Phase evolution in Ni-Nb multilayers upon solid-state reaction

Solid-state amorphization was achieved in the Ni,,Nb,, multilayers upon the rmal annealing by gradually raising the temperature from 250 to 400 degrees C and staying at 400 degrees C for 2 h. More interestingly, before complet e amorphization, a sequential disordering of first Ni and then Nb crystalli ne lattices was observed for the first time, and it was essentially the phy sical origin of an asymmetric growth of the amorphous interlayer during sol id-state reaction reported previously in some binary metal systems. In anot her two multilayered samples with overall compositions of Ni64Nb36 and Ni70 Nb30, thermal annealing under similar conditions resulted in the formation of two metastable crystalline phases with face-centered-cubic and hexagonal -close-packed structures, respectively, although an amorphous phase also ap peared and coexisted with one of the metastable crystalline phases in the i ntermediate states. In the ion mixing experiment, such sequential disorderi ng, as well as formation of metastable phases, was also observed in the res pective Ni-Nb multilayers upon room-temperature 200-keV xenon ion irradiati on. Comparatively, however, ion irradiation eventually induced complete amo rphization in all the multilayers at the respective doses, indicating that ion-induced disordering frequently predominated in the competition between amorphization and the growth of a metastable crystalline phase. A Gibbs fre e energy diagram, including the free energy curves of the newly formed meta stable crystalline phases, of the Ni-Nb system was calculated based on Mied ema's model. The constructed free energy diagram can give reasonable explan ations of the sequential disordering and the thermodynamic possibility of t he formation of either an amorphous or a metastable crystalline phase, of w hich the free energy difference was quite small. It follows naturally that the phase selection, namely, which phase was more favored to be formed even tually than its competitors, was influenced or even determined by the kinet ics involved in the respective processes.