MECHANICAL ALLOYING IN CU-V AND CU-TA SYSTEMS CHARACTERIZED BY POSITIVE HEAT OF MIXING

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 mill ing. The extended bcc solid solution was formed in the Cu-V system, wh ereas an amorphous phase in the Cu-Ta system. The local atomic structu re 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 t he amorphization is found to be common in both systems and to be descr ibed as the preferential penetration of smaller atoms Cu or Ni into th e bcc Ta crystallites. Studies of the ambient temperature effect in th e Cu-Ta system confirmed that no alloying occurs until an average grai n size of Cu and Ta crystallites is reduced to about 10 nm and, once i t reaches this level, an increase in the interfacial energy is large e nough to allow the formation of an amorphous phase. The thermally assi sted amorphization has been observed in the Cu-Ta, indicating that the assembly of line Cu and Ta crystallites behaves like a system with a negative heat of mixing.