SELF-DIFFUSION OF NI IN SINGLE AND POLYCRYSTALS OF NI3AL - A STUDY OF SIMS AND RADIOTRACER ANALYSIS

Diffusion of nickel in the intermetallic compound Ni3Al is measured in the temperature range from 1004 to 1422 K by applying two different techniques for the determination of the concentration profiles. The diffusion of the radioactive Ni-63 tracer in Ni3Al polycrystals (75.2 at% Ni) is analyzed in the temperature region from 1223 to 1422 K by the conventional serial sectioning technique using a precision parallel grinding device and an improved technique of detecting the low-energy beta-decays of Ni-63 With high efficiency by liquid scintillation counting. In the low-temperature region (1004 to 1259 K) the concentration profiles are determined by secondary ion mass spectrometry (SIMS) using the highly enriched stable Ni-64 tracer and Ni3Al single crystals (75.9 at% Ni). The SIMS technique is considered as particularly suitable for profile detection, owing to its potential of measuring also low diffusion coefficients with high accuracy. The temperature dependence of the diffusion coefficient D-V(Ni) follows a perfect linear Arrhenius relation in the investigated temperature range for Ni3Al The frequency factor D-0 = (3.59(-1.50)(+2.59)) x 10(-4) m(2) s(-1) and the activation enthalpy Q = (303.0 +/- 5.3) kJ mol(-1) correspond to a vacancy mechanism. An enhanced diffusivity at lower temperatures, which was previously reported in the literature and ascribed to the presence of constitutional vacancies, is not observed. It is concluded that Ni self-diffusion in Ni3Al only occurs via thermal vacancies. This result is in agreement with recent positron annihilation studies by Badura and Schaefer where, independent of the composition, no constitutional vacancies are detected in the Ni3Al compound.