New insights of solid-state alloying and amorphous-nanocrystalline cyclic phase transitions during Cr-40wt.%Mo powder milling

In this paper, the solid-state alloying of Cr-40wt.%Mo powder by high-energy milling and corresponding amorphous-nanocrystalline cyclic phase transitions were investigated. XRD, TEM and DSC analyses were utilized for phase, microstructure and thermodynamics stability characterization of the milled powders, respectively. The results demonstrate high-energy milling can induce severe lattice distortion and further amorphization of milled powder. Free volume/flow unit widely exists within the amorphous structure, greatly increasing the storage energy of milled powder and subsequently decreasing the thermodynamic barrier of atomic migration. For solid-state alloying of Cr-40wt.%Mo powder, there are two paths of atomic diffusion: diffusion reaction induced by crystal defects and diffusion reaction induced by free volume within the amorphous structure. Coexistence of Cr(Mo)0, Cr(Mo) and Mo(Cr)0 multi-phases is also found in this experiments. Furthermore, the cyclic phase transitions between amorphous alloy phase and nanocrystalline alloy phase are presented and discussed in details. It is concluded that the cyclic phase transitions mainly depend on the evolution of microstructure and storage energy, which are originally influenced by the reciprocating shear deformations during high energy milling.