Ni tracer diffusion in the B2-compound NiAl: Influence of temperature and composition

The effect of composition and temperature on Ni bulk self-diffusion is inve stigated for nine different single crystalline NiAl alloys with well-define d compositions between 46.8 and 56.6 at.% Ni in the temperature range from 1050 to 1630K. The diffusion penetration profiles of Ni in NiAl were determ ined by applying two different techniques of profile detection. Radiotracer experiments have been carried out using the Ni-63 tracer, a serial section ing technique, and sensitive liquid scintillation counting for the high tem perature measurements, while at lower temperatures the diffusion profiles w ere analyzed by secondary ion mass spectrometry (SIMS) using the highly enr iched stable isotope Ni-64. In contrast to the literature data on Ni self-d iffusion in NiAl alloys by Hancock and McDonnell [Phys. stat. sol. A4, (197 1) 143], the present measurements show an unexpected concentration dependen ce of the Ni diffusion coefficients D with nearly constant diffusivities fo r stoichiometric and Al-rich alloys and increasing D values with increasing Ni content on the Ni-rich side of the NiAl composition range. The effectiv e diffusion activation enthalpy Q is equal to about 3.0 +/-0.07 eV for the Al-rich, stoichiometric, and slightly Ni-rich NiAl alloys, while for the co mpositions with larger Ni content a decrease of Q was observed with increas ing Ni content, for example, Q=2.39 eV for the Ni56.6Al42.4 alloy. The pres ent experimental results imply that mainly the same diffusion mechanism ope rates on both sides of stoichiometry in NiAl. This mechanism is identified with the triple defect mechanism. Its contribution is compositionally indep endent. The activation energy of Q=3.18 eV was calculated for the triple de fect mechanism using empirical EAM potentials in agreement with the experim ental data. The decreage of e at large Ni concentrations on the Ni-rich sid e is explained by an additional contribution of the anti-structure bridge m echanism with the activation energy of Q=1.73 eV. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All lights reserved.