Self- and impurity diffusion was investigated in Ni3Al single crystals and polycrystals (table 7 and figure 8). The Ni self-diffusion was investigated by using two techniques for profile detection. At high temperatures, profiles were obtained by using an improved serial sectioning technique while, at low temperatures and small diffusion coefficients, respectively, concentration profiles were determined by secondary ion mass spectrometry. It was concluded from these data that Ni diffusion occurred via nearest-neighbour jumps of thermal vacancies alone in the Ni sub-lattice. Because of the lack of a suitable Al-tracer, Al-diffusion in the compound was simulated by investigating the diffusion behaviour of Ti, Nb, Ge and the homologous element Ga, which were preferentially located on Al sites. The difference in the observed enthalpies QGa and QNi agreed well with the predicted energy (44.4kJ/mol) for antisite defect formation of Al in the Ni sub-lattice. Therefore, it was considered most likely that diffusion of Ga occurred via nearest-neighbour jumps using Ni-vacancies in the Ni sub-lattice. A lower diffusivity of Ti and Nb in comparison to Ga and Ge, combined with a slightly enhanced activation enthalpy, was observed. The smaller activation enthalpy and the 3 to 4 orders of magnitude higher diffusivity of the B atoms indicated that B diffused interstitially in the L12-type Ni3Al lattice. The still relatively high value of QB for such an interstitial mechanism was attributed to the strongly asymmetrical occupancy of the two energetically different types of octahedral interstitial sites in Ni3Al by B atoms and the diffusion path resulting from the corresponding structural and energetic conditions.

Self- and Impurity Diffusion of Ni, Ga, Ge, Ti, Nb and B in the L12-Type Intermetallic Compound Ni3Al. Frank, S., Södervall, U., Herzig, C.: Defect and Diffusion Forum, 1997, 143-147, 245-50

Table 7

Arrhenius parameters for diffusion in Ni3Al