Synthesis of CoAl/Al 2 O 3 nanocomposites and their thermodynamic and kinetic analysis In this research, synthesis of the nanostructured CoAl intermetallic compound and CoAl/Al 2 O 3 nanocomposites were investigated by mechanical alloying (MA). The phase transformations and microstructural changes were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential thermal analysis (DTA). The results showed that MA of the Co 50 Al 50 powder mixture resulted in the direct formation of the nanostructured fully ordered B2-CoAl intermetallic compound by gradual mechanism. Thermodynamics analysis using Miedema model confirmed that the formed phase had the minimum free energy. By incresing MA time, disordering of the CoAl compound was occurred with triple defects formation mechanism and long range order (LRO) parameter reduced to a constant value of 0.82. A general equation was proposed for description of grain growth kinetics of the nanostructured CoAl compound at temperatures above 0.5 T m . During development of CoAl/Al 2 O 3 nanocomposites, it was observed that thermite reaction was occurred abruptly between Co 3 O 4 and Al reactants and Al 2 O 3 /Co nanocomposite was synthesized. Thermal analysis results showed that the reduction reaction of Co 3 O 4 completed in two steps with formation of the intermediate phase of CoO. Mechanical activation of the initial powder mixture decreased the reduction reaction temperature from 1041 to 869 ? C. The activation energies of reduction reactions, calculated using five different model-free methods, were determined to be about 340±5 and 440±7 kJ/mol on a conversion fraction of 0.5. The reaction mechanism was determined to be one dimensional phase controlled reactions and three dimensional surface reactions. The CoAl/Al 2 O 3 nanocomposites containing 10, 25 and 45 vol.% Al 2 O 3 were fabricated by MA of different ratios of Co 3 O 4 -Al-Co powder mixtures. The results showed that the reduction of Co 3 O 4 and CoAl formation reactions occurred simultaneously in a combustion mode.
