The transformation of co-rich alloys produced by mechanical alloying

Elemental powder mixtures of Co and Ti were subjected to high-energy ball milling in order to produce mechanically alloyed powders with nominal compositions Co₆₄Ti₃₆, Co₆₄Ti₃₆, Co₇₀Ti₃₀₃₀, Co₇₆Ti₂₄, and Co₈₅Ti₁₅. The mechanically alloyed powders were treated during 30 minutes in inert atmosphere at temperatures in the range 300 - 700 °C. Both the as-milled powders as well as those subjected to heat treatments have been characterized by x-ray diffraction, scanning electron microscopy, energy-dispersive x-ray spectrometry and differential thermal analysis. As-milled products consist mostly of agglomerated powders with a size between 10 and 80 μm which give an amorphous-like diffraction pattern, except for the Co₈₅Ti₁₅ sample whose pattern presents the characteristic peaks of the Co₃Ti intermetallic phase. The transformation of the asmilled powders occurs at temperatures in the range of about 530 - 670 °C with clearly observed exothermic events. The Co₃Ti phase is found in all heat treated samples, together with fcc-Co (in Co₇₆Ti ₂₄ and Co₈₅Ti₁₅) or the hexagonal Co ₂Ti intermetallic phase (in Co₆₄Ti₃₆, Co ₆₇Ti₃₃; and Co₇₀Ti₃₀); the Co ₇₃Ti₂₇ sample was essentially single-phase Co ₃Ti after heating to 700 °C Our results suggest the occurrence of crystallization of an amorphous phase in two overlapping stages during heating of the mechanically alloyed powders.