Development and characterization of CoAl–Al2O3 intermetallic matrix nanocomposite

CoAl–Al2O3 nanocomposite was synthesized by mechanical alloying (MA) of Co3O4–Al powder mixture. The reduction of Co3O4 by Al and CoAl formation occurred simultaneously in a combustion mode. The resulting CoAl phase had an ordered structure with a long range order (LRO) parameter of 0.99 which was reduced to a constant value of 0.83 by further milling. Lattice parameter measurements showed that disordering of CoAl was caused by triple defect mechanism. Three kinds of structural evolutions namely, reordering and disordering, strain relaxation and grain growth were observed during subsequent isothermal annealing at different temperatures. Isothermal annealing at temperatures below 773 K resulted in a fully ordered CoAl while at higher temperatures disordering occurred again. Decrease of LRO parameter at higher temperatures was interpreted to be due to the thermal activation, which induced triple defects. Grain growth rate of constituent phases was observed to reduce at higher temperatures.