Thermal effects in mechanically induced phase transformations in Co powders

The mechanical processing of Co powders by ball milling is known to induce a phase transformation between the two allotropic forms of Co with hexagonal close-packed and face-centred cubic crystalline structures. The same phase transformation can be also activated and driven by gradual heating or isothermal annealing. For this reason, it is reasonable to expect that temperature plays an important role also during the mechanical processing. In this work, we investigate exactly how temperature affects the phase transformation behaviour by ball milling Co powders at different milling frequencies and temperatures between 300 K and 500 K. Unexpectedly, we find that, regardless of the milling frequency, any temperature increases results in a decrease of the phase transformation rate. We tentatively explain the experimental findings by considering thermally activated recovery processes.