School of materials and metallurgical engineering

In this study, the influence of the most effective parameters of the modified stir casting method such as stirring rate, stirring temperature and master powder type was evaluated on the microstructure and mechanical properties of A356-1 wt% SiC nanocomposites. The mechanical properties were measured by room temperature tensile and compression test and the microstructure was examined by a Scanning Electron microscope and the X-ray diffraction technique. The results showed that there were specific stirring rate and stirring temperature at which the best mechanical properties and nanoparticle distribution in matrix were achieved. The properties and distribution were diminished at stirring rates and stirring temperatures that were higher and lower than the particular values, respectively. These degradations were intense at high stirring temperature and stirring rate. In addition, the use of master powder with lower volume fraction of nanoparticles brought about better mechanical properties and nanoparticle distribution for final nanocomposite. Among different tested values for the parameters, the particular values of stirring temperature and stirring rate were 750 1C and 700 rpm, respectively.

In this article, a new mechanism so-called wet cluster engulfment and the related formula were proposed to predict the critical solidification velocity for nanoparticles engulfment by a solidification front. According to the mechanism, the most important issue in the engulfing of the nanoparticles is the aggregation of nanoparticles, i.e. the formation of wet clusters in front of solidifying phase. In fact, local clustering of nanoparticles created highly viscous zones in the vicinity of the solidification front so that the nanoparticles and the liquid-filled spaces between them move as a unified mass. Thus, the van der Waals and drag forces between a nanoparticles cluster and the solidification front were calculated analytically. The new formulae were derived for the critical solidification velocity and critical diameter of the clusters as a function of the melt viscosity, the nanoparticle size, the cluster size, and the nanoparticle volume concentration inside the cluster. The theoretical results showed that the new model predicted more accurate values for the critical velocity than the earlier models. The proposed model can explain the formation of nanoparticle-rich zones and zones with a low concentration of nanoparticles in the microstructure of the nanocomposites produced via casting.

In this work, nanostructured aluminum titanate (Al2TiO5 or AT) was synthesized by the citrate sol gel method. Then, different volume fractions of this ceramic were blended with Al powder through different durations of the high-energy vibratory milling. The effect of mechanical milling on the thermal degradation of AT in exposure to Al and formation mechanism of in-situ Al2O3 and Al3Ti particles were explored in three conditions: (i) in the powder form; (ii) after annealing of green compact; and (iii) after hot extrusion. In the powder form, it was shown that the mechanical milling is able to significantly diminish the thermal stability of AT, so that the required temperature for the Al3Ti formation decreases from 600 to about 450°C. Also, by extending the mechanical milling duration, the contact surface for interfacial reactions between Al and AT increases and the formation of Al3Ti and Al2O3 is facilitated. By selecting the optimum duration of mechanical milling, this study successfully fixed the microstructure problems of not-milled as-annealed composites i.e. unreacted AT particles and oxygen evolution-arisen cracks, and produced in-situ composite inside each Al particle. However, the induced work hardening of these milled particles hinders the perfect consolidation. To deal with this challenge, the hot extrusion at 535 °C was conducted. The obtained microstructure was composed of very fine (≤ 200 nm) semi-spherical Al3Ti and Al2O3 particles uniformly distributed within the Al matrix. To evaluate the mechanical response of the fabricated nanocomposites, the micro-hardness and nanoindentation testing were carried out. The results proved that the hardness and elastic modulus of the nanocomposites reach 143 HV and 90 GPa, respectively, indicating a significant improvement in the mechanical properties of these systems compared to not-milled extruded composites. Finally, the role of reinforcing particles in strengthening of composite was evaluated.

Carbon nanotube-MgAl 2 O 4 composite powders were successfully prepared through solution combustion synthesis (SCS) followed by catalytic chemical vapor deposition (CCVD) of methane. Catalyst powders were synthesized starting with the stoichiometric ratios of metal nitrates and urea with a small amount of water and different Fe contents followed by subjecting the solution to heat. The obtained powders were placed in a silica tube to react with methane and form carbon nanotubes. It is noteworthy that no hydrogen was used throughout the whole process. Catalysts and composite powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. The quality of products were evaluated by I D /I G ratio obtained from G and D bands intensities in Raman spectra of samples having 10 and 15 wt.% iron. The final product mostly comprised a mixture of single-and double-walled nanotubes on the catalyst containing 10 wt.% Fe, while no carbon product was formed on the catalyst with 5 wt.% Fe.

The continuity and thickness of the coating layer, are the most important factors in wetting properties and strength of carbon fibers. These factors are crucial in the quality of metal matrix composites made with carbon fibers. In this research the Polyacrylonitrail base carbon fibers have been nickel coated with 0.2, 0.5, 0.8 and 11 µ m thickness, by the electroless method. The effect of the thickness of nickel coating on surface condition and also the tensile strength of the carbon fibers has been investigated. The study of surface condition of the coated carbon fibers by SEM showed that the nickel coating at the thickness of about 0.5 µm has the best continuity on the carbon fibers. The results of tensile tests of carbon fibers coated with different thickness of nickel showed that increasing the thickness of coating layer decreases the overall strength of fibers.

This paper describes the development of tough, high reliability sintered carbon materials by the incorporation of graphite flakes into a sintered mesophase-based carbon matrix, a kind of carbon-carbon composite. The bending strengths are lower than those of the matrix alone because of the presence of misaligned flakes at the surface. However, multiple fracture and high work of fracture can be obtained depending upon the size, volume fraction and orientation of the graphite flakes. The fracture toughness is increased by factors of 2-3, due to energy dissipating mechanisms such as crack deflection, crack bridging and pull-out, all caused by the presence of the flakes in the composite. The reliability of the composites in terms of Weibull modulus is also increased by a factor of 2-3, because the defect population is effectively controlled by the flakes, which are mostly un-bonded to the matrix.

Several methods for the synthesis of carbon nanotubes have been developed in the last decade. The CVD process and their associated parameters affect the structure of the resulting nanotubes. In this work CNT growth has been studied on different supported catalysts with different rates of gas flow as one of the critical points. Different supports were prepared by mixing nanosized alumina with tetraethyl orthosilicate (TEOS) by a chemical method at low temperature and iron as the metal catalyst was impregnated by 5%, 10% and 20% weight of the supports. Methane was used as a carbon source for the synthesis of CNTs at 800°C-1000°C. Aluminium-based support, supported catalysts and CNT samples have been characterised by TEM, SEM, BET and XRD.

Bulk Cu−Ti alloy reinforced by TiB 2 nano particles was prepared using in-situ reaction between Cu−3.4%Ti and Cu−0.7%B master alloys along with rapid solidification and subsequent heat treatment for 1−10 h at 900 °C. High-resolution transmission electron microscopy (HRTEM) characterization showed that primary TiB 2 nano particles and TiB whiskers were formed by in-situ reaction between Ti and B in the liquid copper. The formation of TiB whiskers within the melt led to coarsening of TiB 2 particles. Primary TiB 2 particles were dispersed along the grain boundaries and hindered grain growth at high temperature, while the secondary TiB 2 particles were formed during heat treatment of the alloy by diffusion reaction of solute titanium and boron inside the grains. Electrical conductivity and hardness of the composite were evaluated during heat treatment. The results indicated that the formation of secondary TiB 2 particles in the matrix caused a delay in hardness reduction at high temperature. The electrical conductivity and hardness increased up to 8 h of heat treatment and reached 33.5% IACS and HV 158, respectively.

To cite this Article Bodaghi, M. , Mirhabibi, A. R. , Zolfonun, H. , Tahriri, M. and Karimi, M.(2008) 'Investigation of phase transition of γ-alumina to α-alumina via mechanical milling method', Phase Transitions, 81: 6, 571 -580 To link to this Article:

Sol-gel, a commonly used technique, involves the formation of an amorphous gel from a precursor solution. In this research nanocrystalline alumina powders have been synthesized through a new sol-gel simplified method. The route involved using an environmentally low-cost binder and aluminum nitrate in an aqueous medium. The added metal ions were dispersed in the gelled binder matrix. The gelled mass was dried and characterized by simultaneous thermogravimetric and differential thermal analysis (TG-DTA) and calcined at 350, 400, 530, 650, and 900°C for 2 h. The samples were evaluated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The used binder can be applied to form a perfect matrix for the entrapment of metal ions that during heat treatment, gives rise to ultrafine amorphous or crystalline alumina particles. In this study, the transition phases of alumina with particle sizes of 15-25 nm as well as aluminum oxide hydrate phase were produced by the heat treatment of the gelled mass at temperature 650°C. By increasing the calcinations temperature to 900°C, alumina transition phases were changed and aluminum oxide hydrate eliminated. The particle sizes at this temperature were obtained in the range 15-30 nm. The particle size and crystallite size of products measured from XRD and TEM were in good agreement.

The raw materials, condition and the method of preparing the support play an important role in the growth of high quality single-walled carbon nanotubes. These effects have been studied in the catalytic chemical vapor deposition (CCVD) growth of SWNTs on Alumina or Alumina-Silica as a support and iron as a supported catalyst. Tetraethyl Orthosilicate (TEOS) with Alumina were used for support preparation. The supports were prepared at low temperature(less than 90˚C) by chemical method. Methane as a carbon source at 800-1000 ˚C was used for the SWNTs synthesis. Alumina-based supports, supported catalysts and SWNT samples have been characterized by TEM, SEM and XRD.

Copper matrix composite reinforced by 1wt.% TiB 2 particles was prepared using in situ reaction of Cu-1.4wt.% Ti and Cu-0.7wt.% B by rapid solidification and subsequent heat treatment for 1-20 hrs at 900ºC. High-resolution transmission electron microscopy (HRTEM) characterization showed that primary TiB 2 particles were formed in liquid copper. Heat treatment of as-solidified samples led to the formation of secondary TiB 2 particles via spinodal decomposition of titanium-rich zone inside the grains. Mechanical properties (after 50% reduction in area) as well as electrical conductivity of composite were evaluated after heat treatment and were compared with those of pure copper. The results indicated that, due to the formation of secondary TiB 2 particles in the matrix, electrical conductivity increased along with hardness up to 10 hrs of heat treatment and reached 65% IACS and 155 HV, respectively. Moreover, the maximum ultimate (i.e. 580 MPa) and yield (i.e. 555 MPa) strengths of co...