Diffusion in ', at temperatures of between 1298 and 1573K (table 8), was studied by using the diffusion couples: Ni-24.2Al/Ni-24.4Al-2.91at%Co, Ni-24.2Al/Ni-23.1Al-2.84at%Cr and Ni-24.2Al/Ni-20.9Al-3.17at%Ti. The diffusion profiles were measured by using an electron probe micro-analyzer, and the diffusion coefficients of Co, Cr, and Ti in ' which contained 24.2at%Al, were deduced from those diffusion profiles by using the Hall method. The temperature dependences of the diffusion coefficients were given by:

Co:     D(m2/s) = 4.2 x 10-3exp[-325(kJ/mol)/RT]

Cr:     D(m2/s) = 1.1 x 10-1exp[-366(kJ/mol)/RT]

Ti:     D(m2/s) = 5.6 x 101exp[-468(kJ/mol)/RT]

The activation energies increased in the order: Co, Cr, Ti, and were closely related to the substitutional behavior of Co, Cr, and Ti atoms in the L12 lattice sites of '. The Co atoms, which occupied the face-centered sites of the L12 structure, diffused according to the usual activation energy whereas the Ti atoms, which occupied cubic corner sites, diffused according to a larger activation energy that included the energy which was due to local disordering caused by atomic jumps. The Cr atoms, which could occupy both types of site, diffused according to an activation energy which was similar to that for Co atoms.

Diffusion of Cobalt, Chromium, and Titanium in Ni3Al. Minamino, Y., Jung, S.B., Yamane, T., Hirao, K.: Metallurgical Transactions, 1992, 23A[10], 2783-90

Figure 8

Bulk diffusion of B, Ga, Ge, Nb, Ni and Ti in Ni3Al