Non-equilibrium Solid Solution Research Articles

Metastable phase formation during solidification of undercooled melt

Undercooling behaviour of micron-sized droplets entrained in the matrix of the primary phase of Ag–Ge, Al–Ge and Cu–Ag alloys was investigated. Thermal cycle consisted of equilibration of hypoeutectic alloys above the eutectic temperature in two phase liquid–solid region followed by their slow cooling or quenching from the mushy state. Repeated thermal cycling generated a wide size range of droplets. During slow cooling droplets exhibited a large undercooling below the eutectic temperature of alloys. Solidification structure of the undercooled droplets revealed presence of metastable phases in Ag–Ge and Al–Ge alloys and a non-equilibrium solid solution phase in Cu–Ag alloy. Solidification behaviour of undercooled melt and formation of metastable microstructure are discussed.

The Heat Treatment of Al-Cu-Li Alloys Ensured the Stability of Structure and Properties at Long Low Temperature Exposure

The structural transformations and properties changes of sheets from Al-Cu-Li alloys with two levels of Li content (alloy I with 1.7 wt. % and alloy II with 1.5 %) during long low temperature exposure (LLTE) at 85 °C, 1000 h and 90 °C, 4000 h have been investigated. Three-stage ageing ensured the strengthening of alloy II mainly at the expense of T 1 -phase precipitates with the favorable phases morphology on the grain boundaries and in their bodies. The alloy II hot rolled sheets have the high level of properties, including Impact Behavior Energy before and after LLTE The absence of structural changing during LLTE were confirmed by DSC and TEM data. The alloy I sheets, aged on offered regime, had some changes in properties, however they were no more than 5 %. 1. Introduction. The Al-Cu-Li alloys have a unique combination of high strength, low density, weldability, satisfactory damage tolerance characteristics in conditions of static and fatigue loading [1, 2] A lack of these alloys is the reduced plasticity and inherent to the majority of aluminium alloys with lithium insufficient stability of the structure and properties in conditions of long low temperature exposure (LLTE) [3]. The cause of this are the changes in quasiequilibrium solubility at ageing temperature and LLTE temperature and the fine dispersed δ'-precipitates formation from the nonequilibrium solid solution which has stayed supersaturated after ageing [4, 5], and also formation of GPI zones containing Al and Cu [6]. For increase of thermal stability in the previous work [5] the two-stage ageing 120°C, 20 h + 170°C, 12 h with slow cooling from temperature 170°C up to temperature LLTE was offered. This mode has ensured the sufficient stability for the alloy containing 1.75 weight % of lithium in conditions LLTE 85°C, 1000 hours. In the present work it was necessary to check up, as far as the structure and properties of the alloy will stable after the more rigid test - 90°C. 4000 hours. Assume that the reason of structure instability during LLTE is Li solid solution supersaturation [4, 5], for increase of stability it is reasonable to decrease Li content in alloy. Therefore in the given work the alloy with the content of 1.5 weight % lithium was investigated also. The optimum ageing regime was developed and the phase composition, precipitates morphology, complex of properties and their stability during LLTE were investigated. 2. Experimental. The study was carried out on the 10 and 7 mm hot rolled sheets of two Al-Li-Cu (type 1460) alloys with 1.75 wt. Li % (alloy 1) and 1.5 % (alloy II). The hot rolling temperature was 440-470°C. Sheets were solutionised at 530°C, 1.5 h and quenched in water. The time between quenching and stretching was less than 3 h. The residual deformation degree at the stretching was 3 %. After quenching sheets have unrecrystallized structure. The average fiber thickness was 15 μm. The tensile properties, KCU, K e0 , FCPR, Impact Behavior Energy were determined. The changes in the sheets structure were studied by TEM, X-Ray phase analysis, DSC techniques. The alloy thermal stability estimation during LLTE (85°C, 1000 h or 90°C, 4000 h) is difficult due to the limited researches time. There was a necessity to substitute these regimes for more brief high temperature imitative ones.

Alcohol sensor based on a non-equilibrium nanostructured xZrO 2–(1− x)α-Fe 2O 3 solid solution system

Non-equilibrium nanostructured solid solution xZrO 2–(1− x)α-Fe 2O 3 powders have been prepared using the high-energy ball milling technique, and their particle size, structural properties and alcohol gas properties have been systematically characterized using X-ray diffraction (XRD) and gas sensing measurements. Experimental results show that particle size of the powder is drastically milled down to less than 10 nm after 20 h of high-energy ball milling. Our modified structural model, □ 1/3+ZrO 2+ yO 2→Zr 1/3 4++2(1− y)O s 2−+4 yO s −; with □ 1/3 denoting the 1/3 available octahedral sites of the corundum structure; for these non-equilibrium nanostructured solid solution xZrO 2–(1− x)α-Fe 2O 3 materials can explain both the lattice expansion of these high energy milled sample as well as the charge neutrality in terms of additional oxygen dangling bonds at the particle surfaces. The screen-printed thick film gas sensors made from such mechanically alloyed materials demonstrate a very high gas sensitivity value of 1097 at 1000 ppm of alcohol gas in air. It is believed that the high gas sensitivity value is related to the enormous oxygen-dangling bonds at the particle surfaces.

Ion-Bombardment-Induced Decomposition of Nonequilibrium Solid Solution and the Formation of Periodic Structures

A model of forming periodic structures induced on a solid surface by a uniform ion flow (at high implant concentrations) is put forward. It is shown that these periodic structures are a result of the decomposition of the resulting solid solution.

Deposition, microstructure and properties of sputtered copper films containing insoluble molybdenum

The microstructure and properties of Cu films containing insoluble Mo in as-deposited and annealed conditions have been studied. Using magnetron sputtering, Cu films with Mo concentrations up to 25 at. % have been deposited. The Cu-Mo films consist of nonequilibrium supersaturated solid solutions of Mo in Cu and have nanocrystalline microstructures. Upon heating, most of the films studied undergo three major transition events: recovery, crystallite growth and coalescence, and grain growth. Recovery occurs at ∼220–250 °C due to the release of strain energies stored during deposition, while the growth and coalescence of crystallites at ∼490 °C are likely driven by the crystallite-boundary reduction. Upon further annealing at above ∼670 °C, the microstructure alters considerably in most of the films, resulting in extensive growth of crystallites and grains. Rather high strain energies are thought to cause the precipitation of Cu particles in the molybdenum-rich Cu-Mo films (e.g., Cu-25 at. % Mo) during annealing at a temperature as low as 200 °C. The fine structure observed in the 800 °C-annealed Cu-25 at. % Mo film suggests that the extensive crystallite/grain growth is effectively impeded by the presence of molybdenum, signifying a good property of thermal stability. The film’s resistivity and hardness properties correlate well with the microstructure and are governed by the impurity effect of molybdenum. Low-molybdenum Cu-Mo films yield relatively low resistivity, and this is attributable to the improved film microstructure. Hardness results indicate that the strengthening of the films is mainly due to the fine structure and presence of molybdenum.

Microstructure and properties of Cu-C pseudoalloy films prepared by sputter deposition

The microstructure and properties of Cu-C pseudoalloy films prepared by R.F. magnetron sputtering have been investigated. As Cu and C are mutually immiscible, nonequilibrium supersaturated solid solutions of C in Cu with nanocrystalline microstructures were observed in as-deposited films. Upon heating of the films, three major transition events took place. Recovery occurred at ∼280 to 300 °C, while at ∼400 °C, crystallites started to growth and coalescence, due to the release of strain energies stored during deposition. Annealing at above 600 °C led to the occurrence of grain growth and altered the microstructure considerably. Although attempts have been made in this study, a possible annealing-induced phase separation could not be unambiguously identified. Yet, the fact of low twin densities and fine grain structures observed in the annealed films suggests that the extensive grain growth was impeded by the presence of carbon. Resistivity and hardness properties correlated well with the film microstructure and were governed by the impurity effect of carbon. Low-carbon Cu-C films yielded relatively low resistivity, attributable to the improved film microstructure. Hardness results indicated the strengthening of films was mainly due to fine structure, presence of carbon, and grain refinement by annealing twins.

Equipment and methods of surface modification of the microstructure and properties of metals by adsorption assisted ion implantation

Data on phase and structural transformations in the surface layers of metallic alloys as a function of the conditions of ion implantation are reviewed. It is noted that the ion mixing of the surface-absorbed active elements from the implantational gas medium plays an important role in the formation of the element and phase composition of the ion-doped layers. The most important treatment parameters determining the relative role of ionic mixing in structural-phase transformations in the ion-doped layer are: the elementary composition and pressure of the implantational gas medium, the target temperature, the atomic weight of the implanted ions, and the reactivity of these ions and the target ions to the absorbed elements (C, N, O). The formation of high-energy defect structures and the dispersion of the implanted-layer crystal structure in the strong internal-stress fields generated by the highly nonequilibrium solid solutions and in the course of phase transformation are considered. New possibilities for microstructural modification during high-dose implantation are identified with the formation of high-energy defect (including nanocrystalline) structures; heterophase, completely amorphized and other phase-structural states and their combinations in iondoped layers of metallic alloys. Taking advantage of these possibilities makes it possible to reduce by an order of magnitude the implanted dose necessary to upgrade the performance of the surface, and to raise the productivity and lower the cost of the ionbeam technological treatment.

Non-equilibrium W-Cu system alloy powder and bulk body prepared by mechanical alloying and shock compression

Mechanical alloying (MA) has recently been used for material processing methods: the preparation of amorphous phases, non-equilibrium solid solution phases, high pressure phases, etc. [1–4]. The metastable solid solution powders of V–Cu [5], Hg–Cu [6], Al–Fe [7] systems, etc., which can not appear in an equilibrium state, were prepared by the MA treatment. The amorphous powders of Ta–Cu [5], Ni–Zr [8] systems, etc. were prepared also by the MA treatment. The tungsten (W)-copper (Cu) system is a typical immiscible system in the solid and even in the liquid phase. Several studies of the MA treatment of the copper-based W–Cu system were reported for the pre-treatment of sintering, but they did not suggest the formation of solid solutions [9–11]. Gaffet et al. [12] reported that the W–Cu system metastable phase could be prepared by the MA treatment, but the crystal state was not investigated in detail [12]. The preparation of the non-equilibrium bulk body from the MA-treated W– Cu system powder has not been reported. Shock compression has different features from static compression: pulsed short duration, shear stress, heterogeneous state, etc. and can be used as a consolidation method of non-equilibrium materials without recrystallization or decomposition [13, 14]. It is important to consolidate non-equilibrium powders for the evaluation of physical properties and for industrial applications. In this study, the MA treatment and shock compression recovery experiments were performed on a W–Cu mixture powder, to prepare non-equilibrium W–Cu system bulk bodies including solid solutions, which can not appear in an equilibrium state. Starting powders were provided by Rare Metallic Co., Ltd. These tungsten and copper powders consisted of irregular particles of 2–3 and ,45 im diameter with a total impurity value of less than 0.1 and 0.01 wt %, respectively. The starting powder mixture was prepared by mixing tungsten and copper powders in a 60:40 molar ratio. The MA experiments were performed by using the planetary micro ball mill (P-7 of Fritsch Co., Ltd.) in an argon atmosphere [4]. A mill capsule with an inner-diameter of 41 mm and a depth of 38 mm and balls with a diameter of 15 mm, which were both made of silicon nitride, were used. The powder specimen with a weight of about 20 g and seven silicon nitride balls were contained into the capsule. The rotation speed of the ball mill was 2840 r.p.m. The resultant acceleration was estimated to be about 12 g. Small amounts of powder were recovered for several different durations of the MA treatment for X-ray analyses. Shock-compression recovery experiments were conducted using a propellant gun [15]. The MA treatment powder samples were enclosed in an iron capsule (SS-41) with an inside diameter of about 12 mm and with an inside height of about 3.5 mm. The porosity of the powder was about 49%. Shock loading was carried out by impacting the sample capsule with a tungsten flat flyer plate whose thickness was 1 mm. Shock pressures achieved in the capsule and in the sample were estimated by the impedence-matching method from the measured flyer-plate velocity, the Hugoniot parameters of tungsten, steel and copper [16]. The recovered specimens were investigated by chemical analysis and powder X-ray diffraction (XRD). Powder XRD analyses were carried out using monochromatized Cu-KAE radiation with a Rigaku goniometer. Calibration of the goniometer was performed by measuring diffraction peaks of pure silicon powder. The lattice parameter was refined by the least squares method up to an accuracy of greater than 0.01%. The composition mapping analysis was carried out using an electron probe micro analyser (EPMA) apparatus, CXA-733 of JEOL Ltd. Instrumental chemical analysis of nitrogen (N), oxygen (O) and carbon (C) contents were carried out by the inert-gas fusion thermal conductivity method and the combustion in oxygen non-dispersive infrared absorption method using the TC-436 and WR-112 of LECO Corp. Chemical analysis of silicon (Si) was done by the inductively coupled plasma (ICP) method using the SPS-1200 of Seiko Electric Co., Ltd. For each element, the measurements were performed 2–3 times on each specimen. The experimental errors for N, O, C and Si contents were estimated to be less than 0.07, 0.02, 0.01 and 0.0001 wt %, respectively, by the experiments using the Si3N4 and TiN as standard samples. Fig. 1 shows the powder XRD patterns of the starting powder, the MA treated powder (360 min) and the shock-consolidated bulk body with an impact velocity of 0.938 km sy1. The driving pressure in the capsule and the first pressure in the powder were estimated to be 24.5 and 13 GPa,

Mechanical alloying in the Al-Bi alloy system

Mechanical alloying (MA) was carried out to investigate the MA behaviour of the immiscible Al-10, 30 at % Bi alloys. After the MA processing, the Al and Bi were finely and homogeneously alloyed. The Bi crystallite size decreased to 25 nm and 30 nm in the Al-10 at % Bi and Al-30 at % Bi alloys, respectively. By increasing the MA time, the hardness increased up to a value of 80 Hv, which is larger than that obtained from the rule of mixtures. The lattice parameter of Bi decreased by about 0.27%, which shows the formation of a non-equilibrium hcp Bi super-saturated solid solution. The extended solubility of Al in Bi was 1.9% in the Al-30 at % Bi alloy. Due to the extended solubility, depression of the melting temperature of hcp Bi was confirmed in the mechanically alloyed Al-Bi alloys. The maximum depression in the temperature was about 10 K. The measured values corresponded well with those estimated from the extrapolation of the solidus line.

Late stages of phase separation in Al1-xSix solid solutions.

The kinetics of late stages of phase separation in nonequilibrium substitutional solid solutions ${\mathrm{Al}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Si}}_{\mathit{x}}$ was studied by resistivity relaxation. The dynamic scaling exponents have been observed to correspond probably to a transition from coagulation (\ensuremath{\alpha}\ensuremath{\approxeq}0.16\char21{}0.2) to Ostwald-ripening-type behavior. An essential slowing down of the latest stage of relaxation (\ensuremath{\alpha}\ensuremath{\approxeq}0.08) was detected in Al-Si alloys when depletion developed in the Al-based fcc matrix around the Si submicrometer inclusions. \textcopyright{} 1996 The American Physical Society.

Enhancement of superconductivity in vicinity of lattice instability in Al1-xSix

We report on unusual behaviour of the low-temperature electrical resistivity p of non-equilibrium substitutional solid solutions Al1-x, Six. This behaviour is thought to be comparable with anomalies of d2Il dV2 changes observed under disordering in the tunneling or point-contact spectra of metals, and which results from strong electron-phonon interactions. The resistivity data, along with low temperature heat capacity measurements and Al LII-III X-ray emission spectroscopy (XES) and NMR Knight shift experiments, strongly suggest that an enhancement of the electron-phonon interactions due to lattice instabilities is the main reason for the dramatic increase in the superconducting transition temperature Tc to ∼ 11 K in Al1-xSix when x = 0.2.

Some Aspects of Spinodal Decomposition in Spinel Solid Solutions

Abstract Decomposition morphologies of supersaturated spinel solid solutions in the quasi-binary system Co2TiO4-CoAl2O4 at 973 K show all pertinent features of spinodal decomposition. Since the decomposition morphology may not be considered as sufficient evidence for a spinodal process, the thermodynamics of the spinel solid solutions and the linear transport theory of spinodal decomposition were combined to corroborate the experimental results of the early decomposition reaction. Simultaneous ordering, a common property of ternary and higher nonequilibrium solid solutions, was also observed. These ordering processes occur locally in distinct regions of the spinel crystal. Moreover, the spinodal wavelength λ increases in time according to λ ∞ t, which is unusual and will be discussed in the light of cation diffusion in semiconducting oxides with two cation sublattices.

Enhancement of electron-phonon interactions in the nonequilibrium solid solutions Al1-xSix.

We report unusual behavior of the low-temperature electrical resistivity $\ensuremath{\rho}$ of nonequilibrium substitutional solid solutions of the form ${\mathrm{Al}}_{1\ensuremath{-}x}{\mathrm{Si}}_{x}$. This behavior is thought to be similar to anomalies in $\frac{{d}^{2}I}{d{V}^{2}}$ observed in the tunneling or point-contact spectra of metals in the presence of disorder, and to result from strong electron-phonon interactions. The resistivity data, along with low-temperature heatcapacity measurements and Al ${L}_{\mathrm{I}\mathrm{I}\ensuremath{-}\mathrm{I}\mathrm{I}\mathrm{I}}$ x-ray-emission spectroscopy and NMR Knight shift experiments, strongly suggest that an enhancement of the electron-phonon interactions due to lattice instabilities is the main reason for the dramatic increase in the superconducting transition temperature ${T}_{c}$ to \ensuremath{\sim}11 K in ${\mathrm{Al}}_{1\ensuremath{-}x}{\mathrm{Si}}_{x}$ when $x=0.2$.

Nanocrystalline structure of non-equilibrium FeCu alloys obtained by severe plastic deformation under pressure

The nanocrystalline structure and non-equilibrium solid solutions of Fe-20 at% Cu and Fe-80 at% Cu have been observed after severe plastic deformation by shear under pressure. The structure of the deformed samples was studied by electron microscopy, X-ray diffraction and nuclear gamma resonance (NGR) methods. The coarse-grain FeCu alloys were transformed into the fine-grained state during straining up to ϵ = 5–6. Further strain up to ϵ = 7 can be considered as a cold forced superplastic deformation without change of the grain sizes. According to X-ray and NGR data, a redistribution of atoms and formation of non-equilibrium solid solutions are the result of strain from ϵ = 5–6 to ϵ = 7. Based on grain rolling, a mechanism of superplastic deformation has been proposed. Fast diffusion during cold plastic deformation is conditioned by the high stresses acting on the grain boundaries and the volumes of the fine grains. During deformation of the nanocrystalline FeCu alloys the volume diffusion coefficient are equal to about D v = 4.10 −16 cm 2/sec.

Valence fluctuation in nonequilibrium close-packed Ce-Yb alloys produced by sputtering

Using de high-deposition-rate sputtering equipment, we have produced nonequilibrium solid solutions in an immiscible Ce-Yb alloy system. The results of X-ray diffraction and Fourier transform analysis indicate that nonequilibrium close-packed phases are obtained: a randomly-close-packed-layer phase at around the equiatomic concentration and an fcc phase for both the ultimate concentrations. The random atom stacking is also confirmed by the large electrical resistivity. These alloys show rather large electronic specific heat coefficients, indicating a heavy fermion character and contribution of Yb-4f electrons to the density of state at the Fermi level. Magnetic susceptibility obeys the Curie-Weiss law and the effective magnetic moments are larger than the interpolated ones between the trivalent Ce and the divalent Yb metals. The mean valence state of Yb is estimated from the diffraction and magnetization data to be about 2.2.

Non-equilibrium phase formation and valence fluctuation in immiscible CeYb alloys produced by mechanical milling and sputtering

Non-equilibrium solid solutions have been obtained by mechanical milling and sputtering of immiscible CeYb alloys with nearly equiatomic concentrations. The X-ray diffraction and transmission electron micrograph observations indicate the formation of an f.c.c. phase as a result of mechanical milling, and a randomly close-packed layer phase obtained by sputtering. The magnetic susceptibility for these alloys obeys the Curie-Weiss law and the effective magnetic moments are larger than the interpolated values between the trivalent Ce and the divalent Yb, indicating the valence fluctuation of Yb from divalent to trivalent.

Structural?Phase transformations in metal alloys during high-dose ionic implantation

Data on phase and structural transitions in the surface layers of metal alloys as a function of the conditions of ionic implantation (ii) are reviewed. It is noted that ionic mixing of surface-absorbed active elements from the implantational gas medium plays an important role in forming the elementary and phase composition of the ion-doped layers. The dependence of these characteristics on the ii conditions is analyzed. The formation of high-energy defect structures and the dispersion of the implanted-layer crystal structure in the strong internal-stress fields generated by the highly nonequilibrium solid solutions and in the course of phase transformation are considered. New possibilities for microstructural modification in ii are identified, with the formation of high-energy defect (including nanocrystalline), heterophase, and other phase—structural states and their combinations in ion-doped layers of metallic alloys.

Epitaxial solid-solution films of immiscible MgO and CaO

Metastable solid solutions in the MgO-CaO system grow readily on MgO at 300 °C by molecular beam epitaxy. The epitaxy displays both the reflection high-energy electron diffraction oscillations characteristic of layer-by-layer growth and the lattice rotations which have been related to island nucleation. Mg1−xCaxO solid solutions grow despite a larger miscibility gap than in any system for which epitaxial solid solutions have been grown. Epitaxial quenching of nonequilibrium solid solutions is possible because of the very low growth temperatures for rock-salt oxides.

Properties of the Al-Si solid solution: Dynamical properties of the silicon substitutional and the aluminum vacancy.

Using ab initio total-energy and molecular dynamics based on local orbitals and density-functional theory, we present results on the nonequilibrium Al-Si solid solution. We report on relaxations, vibrational spectrum, and superconducting transition temperature. Our results are in excellent agreement with experimental measurements on rapidly quenched samples which show evidence of lattice instabilities induced by the solute. We perform a similar analysis on the vibrational properties of the Al vacancy and discuss the results in terms of the mechanical instability leading to the crystalline-to-amorphous transition.

Methane in neon: Nearly free rotation in a mismatched guest–host system

A nonequilibrium solid solution of 2% CH4 in Ne has been prepared by the matrix isolation technique. The system retains the fcc structure of pure Ne. A broad distribution of methane tunneling frequencies centered at an energy transfer of 0.5 meV is observed by inelastic neutron scattering. All CH4 molecules contribute to this distribution. For geometric reasons, to be consistent with the long range order of the matrix, the guest molecules must appear as trimers or higher multimers. The variety of aggregates with differing local surroundings and the complex level schemes of clusters of interacting molecules produce the broad distribution of tunnel transitions. Annealing leads to a wider potential distribution. In thermal equilibrium methane is insoluble in neon.