Nonequilibrium Alloys Research Articles

Electrodeposition of Al-Mo Alloys from the Lewis Acidic Aluminum Chloride-1-ethyl-3-methylimidazolium Chloride Molten Salt

The electrodeposition of aluminum-molybdenum alloys was examined at copper rotating disk and wire substrates in the Lewis acidic 66.7-33.3 mol % aluminum chloride-1-ethyl-3-methylimidazolium chloride molten salt containing Mo(II) in the form of dissolved The molybdenum content of the electrodeposits depended on the electrode rotation rate, Mo(II) concentration, and bath temperature. It was possible to produce nonequilibrium alloys containing up to 11 atom % Mo. These alloy deposits were compact and chloride-free. Al-Mo alloys containing more than 8 atom % Mo exhibited a chloride corrosion pitting potential of approximately +800 mV against pure aluminum. The corrosion resistance of this alloy is superior to that of all the aluminum-transition metal alloys that have been electrodeposited to date from chloroaluminate molten salts. © 2004 The Electrochemical Society. All rights reserved.

Atomic structures of nonequilibrium alloys in an immiscible Co–Ag system

A Co–Ag potential in the form of TB-SMA (the second moment approximation of the tight-binding scheme) is derived based on some ab initio calculated physical properties. Applying the derived potential, molecular dynamics simulations reveal that a Co-/Ag-based solid solution retains its atomic homogeneity up to a critical ratio of9 Ag/12 Co at.%, over which the Ag/Co solute atoms begin to segregate. Correspondingly, a cage-like configuration is proposed to have 1 Ag/Co atom isotropically surrounded by 9 Co/7 Ag atoms for the homogeneous solid solution. Moreover, when the solute atoms exceed 12 Ag/18 Co at.%, the solid solution turns into an amorphous phase with inhomogeneous atomic structure.

Non-equilibrium Ti-Fe bulk alloys with ultra-high strength and enhanced ductility

ABSTRACTThe high-strength and ductile hypo-, hyper- and eutectic Ti-Fe alloys were formed in the shape of the arc-melted ingots with the dimensions of about 25–40 mm in diameter and 10–15 mm in height. The structure of the samples consists of cubic Pm 3 m TiFe and BCC Im 3 m β-Ti supersaturated solid solution phase. The arc-melted hypereutectic Ti65Fe35 alloy has a dispersed structure consisting of the primary TiFe phase and submicron-size eutectic structure. This alloy exhibits excellent mechanical properties: a Young's modulus of 149 GPa, a high mechanical fracture strength of 2.2 GPa, a 0.2 % yield strength of 1.8 GPa and 6.7 % ductility. The hard round-shaped intermetallic TiFe phase and the supersaturated β-Ti solid solution result in a high strength of the Ti65Fe35 alloy which in addition has much higher ductility compared to that of the nanostructured or glassy alloys. The reasons for the high ductility of the hypereutectic alloy are discussed.

Generalized Ginzburg-Landau functionals for alloys: General equations and comparison to the phase-field method

The statistical methods for nonequilibrium alloys developed earlier are used to generalize the Ginzburg-Landau gradient expansion for the free energy of a weakly inhomogeneous alloy to the case of finite values of order parameters typical for applications. We derive the general expression for the appropriate generalized Ginzburg-Landau functional, and also its explicit form for several typical alloy structures. The functional is used to derive the differential equations which determine the structure of equilibrium antiphase or interphase boundaries and are convenient for both analytical and numerical treatments. We also derive the kinetic equations describing temporal evolution of the local concentration and the local order parameters in a nonequilibrium alloy. We compare both these kinetic equations and the generalized Ginzburg-Landau functionals with their simplified versions used in the conventional phase-field method. The comparison reveals many important differences which are due to a number of unjustified assumptions used in the standard versions of the phase-field approach.

Chemical Leaching of Non-Equilibrium Al(Fe-Co) Powder Produced by Rod Milling

We report on the formation and chemical leaching of non-equilibrium alloy produced by rod milling. X-ray diffractometry, transmission electron microscopy, differential scanning calorimetry, scanning electron microscopy, and vibrating sample magnetometry were used to characterize the as-milled and leached specimens. After 400 h, only the peak of the body-centered cubic type was present in the XRD pattern. The entire rod milling process could be divided into three different stages of milling: agglomeration, disintegration, and homogenization. The saturation magnetization, decreased with increased milling time, the of the powders before milling was about 113.8 emu/g, the after milling for 400 h was about 11.55 emu/g. Leaching of the Al in KOH of the Al at room temperature from the as-milled powders did not induce any significant change in the diffraction pattern. After the leached specimen had been annealed at for 1 hour, the nanoscale crystalline phases were transformed into the bcc Fe, cubic Co, and phases. On cooling the specimen from 85, the degree of magnetization increased slightly, then increased sharply at approximately 364.8, indicating that the bcc phase had been transformed to the Fe and Co phases.

Nonequilibrium alloy powders and bulk alloys in W–Ag system prepared by mechanical alloying and shock compression

W x Ag 100− x alloy powders and bulk alloys consisting of metastable solid solution and nanocrystals were prepared by mechanical alloying (MA) and shock compression. The MA-treated (21 h) powder for x=90 showed the X-ray diffraction (XRD) pattern of a single phase of b.c.c. structure, and those for x≤80 showed the XRD patterns of two-phase mixtures of b.c.c. and f.c.c. structures. The lattice parameters of the b.c.c. structure phases increased with milling time and approached saturation values at about 15 h. The shock-consolidated bulk alloys for x=70 and 80 consisted of two-layer structures of a thick undecomposed layer and a thin decomposed one. The Vickers hardnesses of the undecomposed areas were much higher than those of the decomposed areas in the shock-consolidated bulk alloys.

Role of Oxide/Metal Interface in Corrosion Resistance: Al-W and Al-Mo Systems

Abstract In the past, it has been suggested that a solute-rich layer at the oxide/metal interface was responsible for the significantly enhanced passivity of nonequilibrium Al-Mo and Al-W alloys in...

Molecular dynamics simulation studies of atomic-level structures in rapidly quenched Ag-Cu nonequilibrium alloys

Ag-Cu has been a classical example demonstrating the formation of a single-phase supersaturated solid solution by rapid quenching in a system immiscible in equilibrium at room temperature. This study examines, using molecular dynamics simulations, the local structures and homogeneity of these crystalline and amorphous Ag-Cu alloys produced by quenching from the liquid at different cooling rates $(5\ifmmode\times\else\texttimes\fi{}{10}^{10}--2.5\ifmmode\times\else\texttimes\fi{}{10}^{13}\mathrm{K}/\mathrm{s}).$ It is observed that the retention of amorphous structures requires extremely high quench rates. The amorphous alloys formed are chemically uniform in both long and short ranges. Meanwhile, topologically significant local icosahedral order was able to develop during quenching even at these extreme cooling rates. The Ag-Cu amorphous structures are discussed in comparison with those of Ag-Ni, a system immiscible even in the liquid state. At moderately reduced quench rates, homogeneous crystalline solutions form instead. With further decreasing quench rates, the chemical short-range-order parameter becomes increasingly more positive, suggesting spinodal decomposition. Our simulations point to the need for a careful experimental study of the ultrafine-scale composition modulations in the face-centered-cubic solid solution that is hitherto believed to be homogeneously supersaturated for a wide range of rapid-quench conditions.

Nonequilibrium hcp and fcc phases formed by ion mixing in an immiscible Ni–Ru system

Two nonequilibrium alloy phases were formed in the equilibrium immiscible Ni–Ru system by 200 keV xenon ion mixing of Ni–Ru multilayered films at 77 K, i.e. a metastable hcp phase near Ni 75Ru 25, and a metastable fcc phase with an alloy composition around Ni 25Ru 75. Interestingly, both the hcp and fcc phases were shown out simultaneously in the Ni 50Ru 50 multilayered films upon irradiation. A Gibbs-free energy diagram was constructed based on Miedema’s model and it can well explain the formation as well as the thermal stability of the above nonequilibrium phases. Based on a sliding mechanism, the lattice constants of the metastable phases were accordingly deduced and in good agreement with the experimentally determined values. Furthermore, the metastable hcp phase can be considered as a well-defined Hume-Rothery 7/4 electron compound.

Mechanical alloying and heat treatment of iron silicide powders

Mechanical alloying (MA) is a solid-state powder processng technique involving repeated welding, fracturing, and rewelding of powder particles in a high-energy ball mill. Originally developed to produce oxide-dispersion strengthened (ODS) nickel- and iron-base superalloys for applications in the aerospace industry, MA has now been shown to be capable of synthesizing a variety of equilibrium and non-equilibrium alloy phases starting from blended elemental or prealloyed powders. The non-equilibrium phases synthesized include supersaturated solid solutions, metastable crystalline and quasicrystalline phases, nanostructures, and amorphous alloys. Recent advances in these areas and also on disordering of ordered intermetallics and mechanochemical synthesis of materials have been critically reviewed after discussing the process and process variables involved in MA. The often vexing problem of powder contamination has been analyzed and methods have been suggested to avoid/minimize it. The present understanding of the modeling of the MA process has also been discussed. The present and potential applications of MA are described. Wherever possible, comparisons have been made on the product phases obtained by MA with those of rapid solidification processing, another non-equilibrium processing technique.

Bulk amorphous and nanocrystalline alloys with high functional properties

Recent progress of bulk amorphous, nanocrystalline and nanoquasicrystalline alloys in rod, sheet and ring forms produced by various casting processes has been reviewed by focusing on their formation, structure, mechanical strength, chemical properties, magnetic properties and applications. These bulk nonequilibrium alloys exhibit unique characteristics which cannot be obtained for conventional amorphous, crystalline and quasicrystalline alloys and have been commercialized in some application fields of electrodes and golf clubs, etc. The combination of new compositions, direct production to final material forms, novel structures and unique characteristics is promising for the future development of these nonequilibrium bulk alloys as basic science and engineering materials.

Thermodynamics of Liquid Alloys, and Stable and Metastable Phase Equilibria in the Copper ― Iron System

Enthalpies of formation of liquid Cu ― Fe alloys are studied using high-temperature calorimetry. Measurements are performed at 1673 K in the composition range x Fe = 0-0.12. The temperature dependence of the enthalpy of mixing is established by comparing the results with those obtained previously at 1873 K. There is an increase of alloy heat of formation with decreasing temperature. Thermodynamic models of phases of the system are constructed using the experimental results, published data about the activities of components in the liquid and solid solutions, and also data about stable and metastable phase transformations. Thermodynamic evaluation of the system is performed using the CALPHAD-method. Calculated values are in good agreement with the majority of data for thermodynamic properties and phase transitions. A model of the liquid phase is constructed taking into account the temperature dependence of the enthalpies of mixing. Parameters of stable (L + δ ⇔ γ, L + γ ⇔ e, γ ⇔ e + α) and metastable (L 1 ⇔ L 2, L 1 + L 2 ⇔ e) phase equilibria are calculated by means of the thermodynamic models obtained. The occurrence of a metastable monotectic transformation L 2 ⇔ L 1 + δ with participation of δ-phase in the region of its metastable solidification is suggested. The thermodynamic description of the system obtained is used successfully for theoretical interpretation of the limits of forming supersaturated BCC- and FCC-solutions by quenching from the liquid. A connection between the thermodynamic properties of phases of the system and the limits of forming of supersaturated solutions within it during highly nonequilibrium alloy synthesis is demonstrated.

衝撃波圧力と物質ダイナミクス 衝撃圧縮を用いた非平衡バルク磁性体の創製

Shock compression can be used to consolidate a nonequilibrium alloy or compound powders without recrystallizing or decomposition, taking advantage of the high pressure and short duration, in addition it can be used to synthesize high-pressure phases. Mechanical alloying (MA) has also recently been used for nonequilibrium materials processing, including the preparation of metastable solid solution phases, amorphous phases, nano crystals, etc. We have been preparing nonequilibrium bulk materials consisting of such metastable phases in the transition metal systems or tungsten systems, etc. by using shock compression, and investigating the physical properties. Particularly, the metastable solid solution bulk alloys in transition metal systems showed interesting magnetic properties in relation to the Slater-Poring curve. In addition, a fully-dense Sm2Fe17Nx magnet was for the first time prepared. In this review, the topical magnetic properties of these metastable alloys (Fe-Co, Fe-Cu, Fe-W) and the Sm-Fe-N magnet are shown.

Kinetic study of isothermal crystallization in amorphous Al 33Ni 16Zr 51 produced by mechanical alloying

A non-conventional way of production has been chosen to prepare Al 33Ni 16Zr 51 amorphous alloy: mechanical alloying which allowed the microstructure of the material to be monitored. For this type of non-equilibrium alloys, the evolution toward an equilibrium state and the crystallization kinetics were studied by differential scanning calorimetry. The crystallization processes were interpreted in terms of several theoretical models based on nucleation and growth processes.

Synthesis of metastable silver-nickel alloys by a novel laser-liquid-solid interaction technique

Metastable silver-nickel alloys have been synthesized by chemical wetness and laser-liquid-solid interaction techniques from nitrate and acetate precursors of silver and nickel. Ethylene glycol and 2-ethoxyethanol were used as reductants in the synthesis reactions. Rotating niobium substrates immersed in the liquid precursor were irradiated by a continuos wave CO2 and Nd-YAG laser (λ = 1064 nm). The powders were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and high-resolution transmission electron microscopy (HRTEM). Two-phase alloys containing silver, nickel, and oxygen were fabricated, and the shape of the particles was found to be dependent on laser parameters and the chemical composition of the precursor solution. The synthesis mechanism of non-equilibrium Ag-Ni alloy nanoparticles has been proposed to occur primarily at the laser-liquid-solid interface by a nucleation and growth mechanism.

Characterization of nano-grain iron aluminide materials prepared by mechanical alloying: G.Valdrè et al. (University of Bologna, Bologna, Italy.) Acta Mater., vol 47, no 7, 1999, 2303–2311

Mechanical alloying (MA) is a solid-state powder processng technique involving repeated welding, fracturing, and rewelding of powder particles in a high-energy ball mill. Originally developed to produce oxide-dispersion strengthened (ODS) nickel- and iron-base superalloys for applications in the aerospace industry, MA has now been shown to be capable of synthesizing a variety of equilibrium and non-equilibrium alloy phases starting from blended elemental or prealloyed powders. The non-equilibrium phases synthesized include supersaturated solid solutions, metastable crystalline and quasicrystalline phases, nanostructures, and amorphous alloys. Recent advances in these areas and also on disordering of ordered intermetallics and mechanochemical synthesis of materials have been critically reviewed after discussing the process and process variables involved in MA. The often vexing problem of powder contamination has been analyzed and methods have been suggested to avoid/minimize it. The present understanding of the modeling of the MA process has also been discussed. The present and potential applications of MA are described. Wherever possible, comparisons have been made on the product phases obtained by MA with those of rapid solidification processing, another non-equilibrium processing technique.

Formation of non-equilibrium alloy phases in Cr–Cu multilayered films by ion mixing

The Cr–Cu system is characterized by its positive heat of formation, which leads to a very limited solid solubility between the constituent metals. By ion irradiation of alternately deposited Cr–Cu multilayered films, however, non-equilibrium amorphous, face-centered-cubic (fcc) crystalline phase and Cr-based supersaturated solid solution were formed at the Cr-rich side. The fcc phase was deduced to contain about 62 at.% of Cr and its lattice constant was determined to be 0.335 nm. The formation of the fcc phase was through a bcc–hcp–fcc transition mechanism from the Cr matrix.

Kinetic features of alloy ordering with many types of ordered domain: D03-type orderings

The earlier-described master equation approach to the configurational kinetics of non-equilibrium alloys is used to study kinetic features of `multivariant' orderings for which more than two types of ordered domain are possible. To this end we make extensive simulations of various phase transformations involving D03-type orderings for a number of alloy models. The microscopic structure of various antiphase boundaries in the D03 phase is also studied. A consistent approach to describing the effect of elastic forces on microstructural evolution is outlined and used to study the kinetics of a multivariant ordering accompanied by alloy decomposition. Our simulations reveal a number of peculiar kinetic features of multivariant orderings, many of them agreeing well with experimental observations.

The kinetic cluster-field method and its application to studies of L12-type orderings in alloys

The earlier-described master equation approach to configurational kinetics of non-equilibrium alloys is applied to study L12-type orderings in FCC alloys. We describe the kinetic tetrahedron cluster-field method which generalizes a similar method used for equilibrium systems to the case of non-equilibrium alloys. The method developed is used to simulate A1 L12 and A1 A1+L12 transformations after a quench of an alloy from the disordered A1 phase to the single-phase L12 state or the two-phase A1+L12 state for a number of alloy models with both short-range and long-range interactions. Simulations of the A1 L12 transition show a sharp dependence of the microstructural evolution on the type of interaction, and particularly on the interaction range. The simulations also reveal a number of peculiar features in both the transient microstructures and the transformation kinetics, many of them agreeing well with experimental observations. Microstructural evolution under A1 A1+L12 transition was found to be less sensitive to the type of the finite-range (`chemical') interaction, while in the presence of elastic interaction this evolution shows a number of specific features which were earlier discussed phenomenologically by Khachaturyan and co-workers and are illustrated by our simulations. We also consider the problem of the occurrence of a transient congruent ordering under A1 A1+L12 transformation and discuss the microstructural features of this stage.

Influence of pre-annealing on the thermal stability of a nanocrystalline Hf-Ni alloy

A nanocrystalline (NC) sample was prepared by quenching a Hf 11Ni 89 alloy with the melt-spinning technique. A randomly oriented HfNi 5 nanophase with an average grain size of about 10 nm was formed by this process. As an extension of our previous work on the as-quenched state of this NC Hf-Ni alloy, here we use DSC and XRD to report on a thermal stability study of samples pre-annealed at 340 °C for different periods of time t a up to 510 min. The pre-annealing was performed under conditions that did not result in a change of the grain size. Similarly to the as-quenched state of the NC HfNi 5 alloy, two exothermal peaks appear in the DSC curve for the pre-annealed NC samples on heating at a constant heating rate. The two peaks may also be attributed to a grain-growth process of the HfNi 5 nanophase prior to a precipitation process of a Ni solid solution phase. Activation energies for the two processes were calculated by using the Kissinger equation. It was obtained that a pre-annealing treatment decreases the thermal stability of the NC sample, which was characterized by a reduction of the temperatures and the activation energies for the grain-growth and the Ni precipitation processes. The DSC analysis revealed that the total excess energy of this highly non-equilibrium alloy and, specifically, the grain-boundary energy, decrease with increasing pre-annealing time. These changes in the parameters characterizing the thermal stability of the NC HfNi 5 alloy are discussed on the basis of observed modifications of the microstructure with the help of atomic diffusion processes and, also, of magnetic measurements on pre-annealed samples described elsewhere.