Intensification of a high-temperature phase transformation by ball milling

This work focuses on the allotropic phase transformation occurring in Co powders at high temperature. First, we studied the phase transformation kinetics in Co powders that are exposed to a temperature of 750 K while being stirred. Then, we investigated the phase transformation kinetics in Co powders that are ball milled at the same temperature. We observe that the mechanical processing induces a significant intensification of the phase transformation, which we relate to the effects of individual impacts occurring during the mechanical processing. We developed an analytical kinetic model that provides a reliable description of the phase transformation kinetics while accounting for the discontinuous nature of ball milling. We not only explain the observed intensification on a sound phenomenological basis, but also define an intensification index to measure the extent of intensification. The results can be extended to other cases where ball milling intensifies a transformation process.