Microstructures and their stability in rapidly solidified Al-Fe-(V, Si) alloy powders

Microstructures and their stability in as-atomised Al-6.5Fe-1.5V and Al-6.5Fe-1.5V-1.7Si powders have been investigated using transmission electron microscopy (TEM) equipped with energy dispersive X-ray spectroscopy (EDXS), scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC) techniques. It was observed that microstructures of the as-atomised powder particles showed a close relationship with powder particle sizes. The as-atomised powders exhibited three types of microstructures, namely 'zone A', 'zone B' and 'zone C'. The 'zone A' type microstructure consisted of very fine and homogeneous distributed precipitates in the -Al matrix. The 'zone B' microstructure represented the regions consisting of microcellular structures whereas the 'zone C' microstructure represented the regions consisting of coarse cellular structures and globular quasi-crystalline phase particles. Fine powder particles exhibited both 'zone A' and 'zone B' microstructures. The size of 'zone A' decreased with increasing powder particle sizes. The intercellular phases in 'zone B' of both Al-Fe-V and Al-Fe-V-Si were very fine, randomly oriented microquasi-crystalline icosahedral particles. Microstructures of coarse powder particles exhibited both 'zone B' and 'zone C'. The intercellular phases in 'zone C' of Al-Fe-V powders could be Al₆Fe, whereas in Al-Fe-V-Si powders they were probably silicide phase. Formation of powder microstructures may be explained by the interactions between the growing -Al fronts with the freely dispersed, primary phase particles or the solute micro-segregation. Studies using DSC techniques have revealed the microstructural stability of as-atomised powders. There were three DSC exotherms observed in the as-atomised Al-Fe-V powders. The 'zone A' was stable at elevated temperatures and the exotherm peak corresponding to the transformation reactions occurring in 'zone A' was at 360°C. The exotherm peak, which might correspond to the transformation of the globular clusters of microquasi-crystalline icosahedral phase to single-phase icosahedral particles, was at 450°C. The exotherm peak, which may correspond to the formation of Al₁₃Fe₄ and Al₄₅(V, Fe)₇ phases, was at 500°C. In the as-atomised Al-Fe-V-Si powders, only one exotherm was observed with a peak at 400°C. This exotherm may correspond to precipitation of silicide phase particles.     

Enhancement of mechanical strength of TiO2/high-density polyethylene composites for bone repair with silane-coupling treatment

Mechanical properties of composites made up of high-density polyethylene (HDPE) and silanated TiO₂ particles for use as a bone-repairing material were investigated in comparison with those of the composites of HDPE with unsilanized TiO₂ particles. The interfacial morphology and interaction between silanated TiO₂ and HDPE were analyzed by means of Fourier transform infrared (FT-IR) spectroscopy and scanning electron microscopy (SEM). The absorption in spectral bands related to the carboxyl bond in the silane-coupling agent, the vinyl group in the HDPE, and the formation of the ether bond was studied in order to assess the influence of the silane-coupling agent. The SEM micrograph showed that the “bridging effect” between HDPE and TiO₂ was brought about by the silane-coupling agent. The use of the silane-coupling agent and the increase of the hot-pressing pressure for shaping the composites facilitated the penetration of polymer into cavities between individual TiO₂ particles, which increased the density of the composite. Therefore, mechanical properties such as bending yield strength and Young's modulus increased from 49 MPa and 7.5 GPa to 65 MPa and 10 GPa, respectively, after the silane-coupling treatment and increase in the hot-pressing pressure.     

Synthesis and sintering of ZrO2-CeO2 powder by use of polymeric precursor based on Pechini process

Nanocrystalline ZrO₂-12 mol % CeO₂powders were synthesized using a polymeric precursor method based on the Pechini process. X-ray diffraction (XRD) patterns showed that the method was effective to synthesize tetragonal zirconia single-phase. The mean crystallite size attained ranges from 6 to 15 nm. The BET surface areas were relatively high reaching 97 m²/g. Studies by nitrogen adsorption/desorption on powders, dilatometry of the compacts, and transmission electron microscopy (TEM) of the powders, were also developed to verify the particles agglomeration state. Both citric acid : ethylene glycol ratio and calcination temperature affected the powder morphology, which influenced the sinterability and microstructure of the sintered material, as showed by scanning electron microscopy (SEM).     

Design of composite porous cermets synthesized by hydrothermal treatment of CrAl powder followed by calcination

The microstructure of the porous Cr–Al metal–oxide cermet was studied by means of XRD, SEM, EDX as well as IR and Raman spectroscopy. This cermet was synthesized by mechanical alloying of Cr–Al powders in an AGO-2 planetary ball mill followed by hydrothermal treatment in a special stainless steel die and calcination in air. As a result, a highly porous monolith comprised of metal-like particles randomly distributed in the oxide matrix (Cr₂O₃ and Al₂O₃) was formed. Two types of the composite cores were found in cermets. The first one consisted of chromium phase containing nanoparticles sized from 50 to 140 nm and Al-enriched phase at the interfaces. The second one consisted of new chromium oxide phases with hexagonal Cr₂N-like and fcc CrN-like structures probably with Cr₂O and CrO stoichiometry. These new phases were stabilized within aggregates of the nanocomposite particles containing inclusions of alumina. The relations between different preparation stages and the cermet microstructure are discussed.     

Synthesis and Magnetic Properties of Barium Hexaferrite Powders Using Organic Acid Precursor Method

An investigation of the synthesis of BaFe₁₂O₁₉ powders by the organic acid precursor method is reported by acidic and neutral media. X-ray diffraction (XRD), scanning electron microscopy (SEM), and vibrating sample magnetometer (VSM) are utilized to study the effect of organic precursor type and annealing temperature on the crystal structure, crystallite size, microstructure and magnetic properties of the formed powders. The XRD analysis showed that the crystalline BaFe₁₂O₁₉ phase was obtained at 1200 °C for 2 h using different carboxylic acids in acidic medium. However, pure BaFe₁₂O₁₉ was achieved at low annealing temperature 1000 °C in neutral medium. SEM micrographs showed that the particles were strongly influenced by type of carboxylic acid and the annealing temperature. VSM study indicated that the saturation magnetization was increased with increasing annealing temperature to 1200 °C as the result of formation of single barium hexaferrites phase. High saturation magnetization (M _s =66.5 emu/g) was achieved for the formed powders in neutral medium using tartaric acid as organic precursor. Wide coercivities of the formed powders (H _c =259–5114 Oe) were obtained.     

Preparation and characterization of doped TiO2 nanodandelion

Doped dandelion-like TiO₂ microspheres assembled nanorods were synthesized from rutile powders using either urea or thiourea leading to N- or S-doped TiO₂. The rutile particles reacted in concentrated NaOH and urea (or thiourea) solution under hydrothermal conditions (200 °C for 24 h), yielding N- and S-doped TiO₂ nanodandelions with diameters ranging from 0.7 to 1.3 μm. SEM, HRTEM, X-ray diffraction (XRD) and IR spectra were used to characterize the synthesis of powders. The results show that concentrated urea (or thiourea) and NaOH are used as additives that help in the construction of the dandelion-like structures. The fabricated nanostructures exhibit high photocatalytic activity in the photodegradation of aqueous Methylene Blue solution.     

Fabrication and characteristics of in situ Al12W particles reinforced aluminum matrix composites by reaction sintering

A novel in situ Al12W particles reinforced aluminum matrix composite was synthesized by reaction sintering of tungsten and aluminum powders and followed by hot extrusion. The microstructures were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The tensile tests of composite and pure aluminum materials were measured. The XRD analysis identifies that the in situ Al12W particles are formed by the reaction between tungsten and aluminum powders. Meanwhile, SEM observation shows that the Al12W particles are distributed uniformly in the Al matrix, and TEM observation shows that the interfacial condition of Al12W particles and Al is good. It is found from the tensile tests that the in situ synthesized Al12W particles can significantly enhance the strength of the composite in spite of decreasing elongation. The fracture morphology analysis reveals that the fracture mode of composite is ductile fracture.     

Structure and magnetic properties of ZrO2-coated Fe powders and Fe/ZrO2 soft magnetic composites

Fe particles were coated with ZrO₂ nanopowders using mechanical milling method combined with high temperature recovery annealing process. The effect of milling time on particle size, phase structure and magnetic properties of the core-shell structure powders was studied. Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD) revealed that the surfaces of the composite powders comprised thin and uniform layers of ZrO₂ insulating powders after milling. Also, the SEM images showed the morphology of micro-cellular structured compacts with cell-body of Fe particles and indicated that Fe particles were well separated and insulated by thin ZrO₂ layers. The Fe/ZrO₂ soft magnetic composites displayed much higher electrical resistivity, lower core loss than that of the pure Fe powder cores without ZrO₂ layers at medium and high frequencies. The preparation method of ZrO₂-insulated Fe powders provides a promising method to reduce the core loss and improve the magnetic properties for soft magnetic composite materials.     

Characterization and metastability of alkoxy-derived tetragonal zirconia powder

The zirconia powders were synthesized through the hydrolysis of zirconiumn-butoxides with air moisture as well as with aqueous solutions of different pH values to investigate the metastability of tetragonal (t)-ZrO₂. The SEM and TEM observations reveal that powders prepared with air moisture hydrolysis consist of spherical particles of 0.5–3.5 m diameter, while gel-like powders composed of finer particles were obtained with aqueous solution hydrolysis. The samples obtained show different crystallization and tetragonal/monoclinic transformation temperatures on differential thermal analysis. The metastability of t-ZrO₂, investigated in terms of relative content with X-ray diffraction, was not explicable through the crystallite size effect. Instead, the existence of unpaired electrons, detected using electron paramagnetic resonance, and the strain within the powders, were found to be probably influential in affecting the metastability of t-ZrO₂.     

Synthesis and characterization of the manganese cobaltite spinel prepared using two “soft chemical” methods

Using the coprecipitation method and EDTA gel processes, manganese cobaltite (Mn_1.5Co_1.5O₄) powders were successfully synthesized. The thermal decomposition behavior of the gel precursors, phase formation and morphology of the Mn_1.5Co_1.5O₄ powders were characterized by means of DTA/TGA and MS analyses, X-ray diffraction (XRD), and scanning electron microscopy (SEM), respectively. Well-crystallized dual-phase manganese cobaltite spinel, containing both the cubic and tetragonal phases, was obtained at room temperature for both types of powders calcinated at 800 °C for 10 h in static air, without formation of any intermediate phase. SEM investigations show that the Mn_1.5Co_1.5O₄ powders prepared using two “soft chemical” methods were agglomerates composed of approximately micron-sized particles. The structure and morphology of the bulk samples, as well as their electrical conductivity, were investigated using XRD, SEM, and EIS, respectively. Mn_1.5Co_1.5O₄ spinels with different microstructure exhibited excellent electrical conductivity and structural stability. Major emphasis was placed on structural transformations of the spinel solid solutions with temperature and their effect on the electrical properties of these solutions.     

Effect of external energy on atomic,crystalline and powder characteristics of antimony and bismuth powders

Next to atoms and molecules the powders are the smallest state of matter available in high purities and large quantities. The effect of any external energy on the shape, morphology and structure can thus be studied with relative ease. The present investigation deals with the effect of a non-contact external energy on the powders of antimony and bismuth. The characteristics of powders treated by external energy are compared with the as received powders (control). The average particle sizes, d ₅₀ and d ₉₉, the sizes below which 99% of the particles are present showed significant increase and decrease indicating that the energy had caused deformation and fracture as if the powders have been subjected to high energy milling. To be able to understand the reasons for these changes the powders are characterized by techniques such as X-ray diffraction (XRD), surface area determination (BET), thermal analytical techniques such as DTA-DTG, DSC-TGA and SDTA and scanning electron microscopy (SEM).     

Electron beam irradiated HDPE/EVA/Mg(OH)2 composites for flame-retardant electric cables

The mechanical properties and flammability of high-density polyethylene (HDPE)/ethylene vinyl acetate (EVA) mixed with various amounts of magnesium hydroxide (Mg(OH)₂) as the filler in composites, irradiated with electron beam at an irradiation dose of 150 kGy, have been studied. It is found that high-energy electron beam irradiation has significant effects on the mechanical properties of the HDPE/EVA/Mg(OH)₂ composites. The tensile strength and elastic modulus increased greater than in the unirradiated ones. Meanwhile, with increasing the content of Mg(OH)₂ in the composites, the limiting oxygen index (LOI) value increased sharply. The microstructure of the caves of the unirradiated HDPE/EVA/Mg(OH)₂ composites show poor interface of composites compared with the irradiated ones, as observed in SEM micrographs.     

Development of NiFe-CNT and Ni3Fe-CNT nanocomposites by mechanical alloying

NiFe-CNT and Ni₃Fe-CNT nanocomposites were fabricated by high energy mechanical alloying method. X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and optical microscopy were employed for evolution of phase composition, morphology and microstructure of the powder particles. Ball milled powders were heat treated at 500 °C for 1 h to release the milling induced stresses. Bulk samples were prepared by sintering of cold pressed (300 MPa) samples at 1040 °C for 1 h. XRD patterns of powders, as-milled and after annealing at 500 °C did not show any peak related to CNTs or excess phases due to the interaction between CNTs and matrix. SEM micrographs showed that the addition of CNTs caused a reduction of powder particles size. The hardness value of as-milled NiFe and Ni₃Fe powders reach to 660 and 720 HV, respectively. According to optical microscopy evaluations, the amount and size of the porosities of the composites bulk samples decreased in comparison with matrix ones.     

Capability of mechanical alloying and SPS technique to develop nanostructured high Cr,Al alloyed ODS steels

Abstract

Oxide dispersion strengthened (ODS) Fe alloys were produced by mechanical alloying (MA) with the aim of developing a nanostructured powder. The milled powders were consolidated by spark plasma sintering (SPS). Two prealloyed high chromium stainless steels (Fe–14Cr–5Al–3W) and (Fe–20Cr–5Al+3W) with additions of Y₂O₃ and Ti powders are densified to evaluate the influence of the powder composition on mechanical properties. The microstructure was characterised by scanning electron microscope (SEM) and electron backscattering diffraction (EBSD) was used to analyse grain orientation, grain boundary geometries and distribution grain size. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) equipped with energy dispersive X-ray spectrometer (EDX) were used to observe the nanostructure of ODS alloys and especially to observe and analyse the nanoprecipitates. Vickers microhardness and tensile tests (in situ and ex situ) have been performed on the ODS alloys developed in this work.     

Mechanochemical assisted synthesis of B4C nanoparticles

The present work reports on the preparation of boron carbide nanoparticles by the reduction of boron oxide with magnesium in the presence of carbon using the mechanochemical processing. The phase transformation and microstructure of powders during ball milling were investigated by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The results showed that during ball milling the B₂O₃–Mg–C reacted with a self-propagating combustion mode producing MgO and B₄C compounds. To separate B₄C from the milled powder mixture, an appropriate leaching process was used. After leaching, the purified powder mixture was characterized using XRD and transmission electron microscope (TEM). XRD studies indicated that the prepared particles were single phase crystalline B₄C. Moreover, TEM studies showed the size of B₄C particles were ranging from 10 to 80 nm.     

Macroporous bioactive glass-ceramic scaffolds for tissue engineering

Highly bioactive scaffolds for tissue engineering were synthesized using a glass belonging to the SiO₂-CaO-K₂O (SCK) system. The glass SCK was prepared by a traditional melting-quenching route and its bioactivity was assessed by in vitro tests in a simulated body fluid (SBF). The glass was ground and sieved to obtain powders of specific size that were subsequently mixed with polyethylene particles of two different dimensions. The powders were then uniaxially pressed to obtain a crack free green compact that was thermally treated to remove the organic component and to sinter the inorganic phase. The obtained biomaterial was characterised by means of X-ray Diffraction, SEM equipped with EDS, mercury intrusion porosimetry, density measurements, image analysis, mechanical tests and in vitro evaluations. A glass-ceramic macroporous scaffold with a homogenously distributed and highly interconnected porosity was obtained. The amount and size of the introduced porosity could be tailored using various amounts of polyethylene powders of different size.     

Formation mechanism of B4C–TiB2–TiC ceramic composite produced by mechanical alloying of Ti–B4C powders

In this study, the B₄C–TiB₂–TiC composite powder was synthesized by mechanical alloying (MA) of Ti–B₄C powder mixture. For this purpose, four powder mixtures of Ti and B₄C powders with different molar ratios were milled. In order to study the mechanism of Ti–B₄C reaction during milling, structural changes and thermal analysis of powder particles were studied by X-ray diffractometry (XRD) and differential thermal analysis (DTA). Morphology and microstructure of powder particles during milling were studied by scanning electron microscopy (SEM). It was found that during MA, after decomposition of the outer layers of B₄C particles, first, C reacted with Ti and after that, B was diffused in Ti structure and TiC and TiB₂ phases were formed in gradual reaction mode. Also, the results of DTA and thermodynamic analysis confirmed the suggested mechanism for Ti–B₄C reaction.     

Nanocrystalline TiO<Subscript>2</Subscript> preparation by microwave route and nature of anatase–rutile phase transition in nano TiO<Subscript>2</Subscript>

Nanopowders of TiO₂ has been prepared using a microwave irradiation-assisted route, starting from a metalorganic precursor, bis(ethyl-3-oxo-butanoato)oxotitanium (IV), [TiO(etob)₂]₂. Polyvinylpyrrolidone (PVP) was used as a capping agent. The as-prepared amorphous powders crystallize into anatase phase, when calcined. At higher calcination temperature, the rutile phase is observed to form in increasing quantities as the calcination temperature is raised. The structural and physicochemical properties were measured using XRD, FT–IR, SEM, TEM and thermal analyses. The mechanisms of formation of nano-TiO₂ from the metal–organic precursor and the irreversible phase transformation of nano TiO₂ from anatase to rutile structure at higher temperatures have been discussed. It is suggested that a unique step of initiation of transformation takes place in Ti_1/2O layers in anatase which propagates. This mechanism rationalizes several key observations associated with the anatase–rutile transformation.     

Influence of atomized powder size on microstructures and mechanical properties of rapidly solidified Al–Cr–Y–Zr aluminum alloys

Rapidly solidified powders of Al–5.0Cr–4.0Y–1.5Zr (wt%) were prepared by using a multi-stage atomization-rapid solidification powder-making device. The atomized powders were sieved into four shares with various nominal diameter level and were fabricated into hot-extruded bars after cold-isostatically pressing and vaccum degassing process. Influence of atomized powder size on microstructures and mechanical properties of the hot-extruded bars was investigated by optical microscopy, X-ray diffraction, transmission electronic microscopy with EPSX and scanning electron microscopy. The results show that the fine atomized powders of rapidly solidified Al–5.0Cr–4.0Y–1.5Zr aluminum alloy attains supersaturated solid solution state under the exist condition of multi-stage rapid solidification. With the powder size increasing, there are Al₂₀Cr₂Y (cubic, a = 1.437 nm) and Ll₂ Al₃Zr (FCC, a = 0.407 nm) phase forming in the powders, and even lumpish particles of Al₂₀Cr₂Y appearing in the coarse atomized powders, as can be found in the as-cast master alloy. Typical microstructures of the extruded bars of rapidly solidified Al–5.0Cr–4.0Y–1.5Zr aluminum alloy can be characterized by fine grain FCC α-Al matrix with ultra-fine spherical particles of Al₂₀Cr₂Y and Al₃Zr. But a small quantity of Al₂₀Cr₂Y coarse lumpish particles with micro-twin structures can be found, originating from lumpish particles of the coarse powders. The extruded bars of rapidly solidified Al–5.0Cr–4.0Y–1.5Zr aluminum alloy by using the fine powders eliminated out too coarse powders have good tensile properties of σ_0.2 = 403 MPa, σ_b = 442 MPa and δ = 9.4% at room temperature, and σ_0.2 = 153 MPa, σ_b = 164 MPa and δ = 8.1% at high temperature of 350 °C.     

Magnetic and microwave-absorbing properties of SrAl<Subscript>4</Subscript>Fe<Subscript>8</Subscript>O<Subscript>19</Subscript> powders synthesized by coprecipitation and citric- combustion methods

Al-substituted M-type hexaferrite is a highly anisotropic ferromagnetic material. In the present study, the coprecipitation and the citric-combustion methods of synthesis for SrAl₄Fe₈O₁₉ powders were explored and their microstructure, magnetic properties, and microwave absorptivity examined. X-ray diffraction (XRD), scanning electron microscopy (SEM), a vibrating sample magnetometer, and a vector network analyser were used to characterize the powders. The XRD analyses indicated that the pure SrAl₄Fe₈O₁₉ powder was synthesized at 900°C and 1000°C for 3 h by coprecipitation, but only at 1000°C for the citric-combustion processes. The SEM analysis revealed that the coprecipitation process yielded a powder with a smaller particle size, near single-domain structure, uniform grain morphology, and smaller shape anisotropy than the citric-combustion process. The synthesis technique also significantly affected the magnetic properties and microwave-absorptivity. Conversely, calcining temperature and calcining time had less of an effect. The grain size was found to be a key factor affecting the property of the powder. The powders synthesized by coprecipitation method at calcining temperature of 900°C exhibited the largest magnetization, largest coercivity, and best microwave absorptivity.