Formation of nonequilibrium solid phases by ion irradiation in the Ni-Ta system and their thermodynamic and growth-kinetics interpretations

In the Ni-Ta system, several amorphous alloys were synthesized by room-temperature 200-keV xenon-ion irradiation of multilayered films with various compositions. The metallic-glass-forming range was determined experimentally to extend from 25 to 75 at. % Ni. In addition, a hcp metastable crystalline (MC) phase was observed in the Ni-rich Ni80Ta20 and Ni75Ta25 multilayers at different irradiation stages, and a fcc MC phase was formed in the Ta-rich Ta65Ni25 and Ta75Ni35 multilayers before and after the films were amorphized, respectively. Interesting evolution behavior of the above nonequilibrium solid phases upon thermal annealing was also observed. A free-energy diagram of the system was constructed based on the model of Miedema, de Boer, and de Boer and the method of Alonso and Simozar. The calculation included the free-energy curves of all the competing phases, especially the MC phases, which were considered as compoundlike, and the enthalpies were calculated based on well-identified structural parameters. The calculated energy sequence of the phases was verified at certain compositions by the phase appearance order in the respective as-deposited multilayers upon steady-state thermal annealing, thus confirming the relevance of the diagram in its general outline. The free-energy diagram can explain the formation and the thermal stability of the above solid phases. The growth kinetics of the hcp and fcc MC phases were also discussed in terms of the structural compatibility between the newly formed phase and the matrix in the ion-irradiation process. © 1994 The American Physical Society.