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

Martinelli L., Guerre C., Wilde G., Duhamel C., Divinski S.V., Esin V.A., Belkacemi L.T., Vaidya M., Sevlikar S., Hassanpour A., Jomard F., Irmer D.

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.