Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

Crystal grain refinement in high- and medium-entropy alloys is an effective approach to obtaining good strength as well as ductility. An FeCoNi medium-entropy alloy with a wide range of grain sizes was fabricated by electrodeposition. The alloy had a metastable bcc structure with Fe–36 at.% Co–21 at.% Ni composition and heterogeneous distribution of grain sizes ranging from 20 nm to 15 µm. Moreover, the alloy possessed a tensile strength of 1.2 GPa, and a fracture elongation of 6.3 % without secondary phase. The excellent mechanical properties of the alloy can be attributed to its bcc phase and heterogeneous grain sizes extending to the nanoscale.

DOI： 10.1080/02670836.2023.2187973

Other URL： https://hdl.handle.net/10466/0002000064

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

Laser ablation in liquid (LAL) is a rapid heating and cooling process that enables the formation of a variety of nanomaterials. Extensive LAL studies for metals, semiconductors, insulators, and carbons have been reported. Further, alloys are worthwhile target to investigate the applicability of LAL. We report the LAL of equiatomic FeCoNi medium-entropy alloy (MEA) in water using a high-repetition-rate UV picosecond laser with a galvanometric scanner. The extinction spectra of the colloidal solution, morphology, elemental composition, and size distribution of MEA nanoparticles (NPs) were investigated as a function of laser-irradiation time, laser power, and scanning speed. Electron microscopy revealed that chain-like superstructures longer than 10 µm consisting of Ni-rich FeCoNi MEA-core Fe-shell spherical NPs of approximately 50-nm diameter were formed. These superstructures were not originated from the NP agglomeration during sample preparation for microscopy or the successive laser irradiation of colloids, but from the overstriking ablation on the target surface. Generally, the LAL target was irradiated by spatially and temporally separated near-infrared or visible laser pulses to efficiently produce NPs. Overstriking of UV ultrashort laser pulses was shown to be a valuable strategy for modifying the elemental composition of MEA NPs and for forming their superstructures.

DOI： 10.1016/j.jallcom.2023.169896

Other URL： http://hdl.handle.net/10466/0002001079

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

Crystal-grain refinement is one of the effective approaches to obtaining high-strength materials. A good strength/ductility balance has been reported in fine grains of high- and medium-entropy alloys. However, crystal-grain refinement at the nanometer scale has not been achieved yet. In this study, we used electrodeposition to fabricate 0.2-mm thick equiatomic FeCoNi medium-entropy alloys (MEAs) with 10-nm crystal grains. The nanocrystalline FeCoNi MEAs exhibit the maximum tensile strength of 1.6 GPa, which is the highest reported result to date.

DOI： 10.1038/s41598-022-16086-6

PubMed

Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

Microstructures of electrodeposited Ni were studied from the perspective of growth modes during electrodeposition. The electrodeposited Ni had a heterogeneous microstructure composed of nanocrystalline- and microcrystalline-grains. Electron backscatter diffraction analyses showed that nanocrystalline- and microcrystalline-grains were preferentially oriented to specific planes. Secondary ion mass spectrometry also revealed that coarse-grained regions had higher S content than that of finer-grained regions. Hence, microstructural heterogeneity in electrodeposited Ni is reflected by the overlap of inhibited and free growth modes. Our discussion surrounding microstructural heterogeneity also provides insight into other electrodeposited nanocrystalline systems.

DOI： 10.1038/s41598-020-62565-z

PubMed

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

Electrodeposited nickel usage has been limited owing to thermal embrittlement at high temperatures. Electrodeposited nickel generally contains sulfur, and the grain boundary segregation of sulfur leads to thermal embrittlement. As the main source of sulfur, saccharin (C7H5NO3S) has been suspected to primarily cause thermal embrittlement. In this study, electrodeposited nickel with improved ductility and high heat resistance was fabricated using citric acid instead of saccharin. Using citric acid, a strong (100) plane-preferred orientation parallel to a substrate was obtained over a wide current density range, similar to the case of using saccharin. Therefore, high ductility is expected in as-deposited electrodeposits obtained from electrolyte using citric acid over a wide current density range. The sulfur content in the samples significantly decreased from 0.025 to 0.007wt% using citric acid instead of saccharin sodium. Specimens obtained from the electrolyte using citric acid achieved an elongation to failure of 7.4% even when annealed at 300°C for 24h, whereas specimens obtained from electrolyte using saccharin were brittle and exhibited no plastic deformation at an annealed state.

DOI： 10.1016/j.rsurfi.2020.100001

Presentation type：Oral presentation (general)  

Venue：つくば  

Presentation type：Oral presentation (general)  

Type：Peer review