Formation and Absence of Intermetallic Compounds during Solid-State Reactions in the Ni−Bi System

The reactions between Ni and NiBi 3 at 330, 370, 410, and 450 °C were studied. The compound NiBi 3 was prepared by melting a mixture of 99.9994% Bi shots and 99.996%, −120 mesh Ni powder. It was found that the synthesized NiBi 3 is soft and brittle. This is contrary to the characteristics of common intermetallics, which are often hard and brittle. Such inferior mechanical properties make the formation of NiBi 3 in solder joints very undesirable. The reaction product was a layer of NiBi that grew parabolically, suggesting diffusion-controlled kinetics. The activation energy for the growth of NiBi is 84 kJ/mol. The fact that NiBi formed here, but not in the reaction between Ni and Bi as reported in the literature, suggests that the reason for absence of NiBi is not due to difficulty in nucleation. The more likely reason is that the interdiffusion coefficient of NiBi is much smaller than that of NiBi 3 . The dominant diffusing species in NiBi was analyzed by a marker movement experiment. It was found that the Bi flux through NiBi is 3−9 times greater than Ni flux at 370 °C. An expression relating the marker position to the ratio of Bi flux to Ni flux was proposed.