Welcome to Dr. M. Sherif El-Eskandarany
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Mechanical Alloying for Fabrication of Amorphous, Nanocrystalline and Nanocomposites at Room Temperature | ||||||||||||||||||||||||
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Dr. Eng. M. Sherif El-Eskandarany Professor of Materials and Metallurgy Mining and Petroleum Engineering Department, Faculty of Engineering, Al-Azhar University, Nasr City, Cairo 11371, Egypt | |||||||||||||||||||||||
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Mechanical alloying (MA) has received much attention as a powerful processing tool for fabrication of several advanced materials,using ball-milling and/or rod-milling techniques. The application of this process leads to the formation of new materials, including equilibrium, nonequilibrium (e.g., amorphous, quasicrystals, nanocrystalline, etc.), and composite materials. In addition, MA could be employed for reducing some metallic oxides by milling the oxide powders with certain metallic reducing agents at room temperature. This book commences with a general description of the mechanical alloying process, including starting material requirements, the equipment, characterizations of the milled powders, and consolidation techniques which used to compact the powder into fully-dense bulk materials. BACKGROUND Fundamentally, the term milling may be referred to breaking down the relatively coarse material to the ultimate fineness. Apart from the milling of ores, milling is also used for preparing the materials for industrial applications, such as milling of quartz to fine powder (under 70 mm in diameter), milling of talc to produce body powder, milling of iron ore for preparation of pellets, and many others. About thirty years ago, Benjamin introduced a pioneering development on ball-milling technique for producing complex oxide dispersion-strengthened (ODS) alloys that are used for high temperature structural applications such as jet engine parts. This method, which termed mechanical alloying (MA), could be successfully used for preparing fine, uniform dispersions of oxide particles (Al2O3, Y2O3, ThO2) in nickel-base superalloys. These materials cannot be obtained by the conventional powder metallurgy method. Benjamin and his coworkers at the International Nickel Company (INCO) in New York also investigated the fabrication of other kinds of materials, including solid solution alloys, and metal composites with unique mechanical properties. During the 1970's, the fundamental researches on MA was concentrated on ODS alloys. In 1979, White observed the formation of an amorphous phase by ball-milling elemental Nb and Sn powders at room temperature. In 1983, Koch et al. have reported the first novel technique for formation of Ni60Nb40 amorphous alloy by high-energy ball milling of elemental Ni and Nb powders. Since then, MA method has been successfully employed for formation of large number of amorphous alloys. In 1992, El-Eskandarany et al. were employed a high-energy ball mill for preparing nanocrystalline metal nitrides (TiN and Fe4N) by milling the elemental powder under nitrogen gas flow. Several authors have then prepared metal nitrides and metal hydrides using the same technique. Within the last five years, El-Eskandarany et al. have proposed the ball-milling technique for formation of nanocrystalline refractory materials, including metal nitrides, metal carbides, and noncomposite refractory materials. They have consolidated the end-product of the milled powder into fully-dense nanocrystalline compacts that have unusual unique physical and mechanical properties. In 1998, El-Eskandarany prepared homogeneous nanocomposite Al/SiCp materials by milling the elemental powders of Al and b-SiC in a high-energy ball mill. It should be notified that this composite material is difficult to be obtained by the conventional liquid metallurgy method due to the poor wettability between molten Al (or Al alloys) and the reinforcement material of SiC. In addition, the liquid metallurgy method usually leads to an undesirable reaction between SiC and molten Al, producing a brittle phases of Al4C3 and Si. More recently, it has been demonstrated by El-Eskandarany et al. the formation of ceramic/ceramic nanocomposite WC-14 % (at.) MgO material that combines two contrast properties of high values of hardness and fracture toughness. | |||||||||||||||||||||||
Mechanical Solid State Mixing for Synthesizing | ||||||||||||||||||||||||
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High-energy ball mill was successfully employed to fabricate metal matrix composite (MMC) of Al reinforced with SiC particulate (SiCp) with distinct nanocrystalline characteristics. The powders of the end-products (86 ks of milling time) of the mechanically-mixed nanocomposites SiCxAl100-x (x = 2, 5, 7, 10 vol.%) that have spherical-like morphology (less than 0.5 mm in diameter) were consolidated into bulk compacts, using a plasma activated sintering method. Density measurements show that this consolidation step leads to the formation of fully dense bulk samples. Moreover, the consolidated SiCxAl100-x samples reveal nanocrystalline structure with a homogeneous reinforcement distribution of SiCp into the Al matrix. Reactive products, i.e. Al4C3 and Si, could not be detected on the interfaces between the reinforcement and the matrix in both mechanically mixed powders and consolidated samples. The mechanically mixed powders and consolidated compacts have been characterized by means of x-ray diffraction, scanning and transmission electron microscopes, optical metallography and chemical analyses. The hardness and some mechanical properties of SiCp/Al have been determined as a function of SiCp content. |
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