Münster 1999 – scientific programme

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M: Metallphysik

M 17: Diffusion II

M 17.3: Talk

Tuesday, March 23, 1999, 15:45–16:00, S 9

Tracer Self - Diffusion of Ni in NiAl: II. Diffusion Mechanisms — •S. Divinski, St. Frank, and Chr. Herzig — Institut für Metallforschung, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Stra"se 10, D–48149 Münster

New experimental data were obtained on the Ni self-diffusion in stoichiometric and off-stoichiometric NiAl for a wide temperature range [Frank et al., this meeting]. The most striking feature is that the diffusion of Ni atoms is not enhanced when the Al content increases, despite of the related production a large concentration of structural Ni vacancies in the alloy, e.g. in an amount of about 2 % of constitutional Ni vacancies in Ni₄₉Al₅₁.

Previous explanations involving the next nearest neighbor jumps of Ni vacancies are not confirmed by the present experimental data. A spectrum of probable diffusion mechanisms in NiAl is analyzed in dependence on the composition. Different mechanisms are shown to handle the Ni diffusion on different sides of the stoichiometric composition. On the Ni-rich side the Ni diffusion is operated by the Ni anti-structure atoms. The anti-structure bridge mechanism is proposed to explain the strong increase of the Ni self-diffusion with increasing Ni concentration. A divacancy diffusion mechanism is proposed for the Al-rich side. These divacancies refer to two Ni vacancies occupying adjacent sites within the Ni sublattice and it is shown that a correlated 4-step jump sequence allows a long range diffusion of Ni atoms in the B2 structure. The calculated activation energies of diffusion agree with the experimentally observed values.