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A study has been carried out on non-isothermal and cyclic oxidation behavior of the W1−x Cr x –3 wt pct Y2O3 (x = 0.3 or 0.5) composite samples, and the results have been compared with those of binary W-Cr alloys. The microstructures of the sintered samples showed the presence of W-rich (β1) + Cr-rich (β2) regions and more or less uniformly distributed Y2O3 particles. Protection against high-temperature oxidation is possible due to the formation of a stable mixed oxide scale containing both Cr2WO6 and Y2W3O12 at the oxide–air interface. Melting of the eutectic composition of Y2W3O12 and WO3 leads to the formation of a liquid phase, which helps in closure of pores by sintering and viscoplastic flow of the oxide scale, thereby enhancing its resistance to spallation and further oxidation.

An exploratory study has been initiated on W–Nb alloy for elevated temperature application. The alloy was prepared by mechanical alloying followed by sintering at 2063 K. This is followed by a detailed characterization of microstructure and oxidation resistance properties. Generally, a two phase microstructure comprising W–rich and Nb-rich solid solutions was observed especially in alloys containing higher Nb. Preliminary oxidation study indicated that W–Nb alloy possessed relatively better oxidation resistance as compared to pure W or Nb which may be attributed to the presence of impervious and tenacious complex W–Nb oxides.

The influence of bulk stoichiometry of the nano-dispersoids’ and their surface composition/state on the stability and thermal conductivity of water based nanofluids has been investigated for the first time using mechanically alloyed NiAl intermetallic nanopowders produced from stoichiometric (Ni 50 Al 50 ) and non-stoichiometric (Ni 40 Al 60 and Ni 65 Al 35 ) compositions. X-ray diffraction patterns of the ball milled products have evidenced that only NiAl phase is present in all the three compositions studied after milling for 30 h, and the NiAl crystallite size in these milled powders is in the range of 8–12 nm. The addition of 0.1 vol.% mechanically alloyed Ni 65 Al 35 nanoparticles increases the thermal conductivity of the base fluids by ∼28% and the enhancement in the thermal conductivity increases linearly with the particle loading in the range of 0.1 vol.% to 0.25 vol.%. The visual observation as well as thermal conductivity measurements of the NiAl dispersed aqueous nanofluids manifest longer stability (∼7 days), when the NiAl dispersoid is of Ni 65 Al 35 composition, as compared to the stability of NiAl dispersion of Ni 40 Al 60 composition (∼6 h) or Ni 50 Al 50 composition (∼5 days). The results are interpreted using the FTIR analysis, zeta potential measurements, UV–vis spectrometry and the existing data of the X-ray photoelectron spectroscopy in the literature for the surface states of the ball milled NiAl powders.

A comparative study of high temperature oxidation behavior of the (W1 − xCrx)90Nb10 (x = 0.3, 0.5 and 0.6) alloys has been carried out at 800 °C, 1000 °C and 1200 °C in static air. Microstructural study of the alloy samples conventionally sintered from the nanocrystalline elemental powders of W–Cr–Nb alloys shows the presence of a mixture of W-solid solution (Wss) and Cr2Nb phases. Comparison of the results of isothermal and cyclic oxidation tests shows the damage to be greater during the latter tests. The oxidation resistance during exposure at 800 °C, 1000 °C and 1200 °C is found to be the highest for the (W0.4Cr0.6)90Nb10 alloy. Formation of WO3 appears to be responsible for poor oxidation resistance of (W0.7Cr0.3)90Nb10 alloy at 1200 °C. On the other hand, growth of continuous Cr2WO6 scale appears to have significant role in protection against oxidation for the other two alloys at 1200 °C. The formation of oxide scales containing Cr2WO6 + NbWO5.5 and Cr2WO6 + Nb2O5.3WO3 is found to be responsible for protection against oxidation at 800 °C and1000 °C, respectively.

Nanofluids often exhibit significantly higher thermal conductivity compared to the base fluid. Contributions of the phononic and electronic thermal transport between a heat source and frequently colliding nanoparticles to the enhanced thermal conductivity have been investigated for the first time, through multi-scale modeling. Classical molecular dynamics (MD) model has been used to estimate the phononic component of thermal transport from the heat source to colliding nanoparticles. A meso-continuum model has been used to estimate the same as well as the thermal transport from heat source to colliding nanoparticle due to the combined effect of phononic and electronic mechanisms. The data on thermal pickup by colliding nanoparticles from the heat source, obtained by rationally combining the predictions of MD and meso-continuum approach, have been fed to a higher length scale stochastic model to estimate the enhancement in the conductivity. The stochastic model keeps track of Brownian movement of nanoparticles within the base fluid and the convective heat dissipation of the absorbed thermal energy of the nanoparticles to the surrounding fluid. The present multi-scale model estimates ∼74% thermal conductivity enhancement in water-based silver nanofluid (0.1 vol%) having nanoparticles in the size range of 4–30 nm, and the results are in reasonable agreement with the experimental results reported in the literatures.

Sintering of (W1−xCrx)90Nb10 (x = 0·3, 0·5 and 0·6) alloys, prepared through mechanical alloying of elemental powder blends, has been investigated. Relative density of >98% could be achieved by sintering at 1790°C for 5 h. Characterisation by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy has shown the presence of W rich solid solution and Cr2Nb phases in the microstructures of the sintered alloys. The densification of the W–Cr–Nb alloys is possible at relatively lower temperature due to use of nanostructured powder raw material obtained through high energy milling and liquid phase sintering promoted by the presence of Cr2Nb having relatively lower melting point (1730°C). The matrix grain size of the sintered alloys is decreased, whereas hardness is increased noticeably with the amount of Cr2Nb in the microstructure.

article i nfo Article history: Received 16 June 2012 Accepted 27 August 2012 Available online xxxx Sintering and cyclic oxidation behavior of nanostructured W-Cr alloys having 30, 50 or 60 at.% Cr, prepared through mechanical alloying of elemental blends have been investigated. Relative density of 96-97.5% could be achieved by sintering at 1700 °C. X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy have shown the presence of W-rich and Cr-rich phases in sintered alloys. The oxidation resis- tance during exposure at 800 °C is found as the highest in the W0.4Cr0.6 alloy, while that at 1000 °C and 1200 °C appears to be the best for the W0.5Cr0.5 alloy. Relatively finer and more uniform distribution of the Cr-rich phase in microstructure of the W0.5Cr0.5 alloy appears to be responsible for its superior oxidation resistance. The Cr2O3 is found to be primarily responsible for protection against oxidation at 800 °C, while

Workability and mechanical properties of the ultrasonically cast Al– X wt% Al 2 O 3 ( X =2, 3.57 and 4.69) metal matrix nanocomposites were reported in the present investigation. The Al–Al 2 O 3 (average size ∼10 nm) composites showed maximum reduction ratios of 2, 1.75 and 1.41 at room temperature, and 8, 7 and 6 at 300 °C. The elastic modulus, nanoindentation hardness, microhardness and Vickers hardness were measured on the as-cast, cold and hot rolled specimens. The tensile properties were also evaluated for the as-cast composites for different wt% of reinforcement. The microstructural examination was done by optical, scanning and transmission electron microscopy. The strength and workability of the nanocomposites were discussed in the light of dislocation/particle interaction, particle size and its concentration, inter-particle spacing and working temperature. 2 wt% of Al 2 O 3 reinforcement showed better combination of workability and mechanical properties possibly due to better distribution of particulates in the matrix.

A model reported by the present investigators has earlier shown that the extent of heat pick up by a nanoparticle during its collision with the heat source in a given nanofluid would depend on the thermal conductivity (kp, unit W/m.K), density (ρ, unit kg/m3), elastic modulus (E, unit GPa) and Poissons ratio (μ) of the nanoparticle and heat source. Considering the expression for collision period and thermal conductivity of nanoparticle, a factor χ =kp(ρ/E)0.4 is proposed here and examined for the preliminary identification of the potential of a dispersoid in enhancing the thermal conductivity of a nanofluid. The χ-factor for Ag, Cu, CuO, Al2O3 and SiO2 are 2960, 2247, 116, 14.1 and 5.5, respectively. The higher χ-factor of CuO compared to that of Al2O3 can explain why water and ethylene glycol (EG) based CuO-nanofluid is reported to show higher enhancement in the thermal conductivity, when compared to similar Al2O3-nanofluid. The χ for SiO2 is much smaller than that for Ag, which also corroborates well with the marginal enhancement in thermal conductivity of water based nanofluid containing SiO2 nanoparticles. Therefore, a high value of χ of the nanodispersoid can serve as a parameter for the design of nanofluids for heat transfer applications.

Aluminium alloys reinforced with transition metal aluminide (Al3Ti, Al3Fe, Al3Ni, etc.) particles possess high specific strength both at ambient and elevated temperature. The improved strength of these alloys are the results of slower coarsening rate of the intermetallic particles due to low diffusivity of the transition metals in aluminium. However, the strength can be enhanced further by refining the microstructure of the alloys to nanometer range. The authors have successfully attempted two important non-equilibrium processing techniques i.e. rapid solidification processing (RSP) and mechanical alloying for the refinement of the microstructure in various aluminium alloys. In this report, authors present a short review of their work on RSP of Al–Ti and Al–Fe alloys to produce nanocomposites.

a b s t r a c t Flow behaviour of a tungsten heavy alloy was studied in the strain rate-temperature range of 10−5-1/s and 298-973 K, respectively. It was observed in this study that the dislocation motion in tungsten heavy alloy was controlled by a Peierl's mechanism at low temperatures (up to 573 K). This was confirmed by the magnitude of apparent activation volume and apparent activation enthalpy as well as TEM observations. Apparent activation enthalpy in the Peierls regime, determined by several methods, was found to vary in between 22 and 37 kJ/mol. An Arrhenius type of constitutive equation was also proposed in the Peierls controlled regime for predicting flow stress as a function of temperature and strain rate. Transition tem- perature of rate controlling mechanism—from Peierl's mechanism to forest mechanism—was determined from the strain rate sensitivity and apparent activation volume estimation at several temperatures. The transition temperature was found to be about 673 K.

The effect of 10 at.% Nb on the sintering and high temperature oxidation behavior of W0.5Cr0.5alloy was investigated. Elemental powder blends were made nanostructured by high energy mechanical milling, compacted and finally sintered under reduced atmosphere at 1790°C. The sintered samples were subjected to cyclic isothermal oxidation tests at 800°C to 1200°C in air. The experimental results shows superior sinterability and oxidation protection of W0.5Cr0.5alloy compared to pure W. Characterization of the oxide scales shows porous external W-rich oxide (WO3) formation which is not ideally suitable for oxidation resistance. On the other hand, W-Cr alloy with 10 at.% Nb shows the remarkable sinter densification (~98%) and oxidation resistance up to 1200°C. The oxide scale of the ternary alloy shows formation of stable Cr2O3, Nb2O5oxides and/or complex Cr-Nb oxide (CrNbO4), and there was no evidence of WO3formation in this case.

The irradiation effects of 6 MeV electrons on the electrical properties of Al/TiO2/n-Si metal–oxide–semiconductor capacitors have been investigated. Nine Al/TiO2/n-Si capacitors were fabricated using radio frequency magnetron sputtering and divided into three groups. Groups were irradiated with 6 MeV electrons at 10, 20, and 30 kGy doses, respectively, keeping the dose rate ∼1 kGy/min. The variations in the capacitance–voltage and leakage current–voltage characteristics, in addition to the electrical parameters, such as conductance (G/ω), flat-band voltage, interface trap density and the surface charge density with electron dose were studied. The Poole–Frenkel coefficient of the MOS capacitors was determined from current–voltage characteristics. Possible mechanisms for the enhanced leakage current in the electron irradiated MOS capacitors are discussed.

In the present work the structure and morphology of the phases of nanocomposites formed in rapidly solidified Al–Fe alloys were investigated in details using analytical transmission electron microscopy and X-ray diffraction. Nanoquasicrystalline phases, amorphous phase and intermetallics like Al 5 Fe 2 , Al 13 F 4 coexisted with α-Al in nanocomposites of the melt spun alloys. It was seen that the Fe supersaturation in α-Al diminished with the increase in Fe content and wheel speed indicating the dominant role of the thermodynamic driving force in the precipitation of Fe-rich phases. Nanoquasicrystalline phases were observed for the first time in the dilute Al alloys like Al–2.5Fe and Al–5Fe as confirmed by high resolution TEM. High hardness (3.57 GPa) was measured in nanocomposite of Al–10Fe alloy, which was attributed to synergistic effect of solid solution strengthening due to high solute content (9.17 at.% Fe), dispersion strengthening by high volume fraction of nanoquasicrystalline phase; and Hall–Petch strengthening from finer cell size (20–30 nm) of α-Al matrix.

The present study attempts to investigate room temperature and high temperature flow behaviour of a tungsten heavy alloy (W-7.10Ni–1.65Fe–0.5Co–0.25Mo alloy). The alloy was deformed under compression at room temperature and elevated temperatures (400–700 °C) at different strain rates (1–0.0001 s −1 ) to observe plasticity under compression loading at these temperatures and strain rates. The alloy showed higher plasticity and positive strain rate sensitivity at room temperature. Samples hardness after 70% deformation at room temperature increased from 3.20 ± 0.14 to 5.08 ± 0.03 GPa. Barreling was observed in room temperature compression tested samples. Microstructure of the alloy after heavy compressive deformation at room temperature showed that severe deformation of W grains took place along a direction at 45° to the direction of applied stress. The alloy showed varying (positive and negative) strain rate sensitivity at elevated temperatures. Samples hardness after 70% deformation at elevated temperatures increased from 3.20 ± 0.14 to 4.60 ± 0.23 GPa. At 600 and 700 °C, the specimens failed by shear along the direction which is at an angle 45° to the direction of applied stress. Microstructural evidences indicate that the failure at these elevated temperatures seems to be triggered by the excessive void generation in the matrix and their coalescence under the influence of the applied stress. Room temperature deformation mechanism of tungsten heavy alloy was also studied by carrying out tensile testing at room temperature in strain rates ranging from 0.1 to 0.0001 s −1 . The values of strain rate sensitivity, and apparent activation volume with respect to different level of strain suggest that the deformation mechanism of the alloy is similar to that of bcc metals such as tungsten and molybdenum, i.e. Peierls mechanism controls the dislocation motion.

Two tungsten (W)–nickel (Ni)–copper (Cu) alloys (WNCs) and one W–Ni–iron (Fe) alloy (WNF) were prepared by liquid phase sintering at 1783 K and 1733 K, respectively. The average W-grain size in the sintered WNCs (60–70 μm) was coarser than that in the WNF alloy (30 μm) possibly due to the higher sintering temperature (1783 K) necessary for the former alloys. The volume of the matrix phase in the WNF (25–30 vol.%) was higher than that in WNCs (10–15 vol.%). The tensile properties and hardness of WNF specimens at room temperature were significantly superior to those of WNC specimens apparently due to the finer W-grain size, lesser contiguity and porosity in the former. WNF specimens, in contrast to WNCs, failed under tension by W-grain cleavage fracture, possibly due to relatively stronger matrix phase and W/matrix bonding. At very low strain rate (0.0001/s) the tensile curve of WNF was wavy in nature, but these were absent at higher strain rates (0.001 to 1/s). The tensile strength and elongation of WNF alloy remarkably deteriorated at higher temperatures (773 and 973 K), and the fracture changed to matrix failure mode apparently due to weakening of the matrix phase.

Nano structured carbon nitride thin films were deposited at different RF powers in the range of 50 W to 225 W and constant gas ratio of (argon: nitrogen) Ar:N2 by RF magnetron sputtering. The atomic percentage of Nitrogen: Carbon (N/C) content and impedance of the films increased from 14.36% to 22.31% and 9 × 10−1 Ω to 7 × 105 Ω respectively with increase in RF power. The hardness of the deposited films increased from 3.12 GPa to 13.12 GPa. The increase in sp3 hybridized C–N sites and decrease of grain size with increase in RF power is responsible for such variation of observed mechanical and electrical properties.

Attempts have been made to prepare highly dense bulk Al-based nanocomposite consisting of evenly distributed nano-intermetallic particles and amorphous phase by consolidating mechanically alloyed amorphous Al88Ni6Ti6 powder using spark plasma sintering (SPS), hot-pressing and pressure-less sintering techniques. SPS technique has been most effective in comparison to other two processes in getting better homogeneity of microstructural features, densification and mechanical properties due to lower sintering temperature and time, hindering excessive crystallization of amorphous phase and grain growth of formed nanocrystals. This helps in producing optimum microstructure consisting of homogeneous distribution of nanocrystalline intermetallic phase and remaining amorphous phase as revealed by detailed electron microscopy and nano-indentation tests.

Titanium nitride and titanium oxynitride films were deposited by varying the plasma current density from 10 mA/cm 2 to 40 mA/cm 2 using DC magnetron sputtering at constant gas flow rate and deposition time. Samples were characterized by Grazing Incidence X-Ray Diffraction, XPS, Nanoindentation and colorimetric analysis. Different coloured films like golden, blue, pink and green were obtained at different current densities. At lower current density (10 mA/cm 2 ), golden coloured stoichiometric titanium nitride film was formed. At higher current densities (20, 30 and 40 mA/cm 2 ), non stoichiometric Titanium oxynitride films of colour blue, pink and green were formed respectively. The thickness of the films increased with plasma current density from 43 nm to 117 nm. It was found that the colour variation was not only due to thickness of the film but also due to oxygen atoms replacing the nitrogen positions in TiN lattice. Hardness and Young Modulus of the films were found to decrease from 17.49 GPa to 7.05 GPa and 319.58 GPa–246.77 GPa respectively with increasing plasma current density. This variation of hardness and Young Modulus of the films can be speculated due to change in crystal orientation caused by oxygen incorporation in the films. The film resistivity increased from 16.46 × 10 −4 to 3.28 × 10 −1 Ω cm for increasing plasma current density caused due to oxygen incorporation in the crystal lattice.

Prediction of process parameters in a blast furnace (BF), which is a highly nonlinear, complex and continuous system, is a difficult task. Static models, which are based on first principles and tried in the past, are unable to give satisfactory results because of the dynamic nature of the process. In this paper artificial neural network (ANN) modelling of BF is explored. The concepts of ANNs are briefly discussed, and a few models reported in the literature are reviewed. The areas covered in this review are: (a) process control, viz. prediction of hot metal silicon, (b) process monitoring, viz. prediction of burden distribution, forecasting heat levels, (c) fault detection and diagnosis, viz. forecasting scaffold and scaffolding phenomena of the BF.

An investigation on the structure of an ultrasonically cast nanocomposite of Al with 2 wt.% nano-sized Al{sub 2}O{sub 3} (average size {approx}10 nm) dispersoids showed that the nanocomposite was consisting of nearly continuous nano-alumina dispersed zones (NDZs) in the vicinity of the grain boundaries encapsulating Al{sub 2}O{sub 3} depleted zones (ADZs). The mechanical properties were investigated by nanoindentation and tensile tests. The nano-sized dispersoids caused a marginal increase in the elastic modulus, and a significant increase in the hardness ({approx}92%), and tensile strength ({approx}48%). Subsequent cold rolling to achieve a reduction ratio of 2 resulted in an appreciable increase in the hardness due to change in morphology of the microstructure. Estimation of the strength on the basis of inter-particle spacing, which was measured by transmission electron microscopy, could not be accounted for on the basis of Orowan mechanism, and therefore, strengthening mechanisms like local climb and/or cross slip might have a role in this room temperature (0.32T{sub M}) deformation process.

The nanostructured phases formed in the elemental powder blends of nominal compositions Al 92 Mn 6 Ce 2 and Al 93 Mn 6 Ce 1 during mechanical alloying were investigated by X-ray diffraction and transmission electron microscopy. The difference in Ce content in the two alloys strongly influenced the types of the intermetallic phase(s) formation, including those of the Ce-free phases. Partial amorphization observed in Al 92 Mn 6 Ce 2 after prolonged (60 h) milling was not apparent in the lower Ce bearing alloy. Analysis of the X-ray diffraction data of these alloys along with those reported in the literature indicated the dominant role of the topological factors in the amorphous phase formation by mechanical alloying of the concerned system. The phases present in the mechanically alloyed Al 92 Mn 6 Ce 2 were entirely different from those reported for rapid solidification processing of the same composition [A. Inoue, Nanostruct. Mater. 6 (1995) 53–64.]. Moreover, the phases present in the mechanically alloyed powder showed inferior thermal stability, when compared to that reported for the rapidly solidified product, apparently due to the difference in the nature of structural disorders and strain induced in the products by the two non-equilibrium processing routes.

Hybrid nanocomposites of styrene butadiene rubber (SBR) with nanostructured fly-ash (NFA) were prepared in the laboratory by melt blending technique in an internal mixer. Curatives were added on a laboratory two-roll mill. Curing characteristics as well as physicomechanical properties of the composites were evaluated. A comparison on SBR composites filled with fresh fly-ash (FFA); carbon black (CB) and precipitated silica (PS) has been reported. In general, SBR-NFA composites exhibit higher state of cure and higher strength properties as compared with HAF black-filled and fresh fly-ash-filled SBR composites at equivalent loadings. This may be attributed to the higher reinforcing ability of NFA. This fact has also been supported by the swelling studies and Kraus’ plot. Tear strength and abrasion resistance of the SBR-NFA composites were superior to FFA-filled and precipitated silica-filled vulcanizates, but were inferior to carbon black- filled SBR vulcanizates. The SBR-NFA composites showed lower hardness as compared with both the carbon blackfilled and silica-filled composites. Transmission electron microscopy and scanning probe microscopy studies revealed that the NFA particles are well dispersed in the SBR matrix. These results were further supported by fracture surface analysis under the SEM, which revealed the role of NFA in the prevention of fracture propagation. POLYM. COMPOS., 30:1647–1656, 2009. a 2008 Society of Plastics Engineers

The present study concerns laser surface nitriding of Ti–6Al–4V. The microstructure of the surface nitrided Ti–6Al–4V consisted of TiN dendrites distributed in alpha Ti matrix. The microhardness of the surface was improved to 1142 VHN as compared to the 260 VHN of substrate. The bulk modulus of the surface was increased to 177 GPA as compared to the 114 GPA of the substrate. The pitting corrosion resistance and wettability were significantly improved against simulated body fluid.

Structures of Al-based nanocomposites of Al-Fe alloys prepared by mechanical alloying (MA) and subsequent annealing are compared with those obtained by rapid solidification processing (RSP). MA produced only supersaturated solid solution of Fe in Al up to 10 at.% Fe, while for higher Fe content up to 20 at.% the nonequilibrium intermetallic Al5Fe2 appeared. Subsequent annealing at 673 K resulted in more Al5Fe2 formation with very little coarsening. The equilibrium intermetallics, Al3Fe (Al13Fe4), was not observed even at this temperature. In contrast, ribbons of similar composition produced by RSP formed fine cellular or dendritic structure with nanosized dispersoids of possibly a nano-quasicrystalline phase and amorphous phase along with α-Al depending on the Fe content in the alloys. This difference in the product structure can be attributed to the difference in alloying mechanisms in MA and RSP.

Amorphous and nanocrystalline phases generated in an elemental blend of Al 94.5 Cr 3 Co 1.5 Ce 1 composition (at.%) in the course of mechanical alloying (MA) have been investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and differential scanning calorimeter (DSC). It has been reported that rapid solidification processing (RSP) of the same composition generates a finely mixed unique structure consisting of nanoscale icosahedral (I) particles surrounded by a thin Al phase [A. Inoue, Nanostruct. Mater. 6 (1995) 53–64]. In contrast, MA of Al 94.5 Cr 3 Co 1.5 Ce 1 composition carried out in the present study shows the formation of Al-based solid solutions along with partial amorphisation. It emerges that during MA a wide difference in the atomic size of the alloying elements in solid solution plays a pivotal role in the amorphous phase formation. The possible causes of the difference in microstructure generated by MA and RSP have been discussed. The thermal stability of the mechanically alloyed product has also been investigated by a differential scanning calorimeter (DSC) and XRD.

The optimum milling parameters and energy domains for the synthesis of nanocrystalline NiAl by mechanical alloying was determined. The long-range ordered structure of NiAl under various milling conditions was also evaluated. Dominant role of the total energy of the grinding balls over the impact energy of each ball in the phase formation process was observed. A total energy of 200 kJ kg-1 was found necessary to trigger the NiAl formation from an equiatomic Ni-Al blend during mechanical alloying in stainless steel as well as in tungsten carbide media. The completion of the reaction required 550 and 700 kJ kg-1 of total energy in the stainless steel and tungsten carbide media, respectively. The low-energy requirement for the NiAl synthesis in stainless steel media was attributed to the Fe-contamination during ball milling. A close relation between the enthalpy of formation of the compound and the total energy of the grinding balls for its formation was observed. � 2006 Elsevier B.V. All rights reserved.

The aim of the present work is to produce aluminum matrix nanocomposite powders by displacement reaction between aluminum and ceria and to study the effect of high-energy ball milling on the onset temperature for the reduction reaction. If the onset temperature for the reduction reaction in the milled products during subsequent thermal treatment can be brought down below the melting temperature of the constituents, the reaction product can have very fine structure. This in turn accelerates the consolidation by lowering the sintering temperatures. Two different approaches, pure mechanosynthesis and combined mechanical and thermal synthesis, have been investigated to synthesize aluminum matrix nanocomposite powders by displacement reaction between aluminum and ceria. The ingredients have been subjected to high-energy ball milling up to 100 h in a planetary ball mill with hardened chrome steel grinding media. Differential thermal analysis, X-ray diffraction and electron microscopy have been used to study onset temperature for the reduction reaction and reaction mechanism. High-energy ball milling of the powder modifies the reactivity of the system and refinement of microstructure is accompanied by nanometer range crystallites and an increase in the lattice strain. The onset temperature for the displacement reaction has been reduced from 960 to 690 °C after 5 h of milling. An increase in the milling time up to 30 h changes the reaction to a solid-state reduction reaction. It has been shown that an appropriate combination of mechanical–thermal treatment may lead to the synthesis of aluminum-based nanocomposites by the solid-state processing.

Nanocrystalline L12-Al3(Ti,Zr) intermetallic has been synthesized directly by mechanical alloying of elemental blends of the nominal composition Al75(Ti25-xZrx) for x = 0, 5, 10, 15, 20 and 25. The long range order parameter is found to increase with the Zr content. The lattice parameter difference between the L12-Al3(Ti,Zr) and Al was minimum for the alloy with 10% Zr. A very high hardness of 3.23 GPa was achieved for the L12 phase in the Al75Ti15Zr10 composition in the green compact condition with a relative density of 77.6%. � 2006 Elsevier Ltd. All rights reserved.

A simplified model was developed to determine the temperature of the powder particles at the point of collision during mechanical alloying (MA). The model showed that the temperature reached a maximum of � 473 K within 4–6 h of MA of a ternary blend of Al35Ni35Fe30 with the formation of AlNi phase. 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

An attempt was made to synthesize amorphous and/or nanocrystalline Al-based alloys from elemental powder blends with the stoichiometry Al 65 Cu 35− x Nb x ( x =5–25 at.% Nb) by high energy planetary ball milling. Microstructure of the milled product at appropriated stages of milling was characterized by X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. The Al 65 Cu 20 Nb 15 powder blend seems most amenable to solid state amorphization. This amorphous alloy was subjected to controlled heat treatment to develop a two-phase or composite microstructure of nano-aluminide dispersion in amorphous matrix. The results indicate that the present ternary system is akin to our previously reported results on Al–Cu–Ti system in developing completely/partially amorphous and/or nano-aluminide dispersed Al-rich nanocrystalline or amorphous matrix composites by controlled mechanical alloying and/or subsequent annealing.

The present investigation concerns planetary ball milling of elemental Nb in hardened steel and WC-coated media. Continued milling for about 20 h reduces the grain size below 10 nm with a concomitant lattice expansion. The final product obtained after 30–40 h milling is a pure f.c.c. phase. A detailed micro structural/compositional characterization by X-ray diffraction, resistivity measurements and wet chemical analysis proves that the said f.c.c. phase is neither NbN nor NbC. Instead, it is suggested that elemental Nb undergoes an irreversible b.c.c.→f.c.c. allotropic transformation when the grain size reduces to below 10 nm during high energy ball milling. Furthermore, this f.c.c. phase possibly undergoes an impurity stabilized ordering transformation on annealing at 900°C for 2 h. Finally, the role of impurity in causing this allotropic change is examined.

It is well known that volume diffusivity in nanocrystalline materials is orders of magnitude greater than that in coarse grained materials of identical chemistry. The present study offers a theoretical explanation of this enhancement by attempting to correlate the diffusivity with the excess free volume of the grain boundary atoms in the nanocrystalline materials. Assuming a simplified geometry of the nanocrystalline aggregate, the excess free volume of the grain boundary atoms has been expressed as a function of grain size. Applying the concept of isothermal equation of state, it is shown that the effect of negative hydrostatic pressure on the diffusion kinetics in nanocrystalline materials at a given temperature is equivalent to the same obtained for coarse grained materials at an effective elevated temperature. The predicted results have been compared with the earlier published relevant experimental data from several Cu-based systems.

The present study concerns codeposition of nanocrystalline aluminide particles (NbAl3 and Cu9Al4) along with electrodeposition of Cu on a Cu substrate. It is shown that the success of codeposition primarily depends on the selection of an appropriate electrolyte. Following codeposition under an optimum deposition condition, the microstructure, phase identity and composition of the deposit layer have been studied. In addition, microhardness and electrical resistivity of the deposit have been determined. A suitable correlation of the microstructure and composition of the deposit with its properties suggests that codeposition of NbAl3 is more effective in enhancing the hardness. However, codeposition beyond a limit adversely affects the electrical conductivity. The optimum conditions for codeposition to enhance hardness without adversely affecting conductivity have been determined. Finally, it is predicted codeposition could be a suitable technique for developing a surface composite microstructure with uniform distribution of nanocrystalline aluminide particles.

An allotropic transition from face-centered-cubic (fcc) to hexagonal-close-packed (hcp) Ni(Si) solid solution in Ni95Si5 and Ni90Si10 during nanocrystallization by mechanical alloying is reported. The transformation was identified as a defect-induced melting accompanied by a volume expansion of 8.6% and was observed when fcc Ni(Si) reached a critical crystallite size of 10 nm. Calculation based on equation of state showed that a 37% reduction in tetragonal shear modulus and a negative pressure of about 8.7 GPa were generated at the onset of transformation.

Solid state reactions induced by mechanical alloying (MA) of elemental blends of Ni and Si have been studied over the entire composition range of the Ni–Si system. A monotonous increase of the lattice parameter of the Ni rich solid solution, Ni(Si), is observed with refinement of crystallite size. Nanocrystalline phase/phase mixtures of Ni(Si), Ni(Si)+Ni31Si12, Ni31Si12+Ni2Si, Ni2Si+NiSi and NiSi+Si, have been obtained during MA, over the composition ranges of 0–10, 10–28, 28–33, 33–50, and >50 at. % Si, respectively. The results clearly suggest that only congruent melting phases, Ni31Si12, Ni2Si, and NiSi form, while the formation of noncongruent melting phases, Ni3Si, Ni3Si2, and NiSi2, is bypassed in the nanocrystalline state. The phase formation during MA has been discussed based on thermodynamic arguments. The predicted phase fields obtained from effective free energy calculations are quite consistent with those obtained during MA.

Solid state reactions induced by mechanical alloying followed by isothermal annealing in elemental blends of Ni and Si have been studied over the entire composition range of the Ni-Si system. Above a critical crystallite size, the congruent melting phases, γ-Ni 31 Si 12 and NiSi, become unstable and react with second phase available to form equilibrium non-congruent, β 1 -Ni 3 Si and α-NiSi 2 , after isothermal annealing at 723 and 1023 K, respectively. Formation of equilibrium e-Ni 3 Si 2 has not been observed in the composition range 33-50at.% Si even after annealing at 1073 K. The results also indicate that the crystallite size of non-congruent melting phases is well above 100 nm even at the stage of their evolution, suggesting that they are probably stable only in the bulk state. The grain growth is faster for the non-congruent phases when compared to that of congruent melting compounds. The interfacial energy of the compounds plays an important role in controlling their stability and grain growth in the nanocrystalline state.

Metallurgical and Materials Engineering Department, Indian Institute of Technology,Kharagpur 721 302 (W.B.), India(Received January 7, 1999)(Accepted in revised form May 21, 1999)Keywords: Mechanical alloying; Diffusion; Kinetics; X-ray diffraction; Mathematical modelingIntroductionMechanical alloying (MA) by high energy ball milling has established itself as a versatile solid stateprocessing route for producing metals, alloys and intermetallics in the nanocrystalline state [1]. In MAof ductile metals, powder particles trapped between the colliding balls and vial undergo repeateddeformation, fragmentation and cold welding resulting into agglomerates of multilayered structureswith clean interfaces [2,3]. Intermixing in such an aggregate during MA is actuated by the deformationprocess [1]. Indeed, it is well known over three decades that mechanical deformation enhances thediffusion rate and the process has been termed as mechanical interdiffusion by Balluffi and Rouff [4].Gleiter [5] has shown that a large potential gradient leads to high rate of diffusion in the vicinity of adislocation even at a temperature where self diffusion is practically negligible. In analogy to the masstransport in ion irradiation, Martin and Gaffet [6] have suggested that ballistic diffusion, which isindependent of temperature, may account for the enhanced rate of intermixing during MA. Similarly,Bellon and Averback [7] have considered shear along randomly selected glide planes as one of thepossible mechanisms of intermixing that accompanies MA during ball milling. In a recent work, Pabiet al. [8] have developed a rigorous mathematical model of MA by means of the modified iso-concentration contour migration (MICCM) method to estimate the effective intermixing rate duringMA, which is equivalent to volume diffusion at an elevated temperature (T

The alloying behaviour in a number of systems such as Cu-Ni, Cu-Zn, Cu-Al, Ni-Al, Nb-Al has been studied to understand the mechanism as well as the kinetics of alloying during mechanical alloying (MA). The results show that nanocrystallization is a prerequisite for alloying in all the systems during MA. The mechanism of alloying appears to be a strong function of the enthalpy of formation of the phase and the energy of ordering in case of intermetallic compounds. Solid solutions (Cu-Ni), intermetallic compounds with low ordering energies (such as Ni3Al which forms in a disordered state during MA) and compounds with low enthalpy of formation (Cu-Zn, Al3Nb) form by continuous diffusive mixing. Compounds with high enthalpy of formation and high ordering energies form by a new mechanism christened as discontinuous additive mixing. When the intermetallic gets disordered, its formation mechanism changes from discontinuous additive mixing to continuous diffusive one. A rigorous mathematical model, based on iso-concentration contour migration method, has been developed to predict the kinetics of diffusive intermixing in binary systems during MA. Based on the results of Cu-Ni, Cu-Zn and Cu-Al systems, an effective temperature (T eff) has been proposed that can simulate the observed alloying kinetics. TheT eff for the systems studied is found to lie between 0·42–0·52T 1.

A detailed study of the kinetics of discontinuous precipitation (DP) and type I discontinuous coarsening (DCI) in Zn-4 at % Ag alloy is reported here for the first time. DCI succeeds DP during prolonged isothermal ageing. Both DP and DCI are characterized by a predominantly lamellar morphology of the precipitate phase, statistically constant interlamellar spacing and steady state reaction front (RF) velocity at a given temperature. The interlamellar spacing increases with temperature. The RF velocity shows a C-curve behavior for DP, but increases monotonically for DCI, as a function of temperature. DCI is distinguished from DP by a 3-5 times larger interlamellar spacing and 1-2 orders of magnitude lower RF velocity than those of DP under comparable conditions. DCI may be initiated from an interface between two DP colonies, a former DP-RF, or the free surface intersecting a DP colony. Kinetic analysis of DP using the models of Turnbull, Cahn, Hillert, and Petermann and Hornbogen, and of DCI using the modified Petermann and Hornbogen model (by Fournelle) have yielded grain boundary diffusivity data in the temperature range 353-573 K. Subsequent Arrhenius analysis shows that the activation energy of the DP and DCI processes lies between 50-66 kJ mol -1 . The latter is comparable with the activation energy of grain-boundary self-diffusion of Zn and is nearly half that of tracer impurity diffusion (volume/bulk) of Ag in Zn. Hence, it is concluded that DP and DCI are grain-boundary diffusion-controlled processes in the present alloy.

The β-phase in the Cu-In system is known to decompose into α and δ below 847 K through a eutectoid transformation controlled by interphase boundary diffusion. The present study with the Cu-20.15 at.% In eutectoid alloy investigates the morphology, identity and origin of a duplex microstructure in the prior β region ahead of the eutectoid reaction front. A detailed investigation through optical, scanning and transmission electron microscopy and X-ray diffraction analysis indicates that the prior β region undergoes a time dependent pre-precipitation of a transient phase through a static boundary mode. The latter precedes/accompanies the moving boundary eutectoid transformation initiated at the prior β grain boundaries.