Solid-state reactions during heating mechanically milled Al/TiO 2 composite powders
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12/29/04
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Solid-State Reactions during Heating Mechanically Milled Al/TiO2 Composite Powders D.Y. YING, D.L. ZHANG, and M. NEWBY Solid-state reactions between Al and TiO2 during heating high-energy mechanically milled Al/TiO2 composite powders have been investigated by using a combination of thermal analysis, X-ray diffraction (XRD), and various microstructural characterization techniques. When the TiO2 particles and their interparticle spacing in the Al/TiO2 composite powder particles are sufficiently large, the reaction between Al and TiO2 proceeds by two steps. The low-temperature step is an interfacial reaction, which starts at a temperature close to 660 °C. The high-temperature step is a reaction facilitated by bulk diffusion and starts at a temperature above 820 °C. The first phase formed from the reaction is always Al3Ti irrespective of the starting powder composition or milling time. Al2O3 is difficult to form at temperatures below 800 °C. The formation of the -Ti(Al,O) phase proceeds slowly and requires either continuous heating to a temperature above 1000 °C or holding at a temperature close to 1000 °C for a period of time. Mechanical milling of the Al/TiO2 powder enhances the interfacial reaction between Al and TiO2. This enhancement is originated from the establishment and refinement of Al/TiO2 composite microstructure.
I. INTRODUCTION
CHEMICAL reactions between Al and suitable metal oxides, such as CuO, Fe2O3, and TiO2, have been widely used to produce in-situ metal-matrix composites (MMCs)[1–4] or intermetallic matrix composites (IMCs).[5–10] This process makes use of the fact that the reduction of such oxides by Al results in formation of another metal and aluminum oxide (typically Al2O3). The metal can be a good matrix material by itself, or a good alloying element for Al matrix, or form intermetallic phases with Al, which can also work as a matrix material. On the other hand, the aluminum oxide, especially -Al2O3, is a good ceramic reinforcement material for MMC or IMC. The use of the reaction between Al and TiO2 in producing in-situ MMCs and IMCs is particularly attractive, since Al-Ti or Ti-Al alloys or titanium aluminides are favorable matrix materials. Based on this consideration, a substantial amount of research work in this area has been undertaken and published.[2–10] The work was mainly focused on producing Al alloy–based MMCs and Al3Ti-based IMCs. There has not been much work on producing Ti-rich alloys or intermetallic matrix composites using this method.[5,10] High-energy mechanical milling is a very effective process for producing composite powders.[11,12,13] When the starting phases of the composite powder are not in equilibrium, each of the composite powder particles virtually contains numerous reaction couples. The establishment of the composite structure of the powder particles facilitates intimate and contamination free contact between reactants, while the refinement of the composite structure leads to reduction of the size of the reaction couples. This wil
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