Cobalt-carbon (C:5-67 at.%) alloys were prepared by co-sputtering. Their crystallization behavior was studied by in situ transmission electron microscopy and differential scanning calorimetry. Alloys with more than 10 at.% carbon show an amorphous structure. On heating, two exothermic peaks were observed at 250-350 and 380-420°C for the alloys with less than 35 at.% carbon. These peaks are due to formation of a metastable δ′-phase (orthorhombic, a = 2.84, b = 4.58 and c = 4.42 A ̊) and closed-packed hexagonal (c.p.h.) cobalt, and to decomposition of the δ′-phase to c.p.h. cobalt and graphite, respectively. A new carbide phase was also found whose unit cell is simple cubic with a = 5.82 A ̊. For alloys with more than 40 at.% carbon, the excess free-energy is reduced by precipitation of c.p.h. cobalt while carbon remains amorphous.
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