Amorphous-nanocrystalline Co-Ti alloys consolidated by plasma-assisted hot pressing

Co-Ti powder alloys were prepared by mechanical alloying of Co-Ti elemental powder mixtures of the following nominal compositions: Co₃₃Ti₆₇, Co₅₀Ti₅₀, Co₆₇Ti₃₃, Co₇₅Ti₂₅. The as-milled powders contained a dispersion of nanocrystalline phases within an amorphous matrix, as observed by x-ray diffraction (XRD), transmission electron microscopy (TEM) and electron diffraction and microdiffraction techniques. The mechanically alloyed powders were consolidated by plasma-assisted hot pressing at a temperature of 1173 K under pressures in the range of 50 to 60 MPa. The average grain size of the consolidated products varied from about 80 to 100 nm for the alloys with nominal compositions Co₅₀Ti₅₀, Co₆₇Ti₃₃ and Co₇₅Ti₂₅, being much larger (approx. 500 nm) for the Co₃₃Ti₆₇ alloy. The analysis of phases in the consolidated materials by XRD and TEM indicated the existence of two major constituents in the three alloys with higher Co contents which were identified as Co(Fe)-Ti intermetallic compounds. Microanalysis by energy dispersive spectroscopy (EDS) indicated a systematic deviation of the composition of the consolidated samples towards lower cobalt contents, as well as significant iron and chromium contamination from the milling media. Estimated porosity levels were lower than approx. 5%, while average microhardness values were found between 650 and 840 kg/mm².