Fabrication and characterization of nanostructured Ti6Al4V powder from machining scraps

In recent years researches on properties of nanocrystalline materials in comparison with coarse-grained materials has attracted a great deal of attention. The present investigation has been based on production of nanocrystalline Ti6Al4V powder by means of high energy mechanical milling. In this regard, Ti6Al4V powder was produced by ball milling of machining scraps of Ti6Al4V. The structural and morphological changes of powders were investigated by X-ray diffraction, scanning electron microscopy, and microhardness measurements. The results revealed that ball milling process reduced the size of the coherent-scattering region of Ti6Al4V to approximately 20 nm. Also a remarkable change in morphology and particle size was occurred during ball milling. Moreover, phase evolution during milling was taken into consideration. The as-milled Ti6Al4V powder exhibited higher microhardness comparing to the original samples.     

In-situ fabrication of Al3V/Al2O3 nanocomposite through mechanochemical synthesis and evaluation of its mechanism

In this research, in situ fabrication of Al₃V based nanocomposite and its formation mechanism have been investigated. In order to synthesize Al₃V/Al₂O₃ nanocomposite, a mixture of Al and V₂O₅ powders was subjected to high-energy ball milling and the nanocomposite was produced through a mechanochemical reaction. The produced structure was isothermally heat-treated at 500–600 °C for 0.5–2 h under argon atmosphere. In order to evaluate the structural changes during milling and annealing, the synthesized powders were characterized by X-ray diffraction (XRD). Moreover, the powder morphological changes were studied by scanning electron microscopy (SEM). It was observed that the reaction between Al and V₂O₅ occurred after about 30 min and, the Al₃V and Al₂O₃ were formed in nanocrystalline structure with the continuing mechanical milling. Calculation of adiabatic temperature confirmed that reaction took place in combustion mode. In final stage of milling up to 40 h; it was observed that the Al₃V decomposed to Al and V so that the optimum time of milling to achieve fabrication of nanocomposite was determined to be about 20 h. Calculations based on Miedema’s model verified partial disordering of Al₃V during further milling and annealing of as-milled powder at 600 °C led to the ordering of Al₃V. The crystallite size of Al₃V and Al₂O₃ after annealing at 600 °C for 2 h remained in nanometer scale. So the final product appeared to be stable even after annealing.     

Effect of heat treatment on the microstructure and property of cold-sprayed nanostructured FeAl/Al2O3 intermetallic composite coating

It is difficult to deposit dense intermetallic compound coatings by cold spraying directly using compound feedstock powders due to their intrinsic low temperature brittleness. A method to prepare intermetallic compound coatings in-situ employing cold spraying was developed using a metastable alloy powder assisted with post heat treatment. In this study, a nanostructured Fe(Al)/Al₂O₃ composite alloy coating was prepared by cold spraying of ball-milled powder. The cold-sprayed Fe(Al)/Al₂O₃ composite alloy coating was evolved in-situ to FeAl/Al₂O₃ intermetallic composite coating through a post heat treatment. The effect of heat treatment on the phase formation, microstructure and microhardness of cold-sprayed Fe(Al)/Al₂O₃ composite coating was investigated. The results showed that annealing at a temperature of 600 °C results in the complete transformation of the Fe(Al) solid solution to a FeAl intermetallic compound. Annealing temperature significantly influenced the microstructure and microhardness of the cold-sprayed FeAl/Al₂O₃ coating. On raising the temperature to over 950 °C, diffusion occurred not only in the coating but also at the interface between the coating and substrate. The microhardness of the FeAl/Al₂O₃ coating was maintained at about 600HV_0.1 at an annealing temperature below 500 °C, and gradually decreased to 400HV_0.1 at 1100 °C.     

In situ Al3Ti and Al2O3 nanoparticles reinforced Al composites produced by friction stir processing in an Al-TiO2 system

Al and TiO₂ powders were selected to fabricate in situ Al composites via multiple pass friction stir processing (FSP) based on the thermodynamic analysis. The microstructural investigations indicated FSP would induce reaction between Al and TiO₂. Al₃Ti and Al₂O₃ particles were formed after 4 pass FSP with 100% overlapping. The in situ particles were about 80 nm in size at various FSP conditions, and ultrafine matrix grains 602 nm in size were obtained when water cooling was applied during FSP. Tensile tests indicated that the in situ nanocomposites exhibited pronounced work hardening behavior and a good combination of strength and ductility.     

Ultrafast synthesis of the nanostructured Al59Cu25.5Fe12.5B3 quasicrystalline and crystalline phases by high-energy ball milling: Microhardness,electrical resistivity,and solar cell absorptance studies

In this study, the Al₅₉Cu_25.5Fe_12.5B₃ nanoquasicrystalline alloy and related crystalline phases were synthesized through mechanical alloying using a high-energy ball milling and consolidated by a cold isostatic pressing apparatus. This paper focuses on the synthesis, structural and microstructural evolutions, thermal stability, microhardness, and electrical and optical properties of the Al₅₉Cu_25.5Fe_12.5B₃ nanoquasicrystalline alloy for solar selective absorber usages. The structural evolutions of the mechanically alloyed and heat-treated AlCuFeB powders were investigated by X-ray diffractometry. Accordingly, the effect of milling time and heat treatment on the formation of quasicrystalline and related crystalline phases were studied in the AlCuFeB alloy system. The microstructure, morphology, and chemical microanalysis of the un-milled and as-milled powders were examined by field-emission scanning electron microscopy and energy-dispersive X-ray spectroscopy. The composition of the as-milled AlCuFeB powders was estimated employing inductively coupled plasma-atomic emission spectroscopy. The thermal stability of the AlCuFeB powders was recorded by differential thermal analysis, and the weight gain of the particles during annealing was investigated through thermogravimetric analysis. The nanostructured Al₅₉Cu_25.5Fe_12.5B₃ stable quasicrystalline phase and crystalline Al(Cu,Fe) solid-solution were synthesized by the ultrafast milling procedure in 1 h. The rationale behind using the term ultrafast synthesis is to synthesize the QC i-phase only by the high-energy ball milling procedure in short-term ball milling without subsequent annealing treatment. However, the single quasicrystalline phase could not be obtained even after the annealing treatment. The quasicrystalline size was calculated by the Williamson–Hall method and optimized by the Rietveld refinement procedure, and it was found that the size is varied between 53 and 61 nm. Furthermore, the particle size distribution of the as-milled AlCuFeB powders was measured using laser static light scattering, which ranges from 0.1 to 50 μm. The microhardness of the consolidated as-milled and heat-treated samples was estimated utilizing the Vickers microhardness indenter. At the same time, their electrical resistivity was assessed by the four-point probe method at room temperature. The spectral analyses of absorption on the consolidated as-milled samples were carried out in the ultraviolet, visible, and near-infrared regions. It was found that the presence of the quasicrystalline phase in the AlCuFeB alloy prominently improves the microhardness, electrical resistivity, and particularly sunlight absorptance.     

A modified synthesis method of manganese perovskite La0.67Ca0.33MnO3 by high energy ball milling and post heat treatment

A modified synthesis method of La_0.67Ca_0.33MnO₃ by high energy ball milling and post sintering is reported. The characteristics of samples from this modified method are also studied. The new synthesis method has some advantages including reducing the synthesis procedure into two simple steps, shortening the sintering time, and obtaining similar properties compared with conventional synthesis method. High energy ball milling was employed for 10 h to refine the starting materials into a sub-micron particle size (for comparison, samples milled for 30 and 50 h were also synthesized); during the process, ethanol was added as a milling medium for avoiding amorphization or crystallization, as well as suspending the starting powders for more profound milling. Finally, the as-milled powder was sintered in air to crystallize. The samples show metal-to-insulator transitions at around 260 K, with grain size from 200 nm to 8 μm, and exhibit large magnetoresistance up to 21% at a magnetic field of 3000 Oe near the transition temperature. The possible mechanism is discussed.     

Diffusion bonding of Al2O3-TiC composite ceramic and W18Cr4V high speed steel in vacuum

Al₂O₃-TiC composite ceramic and W18Cr4V high speed steel were joined by diffusion bonding with a Ti-Cu-Ti multi-interlayer in a vacuum of 10⁻⁴-10⁻⁵ Pa. The interfacial microstructures of the Al₂O₃-TiC/W18Cr4V joint were investigated with optical microscope and scanning electron microscopy. The elemental concentration near the diffusion interface was evaluated by electron probe microanalysis. The results indicate that an obvious transition zone was formed between Al₂O₃-TiC and W18Cr4V during the vacuum diffusion bonding. The elements in the transition zone are mainly Ti and Cu with a small amount of Fe. Element Ti concentrates near the two interfaces of the Al₂O₃-TiC/transition zone/W18Cr4V. The microhardness of the transition zone is lower than that of Al₂O₃-TiC and higher than that of W18Cr4V. The formation process of the transition zone consists of five stages: (i) Formation of Cu-Ti liquid phase; (ii) Full melt of Cu; (iii) Full melt of Ti; (iv) Formation of reaction layer; (v) Formation of Cu-Ti solid solution and increment of reaction layer.     

Mechanical properties and microstructure of Al2O3/TiAl in situ composites doped with Cr2O3

Al₂O₃/TiAl composites were successfully fabricated from powder mixtures of Ti, Al, TiO₂ and Cr₂O₃ by a hot-press-assisted exothermic dispersion method. The effect of the Cr₂O₃ addition on the microstructures and mechanical properties of Al₂O₃/TiAl composites was characterized, and the results showed that the Rockwell hardness, flexural strength and fracture toughness of the composites increased as the Cr₂O₃ content increased. When the Cr₂O₃ content was 2.5 wt%, the flexural strength and the fracture toughness attained peak values of 925 MPa and 8.55 MPa m^1/2, respectively. This improvement of mechanical properties was due to the more homogeneous and finer microstructure developed from the addition of Cr₂O₃ and an increase in the ratio of α₂-Ti₃Al to γ-TiAl matrix phases.     

In situ processing and aging behaviour of an aluminium/Al2O3 composite

Reactive sintering involving a displacement reaction between aluminium and CuO powders was applied to fabricate an aluminium based composite. The two powders were mixed in a ball mill and uniaxially pressed before sintering in nitrogen atmosphere at 900 °C. During sintering a displacement reaction between CuO and aluminium occurred, which resulted in in situ synthesis of alumina particles. Differential thermal analysis (DTA), X-ray diffractometry (XRD), optical and scanning electron microscopies were used to investigate the phase and microstructural changes taking place during processing of the composite. Results revealed that no chemical reaction occurred during ball milling and Al₂O₃ phase developed in two stages during sintering of the compact. Below 700 °C, amorphous alumina formed which transformed to crystalline alumina at higher temperature. Aging response of the composite was examined as a function of time in temperature range of 180–220 °C. Composite attained a peak hardness value of 133 H_v after 4 h of aging at 200 °C.     

Microstructure and martensitic transformation in Si-coated TiNi powders prepared by ball milling

Si was coated on the surface of Ti–49Ni (at%) alloy powders by ball milling in order to improve the electrochemical properties of the Si electrodes of secondary Li ion batteries and then the microstructure and martensitic transformation behavior were investigated by means of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Ti–Ni powders coated with Si were fabricated successfully by ball milling. As-milled powders consisted of highly deformed Ti–Ni powders with the B2 phase and amorphous Si layers. The thickness of the Si layer coated on the surface of the Ti–Ni powders increased from 3–5 μm to 10–15 μm by extending the milling time from 3 h to 48 h. However, severe contamination from the grinding media, ZrO₂ occurred when the ball milling time was as long as 48 h. By heating as-milled powders to various temperatures in the range of 673–873 K, the highly deformed Ti–Ni powders were recovered and Ti₄Ni₄Si₇ was formed. Two-stage B2–R–B19′ transformation occurred when as-milled Si-coated Ti–49Ni alloy powders were heated to temperatures below 873 K, above this temperature one-stage B2–B19′ transformation occurred.     

Structure study of Bi4Ti3O12 produced via mechanochemically assisted synthesis

Nanosized bismuth titanate was prepared via high-energy ball milling process through mechanically assisted synthesis directly from their oxide mixture of Bi₂O₃ and TiO₂. Only Bi₄Ti₃O₁₂ phase was formed after 3 h of milling time. The excess of 3 wt% Bi₂O₃ added in the initial mixture before milling does not improve significantly the formation of Bi₄Ti₃O₁₂ phase comparing to stoichiometric mixture. The formed phase was amorphized independently of the milling time. The Rietveld analysis was adopted to determine the crystal structure symmetry, amount of amorphous phase, crystallite size and microstrains. With increasing the milling time from 3 to 12 h, the particle size of formed Bi₄Ti₃O₁₂ did not reduced significantly. That was confirmed by SEM and TEM analysis. The particle size was less than 20 nm and show strong tendency to agglomeration. The electron diffraction pattern indicates that Bi₄Ti₃O₁₂ crystalline powder is embedded in an amorphous phase of bismuth titanate. Phase composition and atom ratio in BIT ceramics were determined by X-ray diffraction and EDS analysis.     

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.     

Effects of copper and Al2O3 particles on characteristics of Cu–Al2O3 composites

High-energy milling was used for production of Cu–Al₂O₃ composites. The inert gas-atomized prealloyed copper powder containing 2 wt.%Al and the mixture of the different sized electrolytic copper powders with 4 wt.% commercial Al₂O₃ powders served as starting materials. Milling of prealloyed copper powders promotes formation of nano-sized Al₂O₃ particles by internal oxidation with oxygen from air. Hot-pressed compacts of composites obtained from 5 and 20 h milled powders were additionally subjected to the high-temperature exposure in argon at 800 °C for 1 and 5 h. Characterization of processed material was performed by optical and scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), microhardness, as well as density and electrical conductivity measurements. Due to nano-sized Al₂O₃ particles microhardness and thermal stability of composite processed from milled prealloyed powders are higher than corresponding properties of composites processed from the milled powder mixtures. The results were discussed in terms of the effects of different size of starting copper powders and Al₂O₃ particles on the structure, strengthening of copper matrix, thermal stability and electrical conductivity of Cu–Al₂O₃ composites.     

Sintering temperature depression in Al<Subscript>2</Subscript>O<Subscript>3</Subscript> by mechanical milling

High-energy milling of Al₂O₃ with hardened steel milling media has confirmed that nanocrystalline powders are readily formed. At a ball to charge mass-ratio of 20:1, the crystallite size falls below 30 nm in just 2 h and below 15 nm in 4 h. The as-milled powders are contaminated with Fe which increases linearly with increased milling time, reaching ∼10 wt% after 16 h. The HCl leaching process of Karagedov and Lyakhov [Karagedov and Lyakhov (1999) Nanostruct Mater 11(5):559] was found to remove a large proportion of the Fe, but residual Fe was found with XRF analysis. Milled and leached samples show significant sintering temperature depression to approximately 1100 °C and produce sintered densities greater than 94% without the application of pressure. Milling induced lattice expansion of the Al₂O₃ is observed which we posit to be due to defect formation rather than Fe absorption. The respective roles of small crystallite size and lattice defects in reducing the sintering temperature are discussed.     

High performance self-aligned top-gate ZnO thin film transistors using sputtered Al2O3 gate dielectric

High performance self-aligned top-gate zinc oxide (ZnO) thin film transistors (TFTs) utilizing high-k Al₂O₃ thin film as gate dielectric are developed in this paper. Good quality Al₂O₃ thin film was deposited by reactive DC magnetron sputtering technique using aluminum target in a mixed argon and oxygen ambient at room temperature. The resulting transistor exhibits a field effect mobility of 27 cm²/V s, a threshold voltage of − 0.5 V, a subthreshold swing of 0.12 V/decade and an on/off current ratio of 9 × 10⁶. The proposed top-gate ZnO TFTs in this paper can act as driving devices in the next generation flat panel displays.     

Reaction sintering fabrication of (AlN, TiN)-Al2O3 composite

The (AlN, TiN)-Al₂O₃ composites were fabricated by reaction sintering powder mixtures containing 10-30 wt.% (Al, Ti)-Al₂O₃ at 1420-1520°C in nitrogen. It was found that the densification and mechanical properties of the sintered composites depended strongly on the Al, Ti contents of the starting powder and hot pressing parameters. Reaction sintering 20 wt.% (Al, Ti)-Al₂O₃ powder in nitrogen in 1520°C for 30 min yields (AlN, TiN)-Al₂O₃ composites with the best mechanical properties, with a hardness HRA of 94.1, bending strength of 687 MPa, and fracture toughness of 6.5 MPa m^1/2. Microstructure analysis indicated that TiN is present as well dispersed particulates within a matrix of Al₂O₃. The AlN identified by XRD was not directly observed, but probably resides at the Al₂O₃ grain boundary. The fracture mode of these composites was observed to be transgranular.     

Tribological properties of Al6061–Al2O3 nanocomposite prepared by milling and hot pressing

Tribological properties of bulk Al6061–Al₂O₃ nanocomposite prepared by mechanical milling and hot pressing were investigated. Al6061 chips were milled for 30 h to achieve a homogenous nanostructured powder. A 3 vol.% Al₂O₃ nanoparticles (∼30 nm) were added to the Al6061 after 15 and 30 h from the beginning of milling. The milling times with Al₂O₃ in these two samples were then 15 h and 30 min, respectively. Additionally, 3 vol.% Al₂O₃ (1 μm and 60 μm) was added to the Al6061 after 15 h of milling; where, the micron size Al₂O₃ in these two samples, was milled 15 h with the matrix. Hot pressing of milled samples was executed at 400 °C under 128 MPa pressure in a uniaxial die. The hot pressed samples were characterized by micro-hardness test, bulk density measurements, pin on disc wear test, and finally scanning electron microscopy observations. Fifteen hour-milled nanocomposite with nanoscale Al₂O₃, showed improvement in wear resistance and bulk density compared with that of 30 min-milled nanocomposites due to better dispersion of Al₂O₃ nanoparticles, improved surface quality of nanocomposite particles before pressing and more grain refinement of Al matrix. Moreover, increasing the reinforcement size increased the wear rate because of reduction in relative density, hardness and inter-particle spacing.     

Synthesis and structural properties of Ni–20Cr–2Y2O3 nanocomposite alloy prepared by a very high energy mechanical milling

Ni–20Cr–2Y₂O₃ nanocomposite alloys were synthesized by a very high energy ball milling (applied milling energy, ∼180 kJ g⁻¹ hit; milling speed, ∼1000 rpm) using the coarse yttria (Y₂O₃; ∼50 μm) starting powders, and their formation behavior was investigated with particular attention paid to Y₂O₃ particles. Homogeneous Ni–20Cr–2Y₂O₃ nanocomposite alloy powders were achieved in a short milling time of 40 min; the Cr elements were almost fully alloyed into the Ni lattice, and simultaneously the original coarse cubic Y₂O₃ particles were transformed into extremely fine monoclinic nanocrystals. Thermal consolidation by spark plasma sintering induced homogeneous precipitations of the Y–Cr–O nano-clustered oxides (mean diameter d_m ∼12 nm), which were identified as an orthorhombic YCrO₃ structure (space group Pnma, a = 0.5523 nm).     

The effect of Ti addition on alloying and formation of nanocrystalline structure in Fe–Al system

In this work, the effect of Ti addition on alloying and formation of nanocrystalline structure in Fe–Al system was studied by utilizing mechanical alloying (MA) process. Structural and morphological evolutions of powder particles were studied by X-ray diffractometry, microhardness measurements, and scanning electron microscopy. In both Fe₇₅Al₂₅ and Fe₅₀Al₂₅Ti₂₅ systems MA led to the formation of Fe-based solid solution which transformed to the corresponding intermetallic compounds after longer milling times. The results indicated that the Ti addition in Fe–Al system affects the phase transition during mechanical alloying, the final crystallite size, the mean powder particle size, the hardness value and ordering of DO₃ structure after annealing. The crystallite size of Fe₃Al and (Fe,Ti)₃Al phases after 100 h of milling time were 35 and 12 nm, respectively. The Fe₃Al intermetallic compound exhibited the hardness value of 700 Hv which is significantly smaller than 1050 Hv obtained for (Fe,Ti)₃Al intermetallic compound.     

Preparation and characterization of (V1−xAlx)2O3 ultrafine powders by chemical doping method

(V_1−xAl_x)₂O₃ ultrafine powders have been synthesized successfully by pyrolysizing poly-crystalline Al-doped precursor, Al-doped (NH₄)₅[(VO)₆(CO₃)₄(OH)₉]·10H₂O in H₂ atmosphere at 1373 K for the first time. The relation between the ratio of Al/V in the VOCl₂ solution and that in polycrystalline Al-doped precursor was studied. The result shows that this chemical doping method is successful, and the Al (III) content in precursor is controllable. The lattice parameters of (V_1−xAl_x)₂O₃ determined by XRD at room temperature and high-temperature reveal that the Al (III) enter the V₂O₃ lattice. The SEM micrograph shows that the size of the powder particles is about 100-200 nm. The DSC and magnetic susceptibility results of the (V_1−xAl_x)₂O₃ demonstrate that the powders exhibit the intrinsic phase transition properties.