Anomalous alloying behavior induced by ion irradiation in a system with a large positive heat of mixing

Amorphization was achieved by room temperature 200-keV xenon ion mixing in two isolated composition ranges, i.e., ${\mathrm{Y}}_{22}$–${\mathrm{Y}}_{27}$ and ${\mathrm{Y}}_{72}$–${\mathrm{Y}}_{83}$, in the Y-Mo system, which has a positive heat of mixing exceeding +26 kJ/mol. In addition, three metastable crystalline (MX) phases, i.e., a Y-rich fcc (fcc-I), a Mo-rich hcp (hcp-I), and another fcc (fcc-II) phase, were observed at different irradiation stages. Kinetically, the fcc-I MX phase was formed through a single-step transition of hcp→fcc, while the hcp-I and fcc-II phases were formed sequentially via a two-step transition of bcc→hcp→fcc. To give further insight into the singularity of the amorphization behavior of the system and the MX phase formation, a Gibbs-free-energy diagram of the system was constructed by calculating the free energies of the amorphous phase, the solid solutions, three MX phases, and of the as-deposited Y-Mo multilayers, for which the interfacial free energies were added. The established diagram can explain well the irradiation-induced amorphization behavior and the MX phase formation in the Y-Mo system.