机械合金化W-Cu固溶体的形成机理

采用W-15%Cu和W-25%Cu两种复合粉,开展了机械合金化诱导W-Cu固溶度扩展机制的研究.研究表明:两种粉末经8 h高能球磨后,Cu固溶入W中形成固溶体.机械合金化诱导W-Cu固溶度扩展的主要机制,一方面是高能球磨过程中所引起的粒子纳米化,形成大量的纳米界面,许多原子"储存"在这些纳米晶界上,诱导W-Cu固溶浓度扩展.另一方面是机械合金化过程晶格严重畸变,晶粒内部生成高密度缺陷,成为溶质快速扩散的网络通道,诱导过饱和固溶体的形成.     

Preparation of the Bi8Sb32Te60 solid solution by mechanical alloying

Bi₂Te₃, Sb₂Te₃ and Bi₈Sb₃₂Te₆₀ thermoelectric materials have been prepared by mechanical alloying using a high energy planetary ball mill. The alloy formation was followed by X-ray diffraction (XRD), the morphology by scanning electron microscopy (SEM) and the composition by electron microprobe. The samples of Bi₈Sb₃₂Te₆₀ were prepared in a reasonable milling time (less than 8 h) by mechanical alloying of binary alloys (Bi₂Te₃ and Sb₂Te₃). The single phase Bi₈Sb₃₂Te₆₀ solid solution obtained presents convenient stoichiometry and good homogeneity in composition. This revised version was published online in November 2006 with corrections to the Cover Date.     

Synthesis of binary copper chalcogenides by mechanical alloying

Copper sulfides (tetragonal Cu_1.92S-Cu_1.99S, rhombohedral Cu_1.71S-Cu_1.89S, CuS), selenides (β Cu_{2 − x}Se, Cu₃Se₂, γ CuSe, CuSe₂), and tellurides (Cu_{2 − x}Te, Cu₃Te₂) were prepared from mixtures of the elements by mechanical alloying (MA), using a high-energy ball mill. The compounds were obtained by 60–120 min of MA. It is notable that two metastable high-pressure phases were obtained by MA: tetragonal Cu_{2 − x}S and pyrite-type CuSe₂. The tetragonal phase gradually transformed to the stable phase of djurleite when kept at room temperature. Cu_1.71S-Cu_1.89S also formed a metastable rhombohedral phase.     

Fabrication of MnSi1.73 thermoelectric material by mechanical alloying

The effect of mechanical alloying (MA) on the formation of MnSi1.73 thermoelectric compound was investigated. Due to the observed larger loss of Si relative to Mn during MA, the starting composition of Mn-Si was modified to MnSi1.83 and MnSi1.88. Sintering was performed in a spark plasma sintering (SPS) machine up to 600-800 degrees C under 50 MPa. The single phase MnSi1.73 has been obtained by MA of MnSi1.88 mixture powders for 200 h. It is also found that the grain size of MnSi1.73 compound analyzed by Hall plot method is reduced to 40 nm after 200 h of milling. Additionally, X-ray diffraction data shows that the SPS compact from 200 h MA powders consolidated at 600 degrees C consists of only nanocrystalline MnSi1.73 compound with a grain size of 90 nm.     

Formation of a Al50Fe50 solid solution by mechanical alloying

This work investigates the alloying reaction undergone by Al₅₀Fe₅₀ powder mixtures submitted to mechanical processing by ball milling. The transformation kinetics was studied by quantitative X-ray diffraction. Experimental evidences indicate that Al gradually dissolves in Fe, finally forming a crystalline solid solution. A phenomenological model was developed to describe the observed kinetics with reference to the number of collisions and to the fraction of powder effectively processed at individual collisions. It is shown that only about 5 μg of powders are involved in the Al dissolution processes at collision. It is also shown that a Al₃₀Fe₇₀ solid solution already forms at the first impact via local dissolution processes.     

Forming luminescent grains via decomposition of supersaturated PbS-CdS solid solution

Nanodimensional luminescent CdS-based grains on the surface of narrow-bandgap inclusions can be formed under certain regimes of annealing of a heterophase film based on a wide-bandgap CdS with narrow-bandgap Pb _x Cd_{1 ? x} S inclusions (with x no less than 0.94). The annealing leads to a change in the luminescent properties of the initial material in the visible spectral range and in the distribution of luminescent regions on the map of cathodoluminescence. Processes leading to these changes in the phase composition are described using data of Auger electron spectroscopy.     

Decomposition of supersaturated solid solution in splat-cooled Sn- 10 at% Sb alloy

A supersaturated solid solution was obtained in an Sn-10 at. %Sb alloy rapidly quenched from the liquid state. Decomposition of this solution, taking place during isothermal annealing at 373 K, was investigated by transmission electron microscopy. In the first stage of decomposition, clustering of Sb atoms in definite planes of the tetragonal lattice of tin was observed. Subsequently, needle-like precipitates of the equilibrium compound SnSb were formed. The sequence of decomposition of the supersaturated solid solution Sn(Sb) and a model for the formation of the compound within the tetragonal lattice of tin are proposed.     

Thermodynamic analysis of the extension of solid solubility in copper-lead immiscible system prepared by mechanical alloying

X-ray diffraction analysis, scanning electron microscopy and thermodynamic calculations in the Miedema's model were used to study the extension of solid solubility in copper-lead immiscible system prepared by mechanical alloying. The result shows that the nanocrystalline supersaturated solid solution formed during the alloying process. The Gibbs free energy change in this system during the formation of solid solution is calculated and shows to be positive, which means that a thermodynamic barrier exists for the formation of this alloy system in solid state. It has been found that the additional energy stored in nanocrystalline copper-lead alloy during the alloying process as a result of crystallite size reduction and increased dislocation density, would be high enough to overcome the thermodynamic barrier in the formation of a solid solution.     

Fabrication of Mg2Si thermoelectric materials by mechanical alloying and spark-plasma sintering process

A mixture of pure Mg and Si powders with an atomic ratio 2:1 has been subjected to mechanical alloying (MA) at room temperature to prepare the Mg2Si thermoelectric material. Mg2Si intermetallic compound with a grain size of 50 nm can be obtained by MA of Mg66.7Si33.3 powders for 60 hours and subsequently annealed at 620 degrees C. Consolidation of the MA powders was performed in a spark plasma sintering (SPS) machine using graphite dies up to 800-900 degrees C under 50 MPa. The shrinkage of consolidated samples during SPS was significant at about 250 degrees and 620 degrees C. X-ray diffraction data shows that the SPS compact from 60 h MA powders consolidated up to 800 degrees C consists of only nanocrystalline Mg2Si compound with a grain size of 100 nm.     

Effect of carbon on the density,microstructure and hardness of alloys formed by mechanical alloying

This work aimed to produce iron-based alloys containing resistant microstructures to improve the mechanical properties of the resulting alloy. The effects of both carbon content and compaction pressure on the microstructure, density and hardness of the alloys were examined. Iron-based alloys with initial carbon contents of 0.5%, 1%, 2% and 3% were produced by powder metallurgy following a process that involved ball milling elemental powders, cold pressing and sintering. The composition, density, microstructure, porosity, hardness and ductility of the alloys depended on both compaction pressure and carbon content. As the carbon content increased, the amount of the resistant microstructure bainite in the alloys also increased, as did their hardness. In contrast, the density and ductility of the alloys decreased with increasing carbon content. This study shows that formation of the resistant microstructure bainite in alloys fabricated by powder metallurgy is influenced by both the initial carbon content of the alloy and compaction pressure during cold pressing.     

Fabrication and properties of carbon fibre-reinforced copper composite by controlled three-step electrodeposition

Fabrication and properties of continuum carbon fibre-reinforced copper materials by controlled three-step electrodeposition are described. The effects of processing parameters, i.e. hot-pressing temperature, pressure and duration, and reinforcement content, on the properties of composites are discussed. An alloying element, nickel, was introduced into the C/Cu interface to enhance the composite strength. Finally, the relatively optimum production parameters (i.e. 30 wt% fibre, 700°C, 40 min and 10 MPa) are suggested. This revised version was published online in November 2006 with corrections to the Cover Date.     

New materials produced by mechanical alloying

The application of mechanical alloying (MA) to alloys based on Fe, Cu, Al, Ti, Co, Ni, Mg, and Nb is reviewed. Enhancement in physical and mechanical behavior, beyond ingot metallurgy and rapid solidification levels, can be achieved by MA, and should lead to commercialization of a number of MA alloys.Conducted under the joint Moscow-Moscow program on Synthesis of Advanced Materials (SAM).     

Recrystallisation of supersaturated copper-cobalt solid solutions

The mutual influence of precipitation and recrystallisation was investigated in supersaturated and cold worked Cu-Co-alloys. For medium amounts of deformation (10 to 20% for 2.8 at. % Co) a discontinuous reaction was found that leads to simultaneous annihilation of dislocations and formation of rows of Co-particles. At lower amounts of deformation, recrystallisation is inhibited by particles, while recrystallisation is complete before precipitation starts at higher amounts of deformation.     

Synthesis of Mg-Ti alloy by mechanical alloying

Mg-based Mg-Ti binary alloys have been synthesized by mechanical alloying of Mg and Ti powder blends. It was found that mechanical alloying of Mg and Ti results in a nanocrystalline Mg-Ti alloy and an extended solubility of Ti in Mg, due to the favorable size factor and the isomorphous structure of Mg and Ti. In the case of Mg-20at.%Ti, about 12.5% Ti is dissolved in the Mg lattice when the mechanical alloying process reaches a stable state. The rest (about 7.5 at.%) remains as fine particles in the size of 50–150 nm in diameter. Dissolution of 12.5 at.% Ti in the Mg lattice causes a contraction of the unit cell volume from 0.0464 to 0.0442 nm³ and a decrease of the c/a ratio from 1.624 to 1.612 of the hexagonal structure. The supersaturated solid solution Mg-Ti alloy decomposes upon thermal annealing at temperatures above 200°C. Hydrogenation enhances the decomposition process at lower temperatures.