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The nanostructured CoAl intermetallic compound was produced by mechanical alloying (MA) of the Co50Al50
elemental powder mixture in a planetary high energy ball mill. The ordered B2-CoAl structure with the grain
size of about 6 nm was formed via a gradual reaction after 10 h of MA. A thermodynamic analysis of the
process was also done. The results showed that the intermetallic compound of CoAl had the minimum Gibbs
free energy compared to solid solution and amorphous states indicating the initial MA product was the most
stable phase in the Co-Al system which was changed to a partially disordered structure with a steady long-range
order of 0.82 at further milling. This amount of disordering caused the enthalpy of ¯nal product to show an
increase of about 5.1 kJ¢mol?1. Calculation of enthalpy related to the triple defect formation revealed that the
enthalpy required for Al anti-sites formation was about 3 times greater than that for Co anti-sites formation.
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