Intrinsic heterogeneity of grain boundary states in ultrafine-grained Ni: A cross-scale study by SIMS and radiotracer analyses

Abstract

Using a correlated diffusion study applying secondary-ion mass spectroscopy (SIMS) and radiotracer analyses, the co-existence of both relaxed and deformation-modified non-relaxed high-angle grain boundaries (GBs) in ultrafine-grained Ni of 2N6 purity processed by equal channel angular pressing (ECAP) is clearly revealed. Due to different depth and lateral resolutions and using experimentally accessible diffusion times, SIMS provides a direct access to the properties of relaxed “slow” GBs (at short penetration depths) while the radiotracer measurements reveal simultaneously the contribution of the deformation-modified “fast” GBs (at large penetration depths). The temperature stability of ultrafine-grained structure of 2N6 Ni is investigated using electron back-scatter diffraction after annealing treatments corresponding to the diffusion experiments. No changes of the ECAP-produced microstructure occur at 403 K, while the ultrafine-grained structure is remarkably evolving to a coarse-grained one at 603 K. The knowledge of the microstructure evolution is used to quantify the diffusion data. The combination of the two complementary techniques allows not only to perform a cross-scale analysis of the mass transport, but also to probe consistently the existence and kinetic properties of different multi-level hierarchic microstructure features. Therefore, the results obtained is a step forward a better understanding of the physics of ultra-fine-grained materials (UFG). For the first time in the case of UFG materials, the SIMS technique is used in a mode with lateral resolution which is correlated with the microstructure characteristics resolving a multi-level hierarchy of diffusion properties of short circuits in severe plastically deformed materials.