The Ni self-diffusion in NiAl:: An experimental investigation of the temperature and composition dependencies and atomistic simulation of diffusion mechanisms

The composition and temperature dependencies of Ni self-diffusion was investigated in a set of NiAl single crystals of different compositions. By using Ni-63 the radiotracer technique was applied in the high-temperature inter val from 1200 to 1630 K, while at lower temperatures (from 1050 to 1200 K) the diffusion profiles were analyzed by secondary ion mass spectrometry (SIMS) using the highly enriched stable isotope Ni-64. A somewhat unexpected compositional dependence of Ni self-diffusion in NiAl has been observed. The Ni diffusivity D occurred to be nearly the same (within experimental errors) for the Al-rich and stoichiometric compositions, while on the Ni-rich side a pronounced increase of D was observed with increasing the Ni content. Within the temperature interval from 1050 to 1500 K the Ni self-diffusion coefficients D show almost linear Arrhenius dependencies in the alloys under investigation. The effective activation enthalpy was found to be about 3.0 eV atom(-1) in Al-rich, stoichiometric, and slightly Ni-rich alloys, whereas for the compositions with larger Ni content a decrease of Q was observed with increasing Ni content. To explain the detailed experimental results atomistic calculations with empirical Embedded-Atom potentials have been carried out. The comparison of experimental data and calculated results supports the triple-defect diffusion mechanism in NiAl, although other diffusion mechanisms may also give a notable contribution.