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چکیده
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An effective method used for the preparation of an intermetallic matrix nanocomposite powder is mechanical alloying realized in a mill using the well-known exothermic reduction reactions of oxides with active metals such as Al [1]. So in this study, the production of CoAl-Al2O3 nanocomposite was conducted by mechanical alloying of a stoichiometric mixture of cobalt oxide and aluminium. The XRD patterns of Co3O4–39wt.% Al powder mixture at various milling times are presented in Figure 1. The formation mechanism is suggested to be two separate reactions which occur simultaneously, i.e. the explosive alominothermic reaction causes high heat release and ignites the formation of CoAl.
After 25 h MA, the grain size for CoAl and Al2O3 was found to be 19 and 26 nm, respectively. In addition, the results showed that the initial MA product was ordered CoAl which was changed to a partially disoredered structure with a steady long-range order of 0.83 at prolonged milling (Table I).
The triple defect mechanism was proposed to be responsible for disordering of CoAl phase [2]. Calculation of enthalpy related to the triple defects formation using Miedema’s extended model [3] revealed that the enthalpy required for Al anti-sites formation was about 3 times greater than that for Co anti-sites formation (Table II).
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