Comparison of metastable phases induced by heat treatment of unmilled and milled cobalt powders

The sintered and water quenched compact samples were prepared from unmilled and milled Co powders. Characterisation was performed by differential scanning calorimetry, scanning electron microscopy and X-ray diffraction techniques. Several metastable phases were obtained upon sintering and quenching. However, more metastable phases were induced on quenching the milled–sintered samples due to introduction of large number of defects in addition to those induced by milling. Micro hardness values for unmilled sintered samples were the lowest while those of 30 h milled-sintered samples were the highest. The current study reveals that the two FCC metastable phases obtained by quenching unmilled powder were similar to those found in milled–sintered samples.

► WQ–unmilled Co yielded HCP, two FCC phases with a = 3.54 Å and 4.25 Å.
► 30 h milled–sintered resulted in two FCC phases with a = 3.55 Å and 4.21 Å.
► WQ of 30 h milled Co yielded MCL, CUB, RHL and FCC2 phases.
► Milled and sintered samples have the highest hardness values.