Non-equilibrium Solid Solution Research Articles

Non-equilibrium solid solution and nanocrystal structure of Fe–Cu alloy after plastic deformation under pressure

Abstract A nanocrystal structure and non-equilibrium solid solution of Fe-20 at.% Cu has been obtained after severe plastic deformation and additional heating. Thin disc samples were pressed at up to 11 GPa between Bridgman anvils and then deformed by rotating one anvil about the other. Cu does not normally form a solid solution with Fe if the Cu concentration exceeds 3 at.%, and only Cu (f.c.c.) and Fe (b.c.c.) lines were revealed by X-ray diffraction from the initial master alloy. After deformation (ϵ = 6.4), broadened Cu and Fe lines were obtained. After further deformation (ϵ = 7.1) only b.c.c. Fe broadened lines were found. On additional heating up to 850–950°C, narrow f.c.c. Cu and b.c.c. Fe lines reappeared. The structure of the samples was investigated by electron microscopy. After deformation to ϵ = 6.4, the sizes of the grains were 10–50 nm and beyond ϵ = 7.1, they were 5–20 nm. According to X-ray and nuclear gamma resonance data, a redistribution of atoms and the formation of a solid solution of Fe-20 at.% Cu took place during deformation. A non-equilibrium solid solution of Fe-20 at.% Cu is also obtained after heating deformed ϵ = 6.4 samples up to 400°C.

Synthesis of ceramic oxide powders by microwave plasma pyrolysis

Ceramic powders prepared pyrolytically exhibit homogeneity and, in most cases, small grain sizes. The energy efficiency of electrically heated systems performing the pyrolysis in a stream of carrier gas is poor. Similar considerations concerning energy demand are valid for spray drying of suspensions. This situation can be improved using a microwave plasma as a source for thermal energy. The process described in this paper works with any aqueous solution of salts used as starting material in ceramics. The process was demonstrated by the synthesis of alumina, zirconia, and zirconia-based ceramic powders; where an energy efficiency of more than 80% was found. For the powder synthesis, aqueous solutions of the nitrates were used as starting materials. Through proper selection of conditions for synthesis, it is possible to obtain nanocrystalline powders, as demonstrated by electron microscopy. Because of the extreme conditions associated with plasma during synthesis, it is possible to prepare nonequilibrium structures and solid solutions in systems in which nearly no equilibrium solubility exists.

Nonequilibrium Phases in Rapidly Solidified High-Carbon Fe–Cr–Mo Alloys

The effect of cooling rate and carbon content on the formation of nonequilibrium phases has been investigated in rapidly solidified 10Cr-5Mo and 10Cr-10Mo iron alloys containing very high carbon contents of 2 to 5% in mass. The cooling rate was varied by changing the nozzle size in a single roller method to result in an approximate value of 6.10 4 to 4.10 5 K/s. Nonequilibrium solid solutions of crystalline γ, e and ψ phases as well as an amorphous phase were formed depending on the cooling rate and chemical composition of alloys. The optimum condition for obtaining the e phase in the 10Cr-5Mo alloy and the ψ phase in the 10Cr-10Mo alloy was determined by the combination of cooling rate and carbon content

Synthesis of nanosized ceramic oxide powders by microwave plasma reactions

This paper describes a novel process using a microwave plasma as a source of energy to synthesize ceramic oxide powders with mean particle size in the range of 5 to 30 nm. The process works without solvents by evaporation of chlorides of the elements used in ceramics. The process was demonstrated by the synthesis of alumina-, titania- and zirconia-based ceramic powders. Air or one of the noble gases mixed with any amount of oxygen and water was used as process gas. Electron microscopy revealed that through proper selection of synthesis conditions, it is possible to obtain nanocrystalline powders. Due to the extreme conditions during synthesis it is possible to prepare non-equilibrium phases or solid solutions in systems exhibiting no equilibrium solubility.

Milling and mechanical alloying of inorganic nonmetallics

The versatility of mechanochemical processing was investigated in a number of nonmetallic inorganic systems. It is shown that high energy grinding can be used to produce amorphous carbon from synthetic graphite and some forms of natural graphite. Elemental sulfur can likewise be amorphized by prolonged high energy grinding. Phase transformations of αFe2O3(hematite) and mechanochemical reactions of this phase with ZnO and NiO are strongly influenced by the presence of iron resulting from wear of the grinding media. Thus spinel type ferrites were obtained by grinding of such mixtures for short times (1–3 h in a Spex mill); however, longer grinding times resulted in the formation of FeO or (Fe,Zn)O (when grinding ZnO) or FeO or (Fe, Ni)O (when grinding NiO), presumably as a result of the reaction of mill wear debris with the mill charge. The suspected (Fe, Zn)O phase is most likely a nonequilibrium solid solution. Negligible were accompanied the mechanochemical synthesis of NiS and ZnS from elemental powders. These sulfides were formed for short milling times. In contrast, sulfides of tungsten were not formed even when rather long milling times were employed. The survey of mechanochemical reactions presented here further reinforces the concept that this low temperature synthesis method is a robust process route for production of a wide range of materials.

Mössbauer Study of Mechanically Alloyed Powders

The microscopic nature of mechanically alloyed powders has been investigated using the 57Fe and 119Sn Mossbauer spectroscopy. The following alloy systems have been treated by a low-energy ball mill; Al-Fe, Fe-Sn, Ag-Fe, Fe-C and Fe-B. To understand the elementary process of kneading, the repeated rolling method has been employed for the Al-Fe system and compared with the results from the ball-milling method. Pure Fe powders have also been ball-milled in order to understand the effects of heavy deformation using the same conditions and investigated by the 57Fe Mossbauer spectroscopy. Hyperfine interaction parameters obtained show that atomic dispersion and the formation of an amorphous, non-equilibrium solid solution and intermetallic compounds occur during mechanical alloying. From the experimental findings for each powder the characteristic features of the mechanical alloying process have been discussed.

Electronic properties and superconductivity of rapidly quenched Al-Si alloys.

The authors present detailed studies of electronic properties of Al-Si alloys prepared in a nonequilibrium state by means of rapid solidification. The quenched alloys exhibit an enhanced superconducting transition temperature up to 6.2 K in an Al-Si 30 at.% alloy as well as an increased thermal slope of resistivity. Using differential scanning calorimetry, a large enthalpy variation (ΔH=4.1 kJ/mole for Al-Si 30 at.%) has been measured during the irreversible transition from the nonequilibrium state to the equilibrium one. This is mainly attributed to the energy difference between the metallic state of silicon atoms trapped in FCC aluminum matrix during quenching and the usual covalent state of silicon precipitates in an equilibrium state. This large energy difference is presented as the origin of a lattice instability which softens the phonon spectrum and gives rise to a stronger electron-phonon coupling. This appears to be a characteristic property of nonequilibrium Al-Si solid solutions, which is associated with the metallic state of silicon atoms. An interpretation of the Tc enhancement is proposed for both Al-Si and Al-Ge alloys based on the phonon softening in these nonequilibrium crystalline alloys

Structural characterization of Cu-Cr films

Cu 80Cr 20 films were deposited by vapor deposition and by ion platings with and without bias. Only the film deposited by ion plating with the bias did form the non-equilibrium single-phase fcc solid solution. By heating above 673 K, phase separation occurred, leading to the formation of two layers, a Cr layer at the surface and a Cu layer in the remaining bulk.

XPS study on the low-temperature CO shift reaction catalyst: I. The unreduced copper-zinc system

X-Ray photoelectron spectroscopy has been used to study CuZn catalysts for the low-temperature CO shift reaction before activation by reduction. Samples with Cu concentrations in the range 3–54% of the total cations and calcination temperatures up to 923 K have been investigated. Single-phase and multiphase precipitates have been compared, showing that several chemico-physical properties are different in the two cases. Evidence has been reached for the existence, in a narrow range of temperatures, of a nonequilibrium solid solution of Cu(II) in ZnO. Also the formation of a solid solution of Zn(II) in CuO has been suggested.

Nonequilibrium solid solutions obtained by heavy ion implantation and laser annealing

Nonequilibrium solid solubility of P, As, and B in single-crystalline silicon annealed by Q-switched ruby laser pulse irradiation was experimentally investigated for residual defects, lattice strain, and electrical activation of implanted impurities. The maximum solubility obtained without any macroscopically extended defect formation was 2–4 times higher than the thermal equilibrium solubility limit. Above this solubility, precipitates, dislocations, and surface cracks were observed. The highest full activation was realized by P implantation, with carrier concentration up to ∼5×1021/cm3 showing no such defects. Formation mechanisms of the defects are discussed and shown to be attributable to the rapid solidification process of the heavily doped layer and to large impurity-induced misfit stress comparable to the fracture stress.

On the evolution of states from a non-equilibrium solid solution

Despite the enormous variety of structural states that evolve from non-equilibrium solid solutions and despite the even larger number of models that have been proposed for them, it is becoming apparent that only a limited number of basic types of evolutionary paths exist.

Preparation of nonequilibrium solid solutions of (GaAs)1−xSix

A method using coincident rf sputtering and rf discharge decomposition is shown capable of producing single-crystal, epitaxial, nonequilibrium solid solutions of Si in GaAs with the composition of Si far exceeding the limits reported for the bulk equilibrium phase diagram. Measured values of alloy composition and lattice parameter indicate close correspondence to Vegard's law. High-temperature annealing of epitaxial films demonstrates the ultimate instability of the alloys.

The processes of establishing short-range order in nonequilibrium solid solutions

The processes of establishing short-range order in nonequilibrium solid solutions