Stability of the metastable phases in the Co-Ta system studied by ab initio and thermodynamic calculations together with ion-beam-mixing experiment

Employing ab initio calculations, the relative stabilities of the possible crystalline structures at compositions of CoTa3, Co3Ta and CoTa were predicted and based on thermodynamic calculations, the possible structural phase transitions were predicted over a broad composition range for the metastable crystalline Co-Ta alloys. To confirm the relevance of the calculations, ion-beam-mixing experiments were conducted using Co-Ta multilayered films with various overall compositions. It turned out that the experimentally observed formation of metastable crystalline Co-Ta phases agreed reasonably well with the theoretical predictions and that the glass-forming range of the Co-Ta system was determined to be within 25-70 at. % of Ta, matching fairly well with that estimated from thermodynamic calculations. Moreover, a reverse martensitic phase transformation was observed in the CoTa2 multilayers upon ion irradiation. In addition, steady-sate thermal annealing of the Co-Ta multilayered films also provided some evidence in support to the ab initio and thermodynamic calculations.