Structure and Properties of Nano-Scale Oxide-Dispersed Iron

Bulk samples of pure iron and yttria dispersed iron with and without titanium (i.e., Fe, Fe-Y₂O₃, and Fe-Y₂O₃-Ti) were prepared by hot extrusion of high-energy ball-milled powders. An examination of the microstructure using TEM revealed that the addition of titanium resulted in the reduction of the dispersoid size with a concomitant increase in the volume fraction of the dispersoids. As a result, Fe-Y₂O₃-Ti exhibited a substantial increase in hardness and tensile properties as compared to Fe and Fe-Y₂O₃. The higher hardness and strength of Fe-Y₂O₃-Ti is shown to be due to the presence of finer and higher number density of Y-Ti-O complex oxides. Dynamic strain aging in the temperature range of 423 K to 573 K (150 °C to 300 °C) was observed in all the compositions studied.     

Ti-Y2O3 Composites with Nanocrystalline and Microcrystalline Matrix

Mechanical milling of a Ti-2 pct Y₂O₃ powders mixture led to the synthesizing of a composite powder with a nanocrystalline Ti matrix having a mean crystallite size of 19 nm. Both the nanocomposite powder prepared through milling and the initial mixture of powders were consolidated by hot pressing under the pressure of 7.7 GPa at the temperature of 1273 K (1000 °C). The transmission electron microscopy (TEM) investigations of the bulk sample produced from milled powder revealed that Y₂O₃ equiaxial particles of less than 30 nm in size are distributed uniformly in the Ti matrix with a grain size in the wide range from 50 nm to 200 nm. The microhardness of the produced nanocrystalline material is 655 HV0.2, and it significantly exceeds the hardness of the microcrystalline material (the consolidated initial mixture of powders), which is equal to 273 HV0.2. This finding confirms that reducing the grain size to the nanometric level can have a beneficial influence on the hardness of titanium alloys. Dispersion hardening also contributes to the hardness increase.     

Microstructural Characterization of Nanosized Ceria Powders by X-Ray Diffraction Analysis

Spherically shaped nanocrystalline ceria powders were prepared by high energy ball milling (HEBM) of plate-shaped as-received ceria powders. Rietveld analysis was used to determine the surface weighted average crystallite size, lattice parameter, and lattice strain. The classical Williamson–Hall as well as modified Williamson–Hall method was used to determine the volume weighted average crystallite size. A comparison of the crystallite size obtained by the classical as well as modified Williamson–Hall method shows that the strain anisotropy consideration in the modified Williamson–Hall method yields a lower value of the crystallite size, and this difference becomes more pronounced with the increase in milling time. The modified Williamson–Hall method indicates that the dislocations present in the ceria powders are edge in character. The ceria powders also were characterized by field emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM) techniques.     

Correlation of the microstructure and mechanical properties of oxide-dispersion-strengthened coppers fabricated by internal oxidation

This study was aimed at the correlation of the microstructure and mechanical properties of oxide-dispersion-strengthened (ODS) coppers fabricated by internal oxidation. Atomized copper powders mixed with Cu₂O oxidant powders were internally oxidized and then hot extruded to fabricate ODS coppers without defects. In order to sufficiently oxidize copper powders, oxidant powders should be added in amounts 30 pct in excess of the stoichiometrically calculated amount. In the extruded ODS coppers, very fine Al₂O₃ dispersoids of 10 nm in diameter were homogeneously distributed inside copper grains of 1 μm in size. The volume fraction of Al₂O₃ dispersoids increased as the Al content in atomized copper powders increased. With increasing volume fractions of Al₂O₃ dispersoids, the yield and tensile strengths increased, while the elongation and electrical conductivity decreased, and all the properties of the ODS coppers were sufficiently above the required properties of electrode materials for spot welding. To understand the mechanism responsible for the improvement of the yield strength of the ODS coppers, yield strength was interpreted using the Orowan’s strengthening model, which was fairly consistent with the experimental results.     

Identification of dispersoid phases created in aluminum during mechanical alloying

Dispersion strengthened aluminum has been produced by mechanical alloying (high energy ball milling) and subsequent extrusion. The production process was modified in several ways in order to determine the changes of the dispersoid structure during annealing. As such, extrusion was performed cold and milling was conducted in part without lubrication. The high strength of the mechanically alloyed aluminum is due to a dispersion of both γ-Al₂O₃ and Al₄C₃-particles with sizes of the order of 10 nm. The dispersoids are amorphous after milling and crystallize during heating in the temperature range of 400 to 500 °C (673 to 773 K). The crystallization coincides with an increase of the room temperature hardness of the material. Possible techniques for improving dispersion strengthened materials have been considered and are discussed. Formerly a Visiting Scholar in the Department of Materials Science and Engineering at Stanford     

Structure and magnetic properties of nanocrystalline Ni0·64Zn0·36Fe2O4 powders prepared by ball milling

Abstract

In this study, nanocrystalline Ni_0·64Zn_0·36Fe₂O₄ powders were prepared using a planetary ball mill. The evolution of the microstructure and magnetic properties during the milling were studied by X-ray diffraction technique, scanning electron microscopy, transmission electron microscopy and vibrating sample magnetometre. It is revealed from the results of the phase analysis that nanocrystalline Ni_0·64Zn_0·36Fe₂O₄ ferrite with average crystallite size of 6·18 nm and non-uniform lattice strain of 0·33% has been formed after 60 h of milling time. A progressive increase of saturation magnetisation and a dramatic decrease in coercivity were also observed with increasing milling time.     

Development of Wear-Resistant Cu-10Cr-3Ag Electrical Contacts with Nano-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Dispersion by Mechanical Alloying and High Pressure Sintering

Cu-10Cr-3Ag (wt pct) alloy with nanocrystalline Al₂O₃ dispersion was prepared by mechanical alloying and consolidated by high pressure sintering at different temperatures. Characterization by X-ray diffraction and scanning electron microscopy or transmission electron microscopy shows the formation of nanocrystalline matrix grains of about 40 nm after 25 hours of milling with nanometric (<20 nm) Al₂O₃ particles dispersed in it. After consolidation by high pressure sintering (8 GPa at 400 °C to 800 °C), the dispersoids retain their ultrafine size and uniform distribution, while the alloyed matrix undergoes significant grain growth. The hardness and wear resistance of the pellets increase significantly with the addition of nano-Al₂O₃ particles. The electrical conductivity of the pellets without and with nano-Al₂O₃ dispersion is about 30 pct IACS (international annealing copper standard) and 25 pct IACS, respectively. Thus, mechanical alloying followed by high pressure sintering seems a potential route for developing nano-Al₂O₃ dispersed Cu-Cr-Ag alloy for heavy duty electrical contact.     

The influence of milling parameters on the characteristics of milled and spray-dried NiCoCrAlY–Al2O3 composite powders

Spray-drying process was selected to agglomerate ball milled NiCoCrAlY–Al₂O₃ composite powders. The effect of the starting alloy powder size on the morphology of composite powder was studied. The parameters of milling were optimised by orthogonal experiment to improve the powder’s flowability and apparent density. Then the optimised powder was sprayed by air plasma spray to prepare NiCoCrAlY–Al₂O₃ composite coating. The results showed that the size distribution of starting particles decided the deformation of alloy particles and the characteristics of agglomerated powders eventually. With the decreasing size range of the starting alloy particles, the sphericity of agglomerated powders increased. The optimised milling parameters were as follows: solid content, 60?wt-%; BPR, 4:1; the rotating speed, 350?rev?min^?1; and milling time, 5?h. And the contribution of solid content was the largest. The Al₂O₃ splats showed good adhesion with alloy matrix when the composite powder melted in good condition.     

Synthesis of Nanocrystalline α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> from Nanocrystalline Boehmite Derived from High Energy Ball Milling of Gibbiste

Dry milling of gibbsite has been carried out for 5 h in planetary ball mill to study the effect of mechanical activation on α-Al₂O₃ formation. Gibbsite undergoes phase transformation during milling and has resulted nanocrystalline boehmite after 5 h of milling. The average crystallite size and the BET surface area of the nanocrystalline boehmite resulted by 5 h milling of gibbsite are 8 nm and 140 m²/g, respectively. The nanocrystalline boehmite has shown reduction in the α-Al₂O₃ formation temperature as well as in the activation energy of α-Al₂O₃ formation. The average crystallite size of nanocrystalline boehmite derived α-Al₂O₃ is measured to be 100 nm by TEM analysis and the BET surface area of resulted nanocrystalline α-Al₂O₃ is 12 m²/g.     

Structural Studies of Dispersoids in Fe–15 wt% Y<Subscript>2</Subscript>O<Subscript>3</Subscript>–5 wt% Ti Model ODS Alloys During Milling and Subsequent Annealing

Oxide dispersion strengthened (ODS) steels have very high thermal stability and creep resistance due to reinforcement of hard and stable nano-sized ceramic dispersoids in metallic matrix which act as barriers to dislocation motion. This study established the role of Ti in the structural evolution of yttria during mechanical milling and subsequent annealing in a Fe–15 wt% Y₂O₃–5 wt% Ti model ODS alloy, using electron microscopy and XRD techniques. The alloy was synthesized in a high energy planetary ball mill in Ar atmosphere by varying the milling durations in the range of 0 (un-milled) to 60 h. The XRD result revealed amorphisation of Y₂O₃/Ti during milling and evolution of YTiO₃ complex oxide upon annealing at 1273 K for 1 h. The electron microscopy studies revealed the refinement of alloy powders from ~50  μm to few nanometers during milling. Electron diffraction analysis and high resolution transmission electron microscopy of 60 h milled as well as and annealed powder showed formation of different types of Y–Ti–O complex oxides such as Y₂Ti₂O₇, Y₂TiO₅ and YTiO₃.     

Effect of Milling Parameters and Cr Content on Morphology and Solubility of Nanostructured Cu–Cr Solid Solutions

In the present work, a set of Cu-based powder mixtures containing up to 6 wt% Cr has been processed through mechanical alloying for a range of milling times up to 96 h. The mixtures were processed in a ball mill with ball to powder ratio of 10:1 and the equal numbers of 1 and 2 cm balls. The processed powder mixtures were investigated by scanning electron microscope, optical microscopic equipped with image analyzer, X-ray diffraction technique and micro hardness in order to determine the particles morphology, distribution of chromium, mean crystallite size, lattice parameter and hardness after milling, respectively. Crystallite sizes were measured by Williamson–Hall method and lattice parameters were determined using an extrapolation function. Results show that the powder behavior varies with milling time, and powder composition.     

粉末冶金制备的Ti35Nb2．5Sn5HA生物复合材料的组织与性能研究

采用高能机械球磨和脉冲电流活化烧结方法制备了一种新型的不含Al、V等有毒元素的口钛合金基体的Ti35Nb2．5Sn5HA生物复合材料。研究了不同机械球磨时间球磨的Ti35Nb2．5Sn5HA粉末以及用这几种粉末烧结制备的样品微观组织和显微硬度变化,球磨时间对烧结复合材料的微观组织和性能的影响。结果表明：随着球磨时间的增加,Ti35Nb2．5Sn5HA粉末的颗粒尺寸逐渐减小,Nh和Sn开始与Ti发生固溶,形成Ti的过饱和固溶体,而且α-Ti也开始向β-Ti转化。当球磨时间达到12h,球磨粉末中α-Ti完全转化为β-Ti,粉末颗粒的平均尺寸为500nm左右。12h球磨的粉末烧结制备的复合材料具有超细晶粒尺寸,晶粒平均尺寸为200nm,这种复合材料的维氏显微硬度可以达到10187．3MPa。     

Mechanical and Thermal Properties of Pulsed Electric Current Sintered (PECS) Cu-Diamond Compacts

In this work, dispersion strengthening of copper by diamonds is explored. In particular, the influence of 50- and 250-nm diamonds at contents of 3 and 6 vol. pct on the mechanical and thermal properties of pulsed electric current sintered (PECS) Cu composites is studied. The composite powders were prepared by mechanical alloying in argon atmosphere using a high-energy vibratory ball mill. The PECS compacts prepared had high density (>97 pct of T.D.) with quite evenly distributed diamonds. The effectiveness of dispersoids in increasing the microhardness was more pronounced at a smaller particle size and larger volume fraction, explained by Hall–Petch and Orowan strengthening models. The microhardness of Cu with 6 and 3 vol. pct nanodiamonds and pure sm-Cu (submicron-sized Cu) was 1.77, 1.46, and 1.02 GPa, respectively. In annealing experiments at 623 K to 873 K (350 °C to 600 °C), the composites with 6 vol. pct dispersoids retained their hardness better than those with less dispersoids or sm-Cu. The coefficient of thermal expansion was lowered when diamonds were added, being the lowest at about 14 × 10^?6 K^?1 between 473 K and 573 K (200 °C and 300 °C). Good bonding between the copper and diamond was qualitatively demonstrated by nanoindentation. In conclusion, high-quality Cu-diamond composites can be produced by PECS with improved strength and better thermal stability than for sm-Cu.     

Fabrication of ultrafine and nanocrystalline WC–Co mixtures by planetary milling and subsequent consolidations

Abstract

In this work ultrafine and nanocrystalline WC–Co mixtures were obtained by low energy milling in planetary ball mill. The effect of the processing conditions on the reduction and distribution of the grain sizes and the internal strains level were studied. The characterisation of the powder mixtures was performed by means of scanning and transmission electron microscopy and X-ray diffraction analysis. Observations through SEM and TEM images showed a particle size below 100 nm, after milling. The X-ray diffraction profile analysis revealed a WC phase refined to a crystallite size of 19 nm.

The mixtures obtained have been consolidated and mechanical and microstructurally characterised. The results show improvements in resistant behaviour of the material consolidated from nanocrystalline powders, in spite of the grain growth experienced during the sintering. The best results were found for the material obtained by wet milling during 100 h, which presents values of hardness higher than 1800 HV.     

Effect of Process Parameters on the Characteristics of Nanocrystalline Alumina Particles Synthesized by Solution Combustion Process

Nanocrystalline ceramic oxide particles with high purity can be synthesized efficiently by the solution combustion synthesis route, which is based on redox reactions between metal salts and reducing agents such as urea and glycine. In the present study, nanocrystalline alumina powders were synthesized using aluminium nitrate nonahydrate as the metal salt and urea as the fuel. The powders were characterized primarily for the phases present and crystallite size (from X-ray diffractometry) and morphology (by scanning electron microscopy). When stoichiometric amounts of the starting chemicals were taken, X-ray diffraction (XRD) analysis of the synthesized powder revealed it to be phase-pure α-Al₂O₃, with a crystallite size of 42 nm. Electron microscopy of the synthesized powder revealed a flaky morphology. Further, the pH value of the solution containing the stoichiometric amounts of the aluminium salt and the fuel was systematically varied by dissolving in liquid ammonia. It was observed (from XRD analysis) that an increase in the pH progressively stabilized the metastable γ-Al₂O₃ phase. An increase in the fuel (urea) content had no effect on the phase stability, but decreased the crystallite size of α-Al₂O₃. A crystallite size of 29 nm could be achieved with an excess fuel ratio of 1.5 over the stoichiometric value.     

Phase transformation,structural evolution and mechanical property of nanostructured FeAl as a result of mechanical alloying

Objective of the work was to synthesize nanostructured FeAl alloy powder by mechanical alloying (MEA). The work concentrated on synthesis, characterization, structural and mechanical properties of the alloy. Nanostructured FeAl intermetallics were prepared directly by MEA in a high energy rate ball mill. Milling was performed under toluene solution to avoid contamination from the milling media and atmosphere. Mixtures of elemental Fe and Al were progressively transformed into a partially disordered solid solution with an average composition of Fe—50 at % Al. Phase transformation, structural changes, morphology, particle size measurement and chemical composition during MEA were investigated by X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Energy dispersive X-ray spectroscopy (EDS) respectively. Vickers micro hardness (VMH) indentation tests were performed on the powders. XRD and SEM studies revealed the alloying of elemental powders as well as transition to nanostructured alloy, crystallite size of 18 nm was obtained after 28 hours of milling. Expansion/contraction in lattice parameter accompanied by reduction in crystallite size occurs during transition to nanostructured alloy. Longer milling duration introduces ordering in the alloyed powders as proved by the presence of superlattice reflection. Elemental and alloyed phase coexist while hardness increased during MEA.     

Synthesis,phase study and magnetic characterisation of Co50Fe40Cu10 ternary alloy nanopowders prepared by mechanochemical alloying process

Abstract

Mechanical milling and hydrogen reduction of pure oxide mixture and magnetic characterisation of Co–Fe–Cu ternary alloy nanopowders were investigated. A powder mixture of Co₃O₄, CuO and Fe₂O₃ with Co₅₀Fe₄₀Cu₁₀ stoichiometry was first milled by a high energy planetary ball mill and then reduced in a hydrogen reduction system.

The optimum condition of the reduction under the hydrogen atmosphere was 650°C and 1 h. The X-ray diffraction patterns exhibit that the powder has ordered bcc structure with b2–bcc space group and 2·87 Å lattice parameter. Mean crystallite sizes calculated from X-ray diffraction results and mean particle size observed from electron microscopes were over 75 nm. Magnetic evaluation of ternary alloy nanopowders showed a saturation magnetisation value about 143 Am² kg^–1 and a low coercivity value of 0·93 Am^–1.     

Mechanical milling of gas-atomized Al−Ni−Mm (Mm=misch metal) alloy powders

Al−14Ni−14Mm (Mm=misch metal) alloy powders rapidly solidified by the gas atomization method were subjected to mechanical milling (MM). The microstructure, hardness, and thermal stability of the powders were investigated as a function of milling time using X-ray diffraction (XRD), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC) methods. In the early stages of milling, a cold-welded layer with a fine microstructure formed along the edge of the milled powder (zone A). The interior of the powder remained unworked (zone B), resulting in a two-zone microstructure, reminiscent of the microstructures in rapidly solidified ribbons containing zones A and B. With increasing milling time, the crystallite size decreased gradually reaching a size of about 10 to 15 nm and the lattice strain increased reaching a maximum value of about 0.7 pct for a milling time of 200 hours. The microhardness of the mechanically milled powder was 132 kg/mm² after milling for 72 hours and it increased to 290 kg/mm² after milling for 200 hours. This increase in microhardness is attributed to a significant refinement of mcirostructure, presence of lattice strain, and presence of a mixture of phases in the alloy. Details of the microstructural development as a function of milling time and its effect on the microhardness of the alloy are discussed.     

Mechanical milling of gas-atomized Al-Ni-Mm (Mm = misch metal) alloy powders

Al-14Ni-14Mm (Mm = misch metal) alloy powders rapidly solidified by the gas atomization method were subjected to mechanical milling (MM). The microstructure, hardness, and thermal stability of the powders were investigated as a function of milling time using X-ray diffraction (XRD), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC) methods. In the early stages of milling, a cold-welded layer with a fine microstructure formed along the edge of the milled powder (zone A). The interior of the powder remained unworked (zone B), resulting in a two-zone microstructure, reminiscent of the microstructures in rapidly solidified ribbons containing zones A and B. With increasing milling time, the crystallite size decreased gradually reaching a size of about 10 to 15 nm and the lattice strain increased reaching a maximum value of about 0.7 pct for a milling time of 200 hours. The microhardness of the mechanically milled powder was 132 kg/mm² after milling for 72 hours and it increased to 290 kg/mm² after milling for 200 hours. This increase in microhardness is attributed to a significant refinement of microstructure, presence of lattice strain, and presence of a mixture of phases in the alloy. Details of the microstructural development as a function of milling time and its effect on the microhardness of the alloy are discussed.     

Phase Evolution and Mechanical Properties of Nano-TiO2 Dispersed Zr-Based Alloys by Mechanical Alloying and Conventional Sintering

The present study deals with the synthesis of 1.0 to 2.0 wt pct nano-TiO₂ dispersed Zr-based alloy with nominal compositions 45.0Zr-30.0Fe-20.0Ni-5.0Mo (alloy A), 44.0Zr-30.0 Fe-20.0Ni-5.0Mo-1.0TiO₂ (alloy B), 44.0Zr-30.0Fe-20.0Ni-4.5Mo-1.5TiO₂ (alloy C), and 44.0Zr-30.0Fe-20.0Ni-4.0Mo-2.0TiO₂ (alloy D) by mechanical alloying and consolidation of the milled powders using 1 GPa uniaxial pressure for 5 minutes and conventional sintering at 1673 K (1400 °C). The microstructural and phase evolution during each stage of milling and the consolidated products were studied by X-ray diffraction (XRD), scanning electron microscopy and transmission electron microscopy (TEM), and energy-dispersive spectroscopy. The particle size of the milled powder was also analyzed at systemic intervals during milling, and it showed a rapid decrease in particle size in the initial hours of milling. XRD analysis showed a fine crystallite size of 10 to 20 nm after 20 hours of milling and was confirmed by TEM. The recrystallization behavior of the milled powder was studied by differential scanning calorimetry. The hardness of the sintered Zr-based alloys was recorded in the range of 5.1 to 7.0 GPa, which is much higher than that of similar alloys, developed via the melting casting route.