The self-diffusion of 63Ni in monocrystals of these ordered intermetallic compounds was investigated (table 122). It was found that the activation energies for diffusion perpendicular to, and parallel to, the hexagonal c-axis were almost the same and lay between 2.14 and 2.34eV:

Ni61Sn39,  c-axis:     D (cm2/s) = 0.51 exp[-2.14(eV)/kT]

Ni61Sn39, || c-axis:     D (cm2/s) = 0.95 exp[-2.24(eV)/kT]

Ni62Sn38,  c-axis:     D (cm2/s) = 0.61 exp[-2.16(eV)/kT]

Ni62Sn38, || c-axis:     D (cm2/s) = 0.70 exp[-2.21(eV)/kT]

Ni64In36,  c-axis:     D (cm2/s) = 63.2 exp[-2.31(eV)/kT]

Ni64In36, || c-axis:     D (cm2/s) = 108 exp[-2.34(eV)/kT]

Moreover, the Do values were direction-dependent and ranged from 0.5 to 0.8cm2/s for Ni-Sn alloys and from 65 to 110cm2/s for Ni-In alloys. The experimental values of the D/D|| ratio were between 1.13 and 1.68 for Ni-Sn alloys and between 0.87 and 0.79 for Ni-In alloys; depending upon the temperature. It was suggested that the principal diffusion mechanism should be a site change of the tracer atom between the Ni chains in the c-direction and the double tetrahedral interstices, with a further jump to another chain. However, 2 other minor mechanisms were possible. With increasing temperature, and filling of the double tetrahedral interstices, the importance of the minority mechanisms increased.

H.Schmidt, G.Frohberg, H.Wever: Acta Metallurgica et Materialia, 1992, 40[11], 3105-11

Table 122

Diffusion coefficients for 63Ni in Ni-In and Ni-Sn compounds

* with respect to c-axis