Series of Ni-base single-crystal superalloys with superior thermal durability have been developed to improve thermal efficiency of gas turbine systems. Microstructural transition during creep so called “raft structure” formation enhances creep properties at lower stress and higher temperature condition. Furthermore, larger perfection degree of the raft structure contributes to better creep properties under the same creep condition. To control the perfection degree of the raft structure, magnitude of a lattice misfit and an elastic misfit between γ and γ′ phases should be controlled. In the current situation, the lattice misfit can be controlled by using alloy design program NIMS has developed. In this review, we focused on the role of the raft structure in alloy design. Observation results and predicted mechanisms about strengthening by the microstructural transition, in addition to the mechanism about microstructural transition itself during creep, were summarized and explained. Finally, under these recognitions mentioned above, our effort to establish a new alloy design approach to control the perfection degree of the raft structure by modifying the elastic misfit was introduced.

The structural changes in electrodeposited palladium (Pd) films with desorption of co-deposited hydrogen were investigated, and the existing states of hydrogen in the Pd film were analyzed. The Pd films were electrodeposited from an alkaline bath consisted of palladium (II) chloride, ammonium chloride, and citric acid. Two pronounced desorption peaks at around 500 K and 880 K were observed in the thermal desorption spectrum of hydrogen from as-deposited Pd film. For the Pd films with lower hydrogen concentrations, the lattice contraction proceeded concurrently with hydrogen desorption at room temperature. The lattice parameter of Pd film decreased with increasing heat treatment temperature up to 500 K, and then increased with grain growth above 550 K. An exothermic peak corresponding to the hydrogen desorption at around 500 K was observed in the differential scanning thermal analysis curve. A large number of nano-voids were observed in the Pd film. These results suggested that the hydrogen desorption peaks observed at around 500 K and 880 K were ascribed to the break-up of vacancy-hydrogen clusters and the desorption of hydrogen molecules from nano-voids, respectively.

Superplastic bonding was performed using electrodeposited Ni and electrodeposited Ni-B to apply to low-temperature diffusion bonding of carbon steel. Electrodeposited Ni and Ni-B were bonded at 450℃ in air and at a strain rate of 1 × 10⁻⁴ s⁻¹. The shear test measured the bond strength, and the maximum bond strength of 69 MPa was obtained. Since the obtained bonding strength at 450℃ is comparable to that obtained by diffusion bonding of carbon steel at 0.5T_m, it is possible to decrease the bonding temperature to 0.4T_m by applying this boning process. The bonding strength depended on the amount of strain and increased as the strain increased in the range of strain 0.2 to 0.4. The voids at the interface shrink by superplastic deformation with grain boundary sliding, and the bonded area increases. In the superplastic bonding of electrodeposited Ni alloys, superplastic deformation effectively shrinks voids with a radius of 1-2 µm.

Fabrication of fine surface textures is one of the most effective technologies to provide new functions for materials. In this research, argon-ion sputter-etching was applied to an α+β type Ti-6Al-4V alloy placed on a SUS304 steel disk at a radio frequency power of 200 W, 250 W and 300 W for 0.9 ks to 21.6 ks. For the 1013 K annealed specimens, fine holes smaller than 500 nm diameters were densely formed on the surfaces when the sputter power was 250 W and sputter time was 0.9 ks. However, when the sputter time increased to more than 1.8 ks, the holes disappeared and the groups of grain-shaped protrusions were formed in flat areas. For the 1323 K solution-treated specimens, the fine holes were also formed at the sputter power of 250 W and at a sputter time shorter than 1.8 ks. The holes connected to ridge-shaped protrusions with increasing sputter time and finally changed to independent and uniformly distributed protrusions. Energy dispersive X-ray analyses revealed an increase in the V and Fe contents on the sputtered surface. This corresponds that the Fe particles arrived from the SUS304 disk dissolved into the alloy and strongly combined with V atoms, which resulted in the selective sputtering of Al and Ti. It is considered that the argon ion radiation has induced the decomposition of surface region to fine areas with different V and Fe contents, and the fine holes are formed by preferential sputtering of the lower V and Fe contents areas.