SELF-DIFFUSION OF NI IN SINGLE AND POLYCRYSTALS OF NI3AL - A STUDY OFSIMS 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 t echniques for the determination of the concentration profiles. The dif fusion 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 gr inding device and an improved technique of detecting the low-energy be ta-decays of Ni-63 With high efficiency by liquid scintillation counti ng. In the low-temperature region (1004 to 1259 K) the concentration p rofiles 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 di ffusion 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 activat ion enthalpy Q = (303.0 +/- 5.3) kJ mol(-1) correspond to a vacancy me chanism. An enhanced diffusivity at lower temperatures, which was prev iously reported in the literature and ascribed to the presence of cons titutional vacancies, is not observed. It is concluded that Ni self-di ffusion in Ni3Al only occurs via thermal vacancies. This result is in agreement with recent positron annihilation studies by Badura and Scha efer where, independent of the composition, no constitutional vacancie s are detected in the Ni3Al compound.