Mechanically activated combustion synthesis of Ti3AlC2/Al2O3 nanocomposite from TiO2/Al/C powder mixtures

Ti₃AlC₂/Al₂O₃ nanocomposite powder was synthesized by mechanical-activation-assisted combustion synthesis of TiO₂, Al and C powder mixtures. The effect of mechanical activation time of 3TiO₂-5Al-2C powder mixtures, via high energy planetary milling (up to 20?h), on the phase transformation after combustion synthesis was experimentally investigated. X-ray diffraction (XRD) was used to characterize as-milled and thermally treated powder mixtures. The morphology and microstructure of as-fabricated products were also studied by scanning electron microscopy (SEM) and field-emission gun electron microscopy (FESEM). The experimental results showed that mechanical activation via ball-milling increased the initial extra energy of TiO₂-Al-C powder mixtures, which is needed to enhance the reactivity of powder mixture and make it possible to ignite and sustain the combustion reaction to form Ti₃AlC₂/Al₂O₃ nanocomposite. TiC, AlTi and Al₂O₃ intermediate phases were formed when the initial 10?h milled powder mixtures were thermally treated. The desired Ti₃AlC₂/Al₂O₃ nanocomposite was synthesized after thermal treatment of 20?h milled powder and consequent combustion synthesis and FESEM result confirmed that produced powder had nanocrystalline structure.     

Synthesis and characterisation of nanostructured Al–Al3V and Al–(Al3V–Al2O3) composites by powder metallurgy

In this study, the formation and characterisation of Aluminium (Al)-based composites by mechanical alloying and hot extrusion were investigated. Initially, the vanadium trialuminide (Al₃V) particles with nanosized structure were successfully produced by mechanical alloying and heat treatment. Al₃V–Al₂O₃ reinforcement was synthesised by mechanochemical reduction during milling of V₂O₅ and Al powder mixture. In order to produce composite powders, reinforcement powders were added to pure Al powders and milled for 5?h. The composite powders were consolidated in an extrusion process. The results showed that nanostructured Al-10?wt-% Al₃V and Al-10?wt-% (Al₃V–Al₂O₃) composites have tensile strengths of 209 and 226?MPa, respectively, at room temperature. In addition, mechanical properties did not drop drastically at temperatures of up to 300°C.     

Microstructure Evolution and Isothermal Compaction in TiO2-Al-C Combustion Reaction

Combustion reaction in the TiO₂-Al-C system was investigated by the combustion wave freezing technique with a wedge-shaped copper-made quenching block. The combustion reaction was a combined process in which the aluminothermic reduction of TiO₂ (3TiO₂ + 4Al 2Al₂O₃ + 3Ti) and TiC formation reaction (Ti + C TiC) occur in series. First, the aluminothermic reduction was activated by wet spreading of molten Al into interspaces between TiO₂ particles to produce rounded Al₂O₃ grains embedded in the Ti-rich liquid phase. In the later combustion stage, the Ti-rich phase reacted with the reactant C to produce TiC grains in the Ti-rich liquid phase. The three-dimensionally interconnected Al₂O₃ structure typically shown in this system mainly originated from interconnection between the rounded Al₂O₃ grains due to the high combustion temperature from the high exothermic TiC formation reaction. With decreasing the combustion temperature and controlling the C and TiC content, the interconnected Al₂O₃ structure changed to isolated. The isolated Al₂O₃ structure showed superior isothermal compaction behavior to the interconnected. Finally, it is suggested that the microstructure of combustion reaction should be one of the important factors in the SHS compaction process.     

Laser processing of ceramics of the SiO2-Al2O3 system

An investigation has been made of the constitution of laser-processed ceramics from the SiO₂-Al₂O₃ system. Samples were produced in the form of pellets a few millimetres in diameter by pulsed laser melting of mixtures of silica and alumina powders containing 40, 60 and 70 mol % Al₂O₃. X-ray diffraction identified the main crystalline phases as Al₂O₃ in the pellets produced from the 70 mol % Al₂O₃ mixture and mullite from the 60 and 40 mol % Al₂O₃ mixtures. The proportion of glassy phase present increased with increasing SiO₂ content. Microstructural observations on the 60 mol % Al₂O₃ pellet showed primary mullite crystals and a lamellar structure interpreted as a eutectic of Al₂O₃ and mullite. Pellets prepared by melting kaolin powder consisted essentially of a glassy phase and much porosity. Cladding of an alumina substrate, carried out using a continuous powder feed into a laser-generated melt pool, was carried out using the same silica-alumina mixtures as those employed for pellet production. A clad layer was also produced by preplacing a kaolin coat on the alumina substrate prior to laser processing. The effects of traverse speed over the range 3.7 to 7.4 mm s^?1 inclusive, power density (44.4 and 111 W mm^?2) and powder flow rate (0.13 to 0.47 g s^?1 inclusive) were investigated. It was found that the phases present in the clad layer depended on the composition of the precursor powder and the processing conditions. Microstructural examination of the clad layers produced from SiO₂-60 mol % Al₂O₃ and kaolin that had completely melted during processing exhibited various growth morphologies.     

Role of intensive milling in mechano-thermal processing of TiAl/Al2O3 nano-composite

In this research a nano-composite structure containing of an intermetallic matrix with dispersed Al₂O₃ particles was obtained via mechanical activation of TiO₂ and Al powder mixture and subsequent sintering. The mixture has been milled for different lengths of time and then as a subsequent process it has been sintered. Phase evolutions in the course of milling and subsequent sintering of the milled powder mixture were investigated. Samples were characterized by XRD, SEM, DTA and TEM techniques.The results reveal that the reaction begins during milling by formation of Al₂O₃ and L1₂ Al₃Ti and further milling causes partial amorphization of powder mixture. DTA results reveal that milling of the powder mixture causes solid state reaction between Al and TiO₂ rather than liquid–solid reaction. Also, it was observed that the exothermicity of aluminothermic reduction is reduced by increasing the milling time and the exothermic peak shifts to lower temperatures after partial amorphization of powder mixture during milling. Phase evolutions of the milled powders after being sintered reveal that by increasing the milling time and formation of L1₂ Al₃Ti in the milled powder, intermediate phase formed at 500 °C changes from D0₂₂ Al₃Ti to Al₂₄Ti₈ phase.     

Formation mechanism of ZrB2–Al2O3 nanocomposite powder by mechanically induced self-sustaining reaction

ZrB₂–Al₂O₃ nanocomposite powder was produced by aluminothermic reduction in Al/ZrO₂/B₂O₃ system. In this research, high energy ball milling was used to produce the necessary conditions to induce a mechanically induced self-sustaining reaction. The ignition time of the composite formation was found to be about 13 min. The synthesis mechanism in this system was investigated by examining the corresponding sub-reactions as well as changing the stoichiometry of reactants. Thermal behavior of the system was also studied.     

Synthesis and characterization of Al (Al2O3–TiB2/Fe) nanocomposite by means of mechanical alloying and hot extrusion processes

Synthesis and characterization of Al–(Al₂O₃–TiB₂/Fe) nanocomposite by means of mechanical alloying and hot extrusion processes was the goal of this study. For this regards, mechanical alloying was done in two steps; formation of Al₂O₃–TiB₂/Fe reinforcements and preparation of Al-base nanocomposite. Results showed that Al₂O₃–TiB₂/Fe nanocomposite powders can synthesis by mechanical alloying and subsequent heat treatment at 700 °C. Hot extrusion of powder samples lead to preparation of fully dense Al-base nanocomposite. With increasing the amount of complex reinforcements, the compression strength was increased and reached to 560 MPa. Consolidated samples show good ductility related to the nature of Al₂O₃–TiB₂/Fe reinforcements.     

Synthesis and structural characterization of nanocrystalline (Ni,Fe)3Al intermetallic compound prepared by mechanical alloying

Synthesis of (Ni, Fe)₃Al intermetallic compound by mechanical alloying (MA) of Ni, Fe and Al elemental powder mixtures with composition Ni₅₀Fe₂₅Al₂₅ was successfully investigated. The effects of Fe-substitution in Ni₃Al alloy on mechanical alloying process and on the final products were investigated. The structural changes of powder particles during mechanical alloying were studied by X-ray diffractometry, scanning electron microscopy and microhardness measurements. At the early stages, mechanical alloying resulted in a Ni (Al, Fe) solid solution with a layered nanocrystalline structure consisting of cold welded Ni, Al and Fe layers. By continued milling, this structure transformed to the disordered (Ni, Fe)₃Al intermetallic compound which increased the degree of L1₂ ordering upon heating. In comparison to Ni–Al system, Ni (Al, Fe) solid solution formed at longer milling times. Meanwhile, the substitution of Fe in Ni₃Al alloy delayed the formation of Ni (Al, Fe) solid solution and (Ni, Fe)₃Al intermetallic compound. The microhardness for (Ni, Fe)₃Al phase produced after 80 h milling was measured to be about 1170HV which is due to formation of nanocrystalline (Ni, Fe)₃Al intermetallic compound.     

Fabrication of Al2O3–20 vol.% Al nanocomposite powders using high energy milling and their sinterability

In this study, alumina-based matrix nanocomposite powders reinforced with Al particles were fabricated and investigated. The sinterability of the prepared nanocomposite powder at different firing temperature was also conducted. Their mechanical properties in terms of hardness and toughness were tested. Alumina and aluminum powder mixtures were milled in a planetary ball mill for various times up to 30 h in order to produce Al₂O₃–20% Al nanocomposite. The phase composition, morphological and microstructural changes during mechanical milling of the nanocomposite particles were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM) techniques, respectively. The crystallite size and internal strain were evaluated by XRD patterns using Scherrer methods.A uniform distribution of the Al reinforcement in the Al₂O₃ matrix was successfully obtained after milling the powders. The results revealed that there was no any sign of phase changes during the milling. The crystal size decreased with the prolongation of milling times, while the internal strain increased. A simple model is presented to illustrate the mechanical alloying of a ductile–brittle component system. A competition between the cold welding mechanism and the fracturing mechanism were found during powder milling and finally the above two mechanisms reached an equilibrium. The maximum relative density was obtained at 1500 °C. The harness of the sintered composite was decreased while the fracture toughness was improved after addition Al into alumina.     

Synthesis of Cu–Zr–Al/Al2O3 amorphous nanocomposite by mechanical alloying

Two routes were used to produce Cu–Zr–Al/Al₂O₃ amorphous nanocomposite. First route included mechanical alloying of elemental powders mixture. In second route Cu₆₀Zr₄₀ alloy was synthesized by melting of Cu and Zr. Cu₆₀Zr₄₀ alloy was then ball milled with Al and CuO powder. It was not possible to obtain a fully amorphous structure via first route. The mechanical alloying of Cu₆₀Zr₄₀, Al and CuO powder mixture for 10 h led to the reaction of CuO with Al, forming Al₂O₃ particulate, and concurrent formation of Cu₆₂Zr₃₂Al₄ amorphous matrix. The thermodynamical investigations on the basis of extended Miedema’s model illustrated that there is a strong thermodynamic driving force for formation of amorphous phase in this system. Lack of amorphization in first route appeared to be related to the oxidation of free Zr during ball milling.     

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.     

The preparation of Al2O3/M (Fe,Co, Ni) nanocomposites by mechanical alloying and the catalytic growth of carbon nanotubes

Mechanical alloying was employed to produce Al₂O₃/M (M=Fe, Co, Ni) nanocomposites. It was found that high-energy mechanical milling could realize not only drastic refinement but also the well dispersion of catalyst precursors in oxide matrixes. After mechanical milling, the solid-state alloying and the accelerated substitutional reactions were observed between the parent oxides. The as-obtained Al₂O₃/M nanocomposites possessed the fine-grained and porous structures and thus high reducibility. Large-scale formation of multiwalled and single-walled carbon nanotubes were achieved by using these mechanical alloying-derived Al₂O₃/M nanocomposites.     

Effect of SiO2 content in AI2O3 on interfacial bonding strength of aiuminium/AI2)3 composite

Abstract

In this study, the interfacial bonding strength between pure aluminium or an aluminium alloy (Al-12Si-1Cu-1Mg-2Ni) and Al₂O₃ containing 0-20 wt-%SiO₂ was measured using a B scale Rockwell hardness indenter. The interface was analysed using scanning electron microscopy, Auger electron spectroscopy, and X-ray diffraction. The results obtained were as follows. In the pure aluminium/Al₂O₃ composite the interfacial bonding strength was not strongly affected by the SiO₂ additions, but the interfacial strength of the aluminium alloy/Al₂O₃ composite could be significantly improved by small additions (2-5 wt-%) of SiO₂ in the Al₂O₃. Magnesium was the only alloying element to segregate at the interface. At the interface of the pure aluminium/Al₂O₃–20SiO₂ specimen, Al₂O₃ and silicon were detected, and MgAl₂O₄ and Al₂O₃ were detected in the aluminium alloy/Al₂O₃–20SiO₂ specimen. The interfacial chemical reaction involving magnesium is believed to be important in increasing the interfacial bonding strength.     

Fabrication of Al-Zn/α-Al2O3 nanocomposite by mechanical alloying

In this study fabrication and characterization of alumina particles reinforced aluminum-based metal matrix nanocomposite by mechanical alloying were investigated. Aluminum and zinc oxide powders mixture milled by a planetary ball mill in order to produce Al-13.8 wt.% Zn/5 vol.% Al₂O₃ nanocomposite. The structural evaluation milled and annealed powders studied by X-ray diffraction, SEM observation and hardness measurement. The aluminum crystallite size estimated with broadening of XRD peaks by Williamson-Hall formula. The results showed that milling of aluminum and zinc oxide for 60 h led to displacement reaction of the zinc oxide and aluminum to produce Zn and Al₂O₃ phases. The milled powder had a microstructure consisting of nanosized Al₂O₃ particles in an Al-Zn solid solution with a nanoscale grain size of 40 nm. Microhardness of this nanocomposite was found to be about 190 HV.     

Use of the Aluminothermic Reaction in the Treatment of Steel Industry By-Products

This is a study of the possibility of using the aluminothermic reaction for the treatment of the steel industry by-products consisting of ferric oxides and contamination. The experimenal method consists of mixing of the powder materials followed by initiation of the aluminothermic reaction. The microstructures of the reaction products are analyzed using X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. Regardless of the composition of the initial oxide mixture, similar reaction products are formed during the reaction. These reaction products are reduced iron and a slag phase consisting of Al₂O₃, Fe, and FeAl₂O₄. As a consequence of the high temperature of the aluminothermic reaction, zinc and lead oxides can be separated from the iron phase during the reaction. The remainder of the contamination is either vaporized or burned during the reaction. It is possible that utilization of the aluminothermic reaction offers an energy-efficient, economic process for the recycling of a variety of different by-products containing ferric oxides.     

The Effect of Excess Aluminum on the Composition and Microstructure of Nb-Al Alloys Produced by Aluminothermic Reduction of Nb2O5

Intermetallic aluminides including those phases of the Nb-Al system are of interest for high-temperature structural applications. Through aluminothermic reduction (ATR) of Nb₂O₅ different alloys of the Nb-Al system can be produced by varying the amount of aluminum (excess aluminum) in the thermit charge. In this work, various Nb-Al alloys were produced by varying Nb₂O₅ and Al powder blends. The resulting alloys were characterized by chemical analysis (Al, O, and C), X-ray diffraction and scanning electron microscopy. The aluminum content of the alloys increased linearly from 14.5 to 50.4 at% as the excess Al was varied from 10 up to 60% over the stoichiometric amount to reduce the Nb₂O₅. The carbon content was lower than 300 wt-ppm. The oxygen content decreases with increasing excess Al, reaching 1300 wt-ppm for the alloy produced with 60% excess Al. The inclusion content (Al₂O₃) decreases significantly as the excess Al is increased. The following metallic phases were identified in the alloys: Nb_ss (niobium solid solution) and Nb₃Al (alloy produced with 10% excess Al); Nb₃Al (alloys produced with 15 and 20% excess Al); Nb₃Al, Nb₂Al, and NbAl₃ (alloy produced with 30% excess Al); and Nb₂Al and NbAl₃ (alloys produced with 40, 50, and 60% excess Al).     

In situ synthesis of Al2O3 particulate-reinforced Al matrix composite by low temperature sintering

The powder mixture of Al-10 wt.% SiO₂ was selected as a research system. Compared with an as-mixed powder, the phase structure and microstructure of an as-milled powder was investigated, and the temperature of the displacement reaction in the two kinds of powder was determined by thermal analysis. The preforms of the two kinds of powder were sintered based on the result of thermal analysis. The results indicate that the particle size of the Al-SiO₂ powder was refined greatly after 4 h of high energy ball milling, and diffusion couples were formed due to SiO₂ particles embedded in the Al matrix. The displacement reaction did not occur between Al and SiO₂ for the as-mixed powder, while it occurred in the range of 560–680°C for the as-milled powder. For the as-milled powder, an aluminum matrix composite reinforced with Al₂O₃ particles, which were homogeneously distributed in the Al matrix, can be fabricated by sintering at 640°C for 2 h.     

Preparation of Al2O3–TiB2 nanocomposite powder by mechanochemical reaction between Al,B2O3 and Ti

Mechanochemical processing is a novel technique for the synthesis of nano-sized materials. This research is based on the production of Al₂O₃–TiB₂ nanocomposite powder using mechanochemical processing. For this purpose, a mixture of aluminum, titanium and boron oxide powders was subjected to high energy ball milling. The structural evaluation of powder particles after different milling times was conducted by X-ray diffractometry (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results showed that during ball milling the Al/B₂O₃/Ti reacted with a combustion mode producing Al₂O₃–TiB₂ nanocomposite. In the final stage of milling, the crystallite sizes of Al₂O₃ and TiB₂ were estimated to be less than 50 nm.     

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.     

Fabrication of Al2O3–ZrB2 in situ composite by SHS dynamic compaction: A novel approach

Al₂O₃–ZrB₂ in situ composites of 97% of theoretical density were successfully fabricated by a novel self-propagating high temperature synthesis (SHS) dynamic compaction, using less expensive raw materials zirconium oxide, boron oxide, and aluminium. The process is fast, energy efficient, where no furnace sintering is required. The process inhibits and controls the grain growth and microstructure. The densification behaviour and correlation with microstructure of the SHS dynamic compacts were compared with the furnace sintered composite samples where the composite powder was prepared by SHS process. The furnace sintered samples showed coarser grain growth and maximum density of 94.5% of theoretical density was achieved. The SHS dynamic compacted in situ composite had much finer grains in the range of 0.5–3 μm with density 95.5% of the theoretical value. The average grain size was found to decrease from 10 μm to 1.4 μm for alumina and from 5.4 μm to 1.0 μm for zirconium diboride from furnace sintering to SHS dynamic compaction, respectively. Addition of Al₂O₃ as a diluent during SHS reaction enhanced the density to 97%. During SHS dynamic compaction, the amount of liquid and the time interval at which the sample stays at high temperature are the controlling factor of the final microstructure and the densification of the composite.