Diffusion in the C15-type intermetallic Laves phase NbCo₂

Abstract

Self-diffusion of both constituents in the Laves phase NbCo₂ has been explored using the tracer method and the radioisotopes ⁵⁷Co and ⁹⁵Nb in combination with sputter and grinder sectioning. The temperature dependences of the Co diffusivity in Nb₃₁Co₆₉ and Nb₂₉Co₇₁ (two compositions inside the phase field of NbCo₂) and of the Nb diffusivity in Nb₃₁Co₆₉ are reported. All activation enthalpies are of similar magnitude. Nb diffusion in NbCo₂ is significantly slower than Co diffusion. Co diffusion itself is considerably faster in the alloy with higher Co content. The diffusion results are discussed in connection with the structure of and the scarce information available about atomic defects in NbCo₂. We propose that diffusion is mediated by thermal vacancies. Furthermore, the results are compared with self-diffusion data for the C14-type Laves compounds MgZn₂ and Tife₂, with interdiffusion data obtained from multiphase diffusion experiments in our laboratory, and with self-diffusion in the also close-packed L1₂ intermetallics.