Tracer diffusion of Ni-63 in nano-gamma-FeNi produced by powder metallurgical method: Systematic investigations in the C, B, and A diffusion regimes

Ni-63 radiotracer diffusion in a well-compacted nanocrystalline (grain size d similar to 80 to 100 nm) gamma-Fe-40wt% Ni alloy was measured by the serial sectioning technique in an extended temperature range from about 610 to 1010 K. Since the material microstructure reveals two different length scales with nano-size grains forming micrometer-size clusters (or agglomerates), three main diffusion paths determine the diffusion behavior: the nanocrystalline grain interior, the nanocrystalline grain boundaries (GB), and the inter-agglomerate interfaces. The systematics of diffusion in a compacted nanostructured material with such a bimodal distribution of interface characteristics was elaborated and the experimental data were analyzed in dependence on the diffusion regime. The absolute values and the Arrhenius parameters of Ni GB diffusion in the nano-gamma-Fe-40wt% Ni alloy (D-0 = 9.3 x 10(-4) m(2) s(-1) and Q = 177 kJ/mol) are similar to the Ni GB diffusivity in coarse-grained poly-crystalline gamma-Fe. Accordingly, the nanocrystalline GBs are concluded to have quasi-equilibrium structures, particularly because of a pronounced grain growth (from about 30 to about 100 nm) during the production stage of the nanomaterial. In contrast, the interagglomerate interfaces, which present the fastest diffusion path in the present investigation (D-0 = 1.9 x 10(-3) m(2) s(-1) and Q = 134 kJ/mol), are likely to be in a non-equilibrium state due to specific features of the applied powder metallurgical process.