Mechanical alloying in Cu-V and Cu-Ta systems characterized by positive heat of mixing (overview)

The mechanical alloying process has been studied on the Cu-V and Cu-Ta systems, both of which are characterized by a positive heat of mixing. The EXAFS and neutron diffraction measurements have been employed as main tools to analyze the structural changes taking place during milling. The extended bcc solid solution was formed in the Cu-V system, whereas an amorphous phase in the Cu-Ta system. The local atomic structure in the amorphous Cu-Ta is compared with that in the amorphous Ni-Ta with a large negative heat of mixing. The elementary process toward the amorphization is found to be common in both systems and to be described as the preferential penetration of smaller atoms Cu or Ni into the bcc Ta crystallites. Studies of the ambient temperature effect in the Cu-Ta system confirmed that no alloying occurs until an average grain size of Cu and Ta crystallites is reduced to about 10 nm and, once it reaches this level, an increase in the interfacial energy is large enough to allow the formation of an amorphous phase. The thermally assisted amorphization has been observed in the Cu-Ta, indicating that the assembly of fine Cu and Ta crystallites behaves like a system with a negative heat of mixing. (author)