No abstract

In aqueous solutions, electro/chemically deposited metals usually grow outward into electrolyte. Here we report that the reduced Al grows inward into the sample, surprisingly, while Mg (in pure Mg and Al 2 Mg 3 alloy) is galvanically replaced with Al in an ionic liquid. The galvanic replacement reaction (GRR) of Al 2 Mg 3 involves a dealloying process that generates a nanoporous Al skeleton, and simultaneously the inward-growth plating of Al that thickens the Al skeleton. The obtained open nanoporous Al might be interesting for many applications including lithium-ion batteries. The anomalous inward-growth of deposited Al is attributed to the low diffusivity of ionic liquid and the negative charge of reduced species. This dealloying/GRR procedure provides a new route to produce hierarchical nanoporous structure in bulk materials.

To minimize pores or voids that left after electro-healing by plain solution without additives, controllable electro-healing was investigated in pure Ni plates with a through-thickness crack in a Watts bath by adding polyethyleneimine (PEI) and saccharin. Controllable electro-healing means that the growth velocity of healing crystals from crack tips is much faster than that from sidewalls of the crack, which reduces the number of pores owing to depletion of healing solution. Rotating disk electrode (RDE) was introduced to evaluate the electrochemical behavior in crack tips and crack center. Voltammograms and galvanostatic measurements indicated that PEI exhibits a convection-dependent inhibition effect on Ni deposition. The addition of saccharin will much enhance the inhibition behavior of PEI, controllable electro-healing was achieved with the presence of both PEI and saccharin.

The performance of 2-Mercapto-5-benzimidazolesulfonic acid (MBIS) in a Watts bath in crack electro-healing was investigated. Potentiostatic and galvanostatic voltammetry, current efficiency measurements were performed for electrochemical investigation. The morphology of the healed crack indicates that the healing crystals grow in a controllable manner with the addition of MBIS under forced convection with the healing crystals fill preferentially in the crack tip to the crack sidewalls. No obvious defects were observed along the interface between the substrate and the healing crystals, which is attributed the much higher current efficiency of MBIS. Annual-ring like defects are observed but with a much longer width compared with PEI. The formation mechanism of annual ring is discussed. Possible approach to get defect-free crack healing is also prospected.

Annealing hardening has recently been found in nanograined (ng) metals and alloys, which is ascribed to the promotion of grain boundary (GB) stability through GB relaxation and solute atom GB segregation. Annealing hardening is of great significance in extremely fine ng metals since it allows the hardness to keep increasing with a decreasing grain size which would otherwise be softened. Consequently, to synthesize extremely fine ng metals with a stable structure is crucial in achieving an ultrahigh hardness in ng metals. In the present work, direct current electrodeposition was employed to synthesize extremely fine ng Ni-Mo and Ni-P alloys with a grain size of down to a few nanometers. It is demonstrated that the grain size of the as-synthesized extremely fine ng Ni-Mo and Ni-P alloys can be as small as about 3 nm with a homogeneous structure and chemical composition. Grain size strongly depends upon the content of solute atoms (Mo and P). Most importantly, appropriate annealing induces significant hardening as high as 11 GPa in both ng Ni-Mo and Ni-P alloys, while the peak hardening temperature achieved in ng Ni-Mo is much higher than that in ng Ni-P. Electrodeposition is efficient in the synthesis of ultrahard bulk metals or coatings.