austenitic steel 05Kh20AG10N3MF under various thermal actions are studied. The transformations are shown to occur only in the Fe-Cr diffusion couple, and they resemble the phase transformations in an Fe-20% Cr binary alloy. At 1200°C, hightemperature phase separation is detected; at 550°C or below, low-temperature separation is detected; and at 700 and 800°C, ordering with the formation of a Laves phase is observed. The ordering-low-temperature separation phase transition in the steel differs substantially from that occurring in Fe-Cr binary alloys.