Allotropic transformation of cobalt induced by ball milling

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We have found that phase transformation can occur in cobalt when subjected to ball milling. The phase formation of cobalt was found to depend on the milling intensity. Under different milling intensity or different milling time, the phase transformations follow the routes of h.c.p. + f.c.c. → h.c.p., h.c.p. + f.c.c. → h.c.p. → f.c.c. + h.c.p. and h.c.p. + f.c.c. → h.c.p. → f.c.c. + h.c.p. → f.c.c., respectively. Our results indicate that the phase formation of cobalt induced by ball milling was determined by the accumulation of structural defects and not by the local temperature rise. Different milling intensity may adjust the rate and level of the accumulation of defects. The as-obtained f.c.c. cobalt is stable and it did not return to the h.c.p. state after annealing at different temperatures. While the h.c.p. phase is not stable, and it partly converted to the f.c.c. phase after annealing at 450°C. The fact was interpreted as being caused by the grain size effect, and it was further proved that the small grain size tends to stabilize the f.c.c. structure of cobalt.