Effect of equal channel and dynamic channel angular pressing of Ni and further heat treatment on its structure and grain-boundary diffusion

Vladimir Popov ¹ Gerrit Reglitz ² Evgeniy Shorohov ³ Dmitriy Kuznersov ¹ Elena Popova ¹ Alexey Stolbovsky ¹ Sergiy Divinski ² Gerhard Wilde ²

¹Ural Branch of RAS, M.N. Miheev Institute of Metal Physics, Russia
²Institute of Materials Physics, Westphalian Wilhelms University of Münster, Germany
³All-Russia Institute of Technical Physics, Russian Federal Nuclear Center, Russia

The structure of Ni after equal channel angular pressing (ECAP) and dynamic channel angular pressing (DCAP) is studied. The energy of powder combustion gasses is used for DCAP that increases the rate of plastic deformation and adds straining by shock enhancing the total impact with respect to ECAP processing.

A sub-microcrystalline structure is shown to be produced and the state of grain boundaries (GBs) is characterized by GB self-diffusion measurements. The GB diffusivities are found to be similar in both ECAP- and DCAP-processed Ni below 400 K, exceeding the self-diffusion rates in coarse-grained Ni by more than 3 orders of magnitude. Thus, the SPD treatment induces characteristic deformation-modified GBs in both states. Both a XRD study and Geometric Phase Analysis of high-resolution TEM images confirm a marginal change of deformation-induced elastic micro-strains. Consequently, a particular GB state hardly affects the level of internal stresses.

Heating up to 473 K does not induce significant changes of both the fine-grained microstructure and microhardness, with their moderate changes being observed after annealing treatments above 573 K. A deviation from the Arrhenius-type temperature dependence of GB diffusion is found to start at temperatures above 400 K - well below the characteristic temperatures of the grain growth - indicating a relaxation of the deformation-modified GBs affected by dislocation reactions.