Microstructure and mechanical properties of mechanically alloyed and spark plasma sintered amorphous–nanocrystalline Al65Cu20Ti15 intermetallic matrix composite reinforced with TiO2 nanoparticles

Multiphase Al₆₅Cu₂₀Ti₁₅ intermetallic alloy matrix composite, dispersed with 10 wt.% of TiO₂ nanoparticles, has been processed by mechanical alloying, followed by spark plasma sintering under pressure in the temperature range of 623–873 K. Differential scanning calorimetry and X-ray diffraction suggest that equilibrium crystalline phases evolve from the amorphous or intermediate crystalline phases. Transmission electron microscopy shows that the composite sintered at 873 K has partially amorphous microstructure, with dispersion of equilibrium, crystalline, intermetallic precipitates of Al₅CuTi₂, Al₃Ti, and Al₂Cu of 25–50 nm size, besides the TiO₂. The composite sintered at 873 K exhibits little porosity, hardness of 5.6 GPa, indentation fracture toughness in the range of 3.1–4.2 MPa√m, and compressive strength of 1.1 GPa. Indentation crack deflection by TiO₂ particle aggregates causes increase in fracture resistance with crack length, and suggests R-curve type behaviour. The study provides guidelines for processing high strength amorphous–nanocrystalline intermetallic composites based on the Al–Cu–Ti ternary system.     

Properties of copper matrix reinforced with various size and amount of Al2O3 particles

Copper matrix was reinforced with Al₂O₃ particles of different size and amount by internal oxidation and mechanical alloying accomplished using high-energy ball milling in air. The inert gas-atomised prealloyed copper powder containing 1 wt.% Al as well as a mixture of electrolytic copper powder and 3 wt.% commercial Al₂O₃ powder served as starting materials. Milling of Cu-1 wt.% Al prealloyed powder promoted formation of fine dispersed particles (1.9 wt.% Al₂O₃, approximately 100 nm in size) by internal oxidation. During milling of Cu-3 wt.% Al₂O₃ powder mixture the uniform distribution of commercial Al₂O₃ particles has been obtained. Following milling, powders were treated in hydrogen at 400 °C for 1 h in order to eliminate copper oxides formed at the surface during milling. Compaction was executed by hot-pressing. Compacts processed from 5 to 20 h-milled powders were additionally subjected to high-temperature exposure at 800 °C in order to examine their thermal stability and electrical conductivity. Compacts of Cu-1 wt.% Al prealloyed powders with finer Al₂O₃ particles and smaller grain size exhibited higher microhardness than compacts of Cu-3 wt.% Al₂O₃ powder mixture. This indicates that nano-sized Al₂O₃ particles act as a stronger reinforcing parameter of the copper matrix than micro-sized commercial Al₂O₃ particles. Improved thermal stability of Cu-1 wt.% Al compacts compared to Cu-3 wt.% Al₂O₃ compacts implies that nano-sized Al₂O₃ particles act more efficiently as barriers obstructing grain growth than micro-sized particles. Contrary, the lower electrical conductivity of Cu-1 wt.% Al compacts is the result of higher electron scatter caused by nano-sized Al₂O₃ particles.     

Properties of ZrO2–Al2O3 composite as a function of isothermal holding time

Zirconia and alumina based ceramics present interesting properties for their application as implants, such as biocompatibility, good fracture resistance, as well as high fracture toughness and hardness. In this work the influence of sintering time on the properties of a ZrO₂–Al₂O₃ composite material, containing 20 wt% of Al₂O₃, has been investigated. The ceramic composites were obtained by sintering, in air, at 1600 °C for sintering times between 0 and 1440 min. Sintered samples were characterized by microstructure and crystalline phases, as well as by mechanical properties. The grain growth exponents, n, for the ZrO₂ and Al₂O₃ were 2.8 and 4.1, respectively, indicating that different mechanisms are responsible for grain growth of each phase. After sintering at 1600 °C, the material exhibited a dependency of hardness as function of sintering time, with hardness values between 1500 HV (120 min) and 1310 HV (1440 min) and a fracture toughness of 8 MPa m^1/2, which makes it suitable for bioapplications, such as dental implants.     

Friction characteristic of micro-arc oxidative Al2O3 coatings sliding against Si3N4 balls in various environments

The alumina ceramic coatings were prepared on 2024Al alloy by micro-arc oxidation (MAO) technique. The phase structure of the MAO Al₂O₃ coating was determined using X-ray diffraction. The thickness and micro-hardness of the MAO Al₂O₃ coatings was measured using eddy current thickness equipment and micro-hardness tester. The friction property of MAO Al₂O₃ coatings sliding against Si₃N₄ ceramic balls were investigated in air, water and oil by a ball-on-disk tribo-meter, and the worn surfaces of the MAO Al₂O₃ coatings were observed using scanning electron microscope (SEM). The results showed that the MAO Al₂O₃ coatings mainly contained -Al₂O₃ and γ-Al₂O₃ phase. The micro-hardness of the polished MAO coatings was HV1740 ± 87. With an increase in normal load and sliding speed, the friction coefficient in air increased from 0.74 to 0.87, while decreased from 0.72 to 0.57 in water and 0.24 to 0.11 in oil. This indicates that the fluid lubrication could improve the friction behavior of the MAO Al₂O₃ coatings. The worn surfaces' observation indicated that the wear mechanism of the MAO Al₂O₃ coatings changed from abrasive wear in air to mix wear in water, and became microploughing wear in oil.     

Formation and chemical leaching effects of nonequilibrium Al0.6(Fe25Cu75) alloy powder produced by rod milling

The formation and chemical leaching effects of a nonequilibrium Al_0.6(Fe₂₅Cu₇₅)_0.4 powder produced by rod milling is described. X-ray diffraction, transmission electron microscopy, differential scanning calorimetry and vibrating sample magnetometry were used to characterize both the as-milled and leached specimens. After 400 h of milling, only the bcc AlFe phase with an amorphous phase was detected in the XRD patterns. The crystallite size for the bcc AlFe phase (110) after 400 h of milling was about 5.3 nm. The peak temperature and the crystallization temperature of the as-milled powders were 448.7 and 428.0 °C, respectively. Al atoms leaching from the as-milled bcc AlFe powders in the L1 condition did not alter the diffraction pattern significantly, even though Al atoms had been removed. After the L1 specimen was annealed at 500 °C for 1 h, the bcc AlFe phase transformed to the fcc Cu, Fe, and CuFe₂O₄ phases. The peak widths of L1 and L2 specimens were similar, but became broader than that of the as-milled powder. The saturation magnetization decreased with increasing milling time, and a value of 10.4 emu/g was reached after 400 h of milling. After cooling the specimen from 750 °C, the magnetization slowly increased at approximately 491.4 °C, indicating that the bcc AlFe phase had transformed to the fcc Cu and Fe phases.     

Production of niobium carbide ceramic composites derived from polymer/filler mixtures: preliminary results

Studies have shown that Al₂O₃–NbC composites present a good potential to be used for metalworking. Manufacturing of composite ceramic material derived from polymer reactive filler mixtures were investigated. The present study reports the preliminary results of reaction bonded niobium carbide derived from polymer (polysiloxane), inert filler (Al₂O₃) and reactive filler (Nb). Niobium powder, alumina and polysiloxane mixtures were homogenized in a planetary ball milling and pressureless sintered in inert atmosphere at temperatures up to 1600 °C. Depending on the niobium content and pyrolysis conditions, ceramic materials with a porosity of 20–40%, a weight loss of 5–15%, a linear shrinkage of 2–4% and a flexural strength of maximum 80 MPa were obtained. X-ray diffraction (XRD) of a sample containing 60 wt% polymer + 40 wt% Nb showed the presence of new crystalline phases such as NbC, Nb₃Si and Nb₅Si₃.     

Direct synthesis of La9.33Si6O26 ultrafine powder via sol–gel self-combustion method

Single phase La_9.33Si₆O₂₆ ultrafine powder, as a kind of highly activated precursor to prepare medium-to-low temperature electrolyte for solid oxide fuel cells (SOFCs), has been successfully synthesized via a non-aqueous sol–gel and self-combustion approach from the starting materials: lanthanum nitrate (La(NO₃)₃·6H₂O), citric acid, ethylene glycol (EG), tetraethyl orthosilicate (TEOS) and ammonium nitrate. The details of gel's self-combustion were investigated by DTA–TG and the structural characterization of as-synthesized powder from self-combustion was performed by XRD and SEM. The results show that La_9.33Si₆O₂₆ single phase of apatite-type crystal structure can be directly synthesized by sol–gel self-combustion method without further calcinations on the condition that the molar ratio (R) of NO₃⁻ to citric acid and ethylene glycol being 6:1. Such powders composed of well-dispersed particles with an average size of 200 nm and a specific surface area of 5.54 m²/g. It can be sintered to 90% of its theoretical density at 1500 °C for 10 h, about 200 °C lower than the sintering temperature for the powder derived from traditional solid reactions. The sintered material has a thermal expansion coefficient of 9.2 × 10⁻⁶ K⁻¹ between room temperature and 800 °C.     

Preparation and luminescent properties of europium-activated YInGe2O7 phosphors

Effects of precursor milling on phase evolution and morphology of mullite (3Al₂O₃·2SiO₂) processed by solid-state reaction have been investigated. Alumina and silica powders were used as starting materials and milling was taken place in a medium energy conventional ball mill and a high-energy planetary ball mill. Milling in a conventional ball mill although decreases mullite formation temperature by 200 °C, but does not considerably change mullite phase morphology. Use of a planetary ball mill after 40 h of milling showed to be much more effective in activating the oxide precursors, and mullitization temperature was reduced to below 900 °C. Whisker like mullite was formed after sintering at 1450 °C for 2 h and volume fraction of this structure was increased by increasing the milling time. XRD results showed that samples mechanically activated for 20 h in the planetary ball mill were fully transformed to mullite after sintering at 1450 °C, whereas Al₂O₃ and SiO₂ phases were still detected in the samples milled in the conventional ball mill for 20 h and then sintered at the same conditions.     

Three-point bending behavior of surface composite Al2O3/Ni on bronze substrate produced by vacuum infiltration casting

Al₂O₃/Ni surface infiltrated composite layer is a protective surface layer. It was fabricated on bronze substrate through vacuum infiltration casting technique using Ni-based powder and Al₂O₃ powder with different content as raw materials. With an appropriate choice of processing condition, a compact infiltrated layer is achievable as conformed through SEM observation. The infiltrated layer consists of surface composite layer and transition layer, and the thickness of transition layer decreases with increasing content of Al₂O₃. Metallurgical fusion formed at the interface between the surface infiltrated composite layer and substrate. Three-point bending tests were carried to investigate the mechanical and bonding properties of the surface infiltrated layer. It was found that load-holding circumstance appeared on the load–displacement curve of specimen with surface infiltrated layer comparing with that of the substrate. The peak load reduces with increasing content of Al₂O₃. The fracture extended to the substrate for specimen with Al₂O₃ content less than 20 (wt%). The fracture direction of specimen more than 30% with Al₂O₃ (wt%) is along with the interface of the surface composite and the substrate because of the thinner transition layer.     

Toughened ZrO2 ceramics sintered with a La2O3-rich glass as additive

In this study, the influence of the glass addition and sintering parameters on the densification and mechanical properties of tetragonal zirconia polycrystals (3Y-TZP) ceramics were evaluated. High-purity tetragonal ZrO₂ powder and La₂O₃-rich glass were used as starting powders. Two compositions based on ZrO₂ and containing 5 wt.% and 10 wt.% of La₂O₃-rich glass were studied in this work. The starting powders were mixed/milled by planetary milling, dried at 90 °C for 24 h, sieved through a 60 mesh screen and uniaxially cold pressed under 80 MPa. The samples were sintered in air at 1200 °C, 1300 °C, 1400 °C for 60 min and at 1450 °C for 120 min, with heating and cooling rates of 10 °C/min. Sintered samples were characterized by relative density, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Hardness and fracture toughness were obtained by Vickers indentation method. Dense sintered samples were obtained for all conditions. Furthermore, only tetragonal-ZrO₂ was identified as crystalline phase in sintered samples, independently of the conditions studied. Samples sintered at 1300 °C for 60 min presented the optimal mechanical properties with hardness and fracture toughness values near to 12 GPa and 8.5 MPa m^1/2, respectively.     

Oxidation behavior of the (Cu78Y22)98Al2 bulk metallic glass containing Cu5Y-particle composite at 400–600 °C

The oxidation behavior of the (Cu₇₈Y₂₂)₉₈Al₂ bulk metallic glass containing 55% Cu₅Y particles (CYA-composite) was studied over the temperature range of 400–600 °C in dry air. The results generally showed that the oxidation kinetics of the composite obeyed a two-stage parabolic-rate law, with its steady-state parabolic-rate constants (k_p values) increased with temperature. In addition, the oxidation rates of the composite were significantly lower than those of the polycrystalline Cu–20%Y alloy. The scales formed on the composite consisted mostly of hexagonal-Y₂O₃ (h-Y₂O₃) and minor CuO, while significant amounts of Cu₂O and CuO, with minor amounts of Y₂O₃ were detected for the Cu–20%Y alloy. It was found that the absence of Cu₂O is responsible for the slower oxidation rates of CYA-composite.     

Synthesis and characterization of molybdenum aluminide nanoparticles reinforced aluminium matrix composites

Aluminium matrix composites reinforced with molybdenum aluminide nanoparticles were synthesized by ball milling and reactive sintering of the mixture of aluminium and 10 wt% hydrated molybdenum oxide powders. Sintering the as milled powder in air below 750 °C produced MoAl₁₂ intermetallic compound nanoparticles, at 750 °C produced a mixture of MoAl₅ and MoAl₄ nanoparticles and at 800 °C under Argon atmosphere produced predominantly MoAl₄ intermetallic nano-particles in the Al matrix. The powder compacts sintered in air below 750 °C produced MoAl₁₂ whereas at 750 °C or above formed the Al matrix composite reinforced with the MoAl₅ nanoparticles. These nanoparticles become agglomerated to take up some irregular shaped flakes in the metal matrix. The reaction between Al and hydrated Mo oxide powders was found to be a favorable way to produce predominantly a particular Mo–Al intermetallic compound at a particular temperature. The Al₂O₃ particles formed as another reaction product, in all the above reactions, remain distributed in these composites. The composites thus formed were characterized by SEM-EDS, DTA, XRD and TEM analysis.     

Microstructural evolution of some MgO–TiO2 and MgO–Al2O3 powder mixtures during high-energy ball milling and post-annealing studied by X-ray diffraction

High-energy dry ball-mill and post-anneal processing were applied to synthesize MgTiO₃ and Mg₂TiO₄ single crystalline phases from the predetermined compositions of MgO–TiO₂ powder mixtures. Also, the experiments were performed to show that it is possible to prepare MgAl₂O₄ single crystalline phase from the predetermined composition of MgO–Al₂O₃ powder mixture only by employing high-energy dry ball milling, i.e. without post-annealing the milled samples. In contrast, fully developed single crystalline powders of MgTiO₃ and Mg₂TiO₄ were obtained after post-annealing the milled samples for 1 h at 900 and 1200 °C, respectively.     

Development of moderate temperature CVD Al2O3 coatings

Chemically vapor deposited Al₂O₃ coatings, due to their high hardness and chemical inertness, are currently the state of art in the cutting tool industry. The conventional high deposition temperature of about 1050 °C for Al₂O₃ coatings, based on the water–gas shift process, has to a great extend restricted the development of several hybrid coatings, such as TiC/TiN/TiCN/Al₂O₃. To overcome this limitation, alternate systems to deposit Al₂O₃ at moderate temperatures have been investigated. Systems using NO–H₂, H₂O₂, NO₂–H₂ and HCOOH were identified and thermodynamic calculations were performed to evaluate them as potential sources of oxygen donors to form Al₂O₃ in the moderate temperature range of 700–950 °C. Preliminary results have clearly demonstrated that it is possible to grow moderate temperature alumina (using such alternate sources) on the TiC/TiN coated cemented carbide substrates.     

Synthesis of TiC–Al2O3 nanocomposite powder from impure Ti chips, Al and carbon black by mechanical alloying

The possibility of providing TiC–Al₂O₃ nanocomposite as a useful composite from low-cost raw materials has been investigated. Impure Ti chips were placed in a high energy ball mill with carbon black and aluminum powder and sampled after different times. XRD analysis showed that TiC has been synthesized after 10 h of milling. It could be observed from the width of XRD patterns’ peaks that the size of produced TiC crystallites is in the order of nanometer. In order to forming of TiC–Al₂O₃ composite, heat treatment was performed in different temperatures. Investigations have revealed that formation temperature of TiC as the dominant phase decreased for the milled specimens during heat treatment, also nanocrystalline TiC–Al₂O₃ composite was formed in this situation. Furthermore milling led to increase of strain and decrease of TiC lattice parameter while during heat treatment nanocrystalline grains grow up and strain decreases.     

Formation of Al–Y oxide during processing of γ-TiAl

While processing Y₂O₃ dispersed γ-TiAl, Y₂O₃ particles which dissolved during hot isostatic pressing (HIP’ing) were found to precipitate during the heat treatment in the form of a mixed Al–Y oxide. To understand the chemical reaction that occurs between Y₂O₃ and γ-TiAl during the heat treatment cycle, a powder mixture comprising of γ-TiAl and 10 wt.% Y₂O₃ was mechanically alloyed (MA’d) for 8 h and the milled powder was subjected to differential thermal analysis (DTA) at 1150 °C prior to analyzing it using X-ray diffraction technique. The present study clearly demonstrates that aluminum in the combined form either as γ-TiAl or Al₂O₃ reacts in a similar manner with Y₂O₃ when milled and heat treated at 1150 °C. In either case there is formation of Al₂Y₄O₉ (2Y₂O₃.Al₂O₃).     

Oxidation behavior of ceramic coatings on Ti–6Al–4V by micro-plasma oxidation

Compound ceramic coatings prepared on Ti–6Al–4V alloy by pulsed bi-polar micro-plasma oxidation (MPO) in NaAlO₂ solution were oxidized under different temperature in air. The phase composition and surface morphology of the coatings before and after oxidation were investigated by X-ray diffractometry and scanning electron microscopy, respectively. Meantime, the weight gains and the high temperature oxidation characteristics of the coated samples were investigated. The results show that the coatings prepared by MPO were composed of a large amount of Al₂TiO₅ and a little -Al₂O₃ and rutile TiO₂. And the oxidation process of the coated samples included the decomposition of the Al₂TiO₅ in the coating, the oxidation of the substrate and the changes of the coating structure. After high temperature oxidation, the increase of -Al₂O₃ in the coating was due to the decomposition of Al₂TiO₅, whereas the increase of rutile TiO₂ in the coating was attributable to both the decomposition of Al₂TiO₅ and the oxidation of the Ti substrate. The main crystalline of the coatings became rutile TiO₂ after the oxidation of 1000 °C for 1 h. The decomposition of Al₂TiO₅ in the coating occurred at 900 and 1000 °C, and its half decomposition time was less than 1 h at 1000 °C. Increasing oxidation temperature or extending oxidation time, the weight gains of coated samples was increased to different extent. However, the weigh gains of the coated samples was much lower than that of the substrate, so the ceramic coatings improved the oxidation resistance of Ti alloy greatly under the experimental conditions.     

Microstructure and magnetic properties of Co/Al2O3 nanocomposite powders

Nanocomposite powders of magnetic cobalt nanoparticles dispersed by nonmagnetic Al₂O₃ particles have been prepared by planetary ball milling. Ball milling of the CoO and Al mixture powder after a certain milling duration reduces CoO to (fcc and hcp) Co completely and oxidizes Al to -Al₂O₃ simultaneously. The average grain sizes of the nanocomposite powders are 19 nm for Co and 28 nm for -Al₂O₃ after the completion of the reduction reaction. By direct ball milling of the mixture of Co and Al₂O₃, the allotropic phase transformation of Co was observed and the average grain size of Co is reduced to 5 nm. For both the samples of the mechanochemical series and the direct milling series, the saturation magnetizations of the nanocomposite powders decrease with decreasing average grain size of Co. This may be due to the enhancement of the interface effects and the increase of the superparamagnetic particles with decreasing Co grain size. The coercivities of the Co/Al₂O₃ nanocomposite powders increase up to 380 Oe. The increasing grain boundaries with decreasing Co grain size result in the domain wall pinning which predicts the coercivity enhancement. In addition to the grain size effects, the reduction of the particle size toward the size region of single domain also contributes to the increase of coercivity.     

Porous AlN ceramic substrates by reaction sintering

A novel approach was undertaken in producing porous AlN microelectronics tapes with high thermal conductivity and low dielectric constant. This method essentially utilised polymer micro-spherical powders that were used as a sacrificial mould to introduce controlled porosity into the green tapes during pyrolysis. The Al₂O₃-rich porous green tapes were then reaction sintered at 1680 °C for 12 h to achieve porous AlN tapes. This work builds upon the previously developed novel reaction sintering process that densified and converted Al₂O₃-rich tapes (Al₂O₃–20 wt.% AlN–5 wt.% Y₂O₃) to AlN tapes at a relatively low sintering temperature of 1680 °C. The sintering behaviour of the porous tapes was investigated, and the effects of the microspheres particle size and volume addition were studied. The microspheres successfully contributed to the significant reduction of tape density by porosity, and this contributed to lowering its dielectric constant. Dielectric constant of the AlN tapes were reduced to about 6.8–7.7 whilst thermal conductivity values were reasonable at about 46–60 W/m K. Coefficient of thermal expansion (CTE) values showed a linear trend according to phase composition, with the porous AlN tapes exhibiting CTE values of (4.4–4.8)×10⁻⁶ °C⁻¹, showing good CTE compatibility with silicon, at 4.0×10⁻⁶ °C⁻¹. The added porosity did not significantly affect the CTE values.     

Mechanochemical reduction of MoO3/SiO2 powder mixtures by Al and carbon for the synthesis of nanocrystalline MoSi2

In this investigation, MoSi₂ intermetallic compound has been synthesized by reducing of MoO₃/SiO₂ powder mixtures by Al and carbon via mechanical alloying (MA). Powder mixtures were ball milled for 0–100 h and structural evolutions have been monitored by X-ray diffraction. In the Al system, both β-MoSi₂ (high temperature phase) and -MoSi₂ (low temperature phase) were obtained after 3 h of milling and after 70 h of milling the β-phase transformed to -phase. The crystallite size of -MoSi₂ and Al₂O₃ after milling for 100 h was 12 and 17 nm, respectively. In reducing with carbon, two different compositions with nominal carbon content of 13.7 and 24 wt.% were used that in both compositions, -MoSi₂ forms during 10 h of milling. Higher carbon content increases the amount of MoSi₂.