The time-dependent variation of effects of thiourea addition on Cu electrodeposition, and the electrochemical oxidation behavior of thiourea, were investigated by impedance measurements and cyclic voltammetry. In an aqueous sulfuric acid solution containing thiourea, an oxidation current of thiourea was observed at approximately 0.15–0.20 V on a Cu electrode and at approximately 0.7 V on a Pt electrode. The addition of thiourea or formamidine disulfide (FDS) decreased the charge transfer resistance of Cu deposition, thereby promoting Cu deposition. The FDS-added solution exhibited a lower charge transfer resistance than the thiourea-added solution. In the thiourea-containing solution, the charge transfer resistance further decreased with time from initial stage. Thereafter, it increased, and the additive effect gradually diminished. In the FDS-containing solution, the charge transfer resistance increased with time from the initial stage, and the additive effect gradually diminished. The loss of the additive effect occurred more rapidly under electrolysis conditions. This may be due to the acceleration of FDS decomposition induced by electrolysis. The enhancing effect of thiourea at initial stage is attributed to its conversion into FDS via electrochemical or chemical oxidation, which promotes the deposition reaction via a Cu[FDS]⁺ complex. In contrast, FDS may undergo hydrolysis, ultimately leading to the loss of its additive effect.

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Colored coatings produced by electrodeposition can significantly enhance the value of materials. The polarization curves for composite deposition from a Zn–Ni plating solution were measured, and the deposition potential region in which color appeared was investigated. In addition, the structure of the colored deposited films was analyzed by transmission electron microscopy. Colored deposited films were obtained at potentials close to the equilibrium potential for Zn deposition. TEM observations and EDX analysis revealed that the colored films consisted of a composite structure with ZnO and ZnS as the matrix phases, in which fine metallic Ni nanoparticles with diameters of 3–5 nm were uniformly dispersed.

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Since the properties of electrogalvanized steel sheets depend on the crystal orientation of the deposited Zn, the effects of current density and substrate characteristics on the orientation relationship between Zn and Fe were investigated. Chemically polished Al-killed steel sheets, IF steel sheets, and high-purity electrolytic Fe were used as substrates. The crystal orientation relationships between Zn and Fe were analyzed by electron backscatter diffraction. On Al-killed steel sheets, Zn grew epitaxially at 1500 to 9000 A·m⁻² according to the Burgers orientation relationship, {101}Fe // {0001}Zn. In contrast, at 15000 A·m⁻², the orientation relationship {111}Fe // {0001}Zn was observed. On IF steel sheets, Zn was predominantly oriented with its {0001} planes over the entire current density range, and the orientation relationship {111}Fe // {0001}Zn was maintained. On high-purity electrolytic Fe, the orientation relationship {111}Fe // {0001}Zn was observed at 1500 A·m⁻², whereas it changed to {101}Fe // {0001}Zn at 15000 A·m⁻². These results indicate that the Zn/Fe orientation relationship and the crystal orientation of Zn are determined by the competition between epitaxial growth and electrodeposition overpotential, and that the manifestation of these effects depends on the grain size of the substrate.

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This study presents tensile properties of Al-Mg-Si alloys after processing by severe plastic deformation through high-pressure torsion (HPT). The alloys were fabricated so as to include excess Fe and Si with different additions of Cu as model alloys for recycling purpose. The tensile strength well exceeded 500 MPa with the total elongation more than 15% after HPT-processing under 2 GPa for 1 turn in all the model alloys. The strength further increases to more than 700 MPa with increasing addition of Cu while maintaining reasonable ductility (~8%). Transmission electron microscopy confirmed that the grain size was reduced to 180 nm and further reduced to 160 nm with increasing Cu addition. Microstructural analyses using scanning transmission electron microscopy and atom probe tomography revealed that Cu was segregated at grain boundaries, contributing to the increase in the tensile strength. The high strength with enhanced ductility is discussed in terms of strain rate sensitivities measured from strain rate change tests.

Mater. Trans. 67（2026）748-756 に掲載

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