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Rhenium alloys exhibit a unique combination of chemical, physical, and mechanical properties that makes them attractive for a variety of applications. Herein, we present atomic-scale structural and atomic part-per-million level three-dimensional (3D) chemical characterization of a Re-Ni coating, combining aberration-corrected scanning transmission electron microscopy (STEM) and atom-probe tomography (APT). A unique combination of a columnar and multilayer structure is formed by singlebath dc-electroplating and is reported here for the first time. Alternating thicker Re-rich and thinner Ni-rich layers support a mechanism in which Ni acts as a reducing agent. The multilayers exhibit hetero-epitaxial growth resulting in high residual shear stresses that lead to formation of corrugated interfaces and an outer layer with mud-cracks. IntroductionRhenium (Re) is a refractory metal with a unique combination of chemical, physical, and mechanical properties that makes it attractive for high-temperature, catalytic, energy, electrical, biomedical, and other applications, in spite of its high cost. [1,2] Interest in electroless deposition [3,4] and electrodeposition [5][6][7][8][9][10][11][12][13][14][15][16] of Re and its alloys has recently emerged. Electroplating of pure Re yields poor deposition results. The addition of salts of the iron-group metals (Ni, Fe, and Co) to the plating bath improves, however, the coating dramatically, allowing deposition of thick layers of the

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