SOLID-STATE REACTION INDUCED BY MILLING OF A MIXTURE OF COBALT AND BORON POWDERS

A mixture of polycrystalline elemental cobalt and boron powders in the atomic ratio Co67B33 was mechanically milled for 150 h. The milled powders were examined by X-ray diffraction, extended X-ray absorption fine structure spectroscopy and differential scanning calorimetry. Two steps where energy was regulated by different full/void volume ratio in the vials were carried out. In the first milling step (f/v = 1/1) amorphization involves only a portion of the starting powder and reaches a steady state after 25 h. More energetic conditions (f/v = 1/5) lead to almost complete amorphization of the sample after 70 h, and in the final stages the formation of crystalline t-Co2B is observed. Then a steady state, in which both amorphous and crystalline phases coexist, is reached.