Comparative Study of Mechanical Alloying Induced Nanocrystallization and Amorphization in Ni-Nb and Ni-Zr Systems

摘要

The nanocrystallization and amorphization processes in Ni_(60)Nb_(40) and Ni_(60)Zr_(40) binary alloys during mechanical alloying (MA) were studied in detail. The mechanical alloying behavior of these alloy systems was compared with respect to the rate of refinement of grain size, ultimate grain size, and rate of amorphization reaction. For both compositions, MA leads to the refinement of grain size and enhancement of internal strain, followed by the amorphization reaction. The higher melting temperature metal Nb exhibits smaller grain size and greater internal strain, although the Ni and Nb grain size approach a similar value of ～15 nm after 20 hours of milling time. The refinement of grain size and enhancement of internal strain was observed to occur with a slower rate during MA of Ni_(60)Zr_(40) alloy compared to Ni_(60)Nb_(40) alloy. In all cases, an ultrafine layered structure with a typical thickness of 30 nm, containing nano-scale size grains with a typical size of 15 nm and a high density of dislocations, develops prior to the amorphization reaction. This observation suggests that numerous high-speed diffusion paths such as grain boundaries and dislocations are necessary to allow a high diffusion rate at low temperature and therefore permits the amorphization reaction to take place kinetically. The Ni-Zr system is a better glass former in MA than Ni-Nb system; i.e., the start time of amorph ization reaction for Ni_(60)Zr_(40) was about half that for Ni_(60)Nb_(40). These results were discussed in terms of physical and chemical characteristics of the constituent elements of the alloy systems. Furthermore, the thermodynamically stable phase in each system was predicted using a semi-empirical Miedema model, and the results were compared with the structure formed in MA of Ni-Zr and Ni-Nb powder mixture.