Isothermal compression tests at 1000°C and 0.1 s^–1 strain rate, in which mica or glass sheets were used as a lubricant, were conducted. Isothermal condition was achieved by placing high-heat-resistant (HHR) alloys between a workpiece and ceramic tools in the induction-heating configuration to prevent heat from escaping to the ceramic tools. To perform high compression tests, it was necessary to increase the diameter of the HHR alloy, for which a new single-turn coil was designed using FEM calculation coupled with deformation-temperature electromagnetic fields. In order to obtain the correct flow stress, inverse analysis was conducted using the FEM calculation, in which temperature and strain rate fluctuations were compensated. However, the compensation was insufficient when the distribution of temperature and strain rate was large. The use of glass sheets as a lubricant considerably reduced friction and uniform deformation was achieved. Thus, flow stress obtained using the inverse analysis became extremely reliable. The flow stress of mica obtained using inverse analysis with a constant friction coefficient was different from that of glass. Introducing new friction model that the friction coefficient changed from 0.02 to 0.3, the flow stress of mica was consistent with that of glass. Therefore, the flow stress obtained using the inverse analysis for the new configuration proposed in this study proved to be reliable.

Zn–Ni alloys were electrodeposited on a Cu electrode at 10–5000 A·m⁻² and 5 × 10⁴ C·m⁻² in an unagitated zincate solution at 293, 313, and 333 K. The effect of solution temperature on the electrodeposition behavior of Zn–Ni alloys from alkaline zincate solutions was investigated. The transition current density at which the deposition behavior shifted from a normal to anomalous codeposition was almost similar at 293 and 313 K but increased at 333 K. The transition current density increased at 333 K due to the enhanced hydrogen evolution and Ni deposition. The current efficiency for alloy deposition increased with solution temperature in both normal (10–50 A·m⁻²) and anomalous (500 A·m⁻²) codepositions region. In a normal codeposition region, Ni deposition and hydrogen evolution mainly occurred, and the current efficiency increased with solution temperature due to a larger promotion effect of increase in solution temperature on the Ni deposition. In an anomalous codeposition region at 500 A·m⁻², Zn deposition and hydrogen evolution mainly occurred, and Zn seems to proceed under a mixed rate-determining process of the charge transfer and diffusion of Zn ions. The current efficiency increased with solution temperature since the diffusion of Zn ions was accelerated. The Ni content in the deposited films increased with solution temperature at all the current densities, since Ni deposition was more accelerated than Zn deposition with increasing solution temperature in the region where the charge transfer process was rate-limiting. The γ phase of the deposited films increased with increasing solution temperature.

This study examined cold rolling (20%) effects on the creep rupture strength of 12Cr-5.4W ferritic steel with martensite and δ-ferrite at 700 °C. Creep rupture strength of the as-received steel was equal to or greater than that of Gr.92 steel under 100-50 MPa, but cold rolling decreased the creep rupture strength by as much as 10%. Microstructure observations of crept steels revealed coarsening of the Laves phase by cold rolling in δ-ferrite but not in martensite. This finding suggests that the Laves phase coarsening in δ-ferrite is related to short-circuit diffusion because fine sub-grain structures were observed inside δ-ferrite grains in the cold-rolled steel after creep. Also, the martensite lath structure in the cold-rolled steel recovered quickly. Collectively, these cold-rolling-related phenomena of microstructural degradations are inferred as factors decreasing the creep rupture strength. In the as-received steel, some creep voids were observed in martensite: most were observed adjacent to coarse vanadium nitrides. By contrast, creep voids in the cold-rolled steel were most numerous near the martensite – δ-ferrite interface. It was suggested that nucleation of creep voids near the interface in the cold-rolled steel was attributed to mechanisms such as the Laves phase size and distribution at the interface and stress concentration effects of dislocations that were piled up toward the interface.

To develop microstructure control concepts for ensuring the toughness of high-strength steel plates, basic research was conducted using ferrite single-phase steels with different amounts of C, and the effects of the states of C were investigated along with those of solute N. In this study, Fe-0.017C (mass%) alloy, wherein the state of C was changed to a solid solution, intragranular cementite, and intergranular cementite, were used for microstructural observation, Charpy testing, and fracture surface investigation. The results reveal that the toughness of the intragranular cementite steel was the best, followed by that of solute C steel and intergranular cementite steel. In intergranular cementite steel with significantly inferior toughness, the coarse intergranular cementite leads to dislocation pile-up, initial crack formation, and macroscopic brittle fracture. The brittle fracture of intragranular cementite steel was caused by the deformation twins. It is thought that the fine intragranular cementite only had a minor effect on the crack initiation and dislocation mobility. Twin was also confirmed at the initiation point of brittle fracture in the solute C steel. Hence, it was deduced that the deterioration of toughness caused by solute C resulted from the promotion of twinning, which replaced the dislocation movements. However, the deterioration of toughness caused by solute C was smaller compared with that caused by solute N, which partly caused intergranular fracture. This is attributed to the suppression of intergranular fracture by the presence of a small amount of solute C.

The effects of Ni concentration and dispersed conditions of vanadium carbide (VC) nano-particles on the hydrogen embrittlement (HE) behavior of precipitation-hardened high-Mn austenitic steels were investigated under the presence of thermally pre-charged hydrogen. Slow strain-rate tensile tests revealed that HE susceptibility decreased with an addition of Ni. VC precipitates functioned as the trapping sites for dissolved hydrogen, though its effect on resisting the HE was trivial. Hydrogen enhanced intergranular (IG) fracture wherein its area fraction was increased with the hardening by VC as well as with the escalation of internal hydrogen concentration. The IG fracture was the primary rationale for the hydrogen-induced loss of ductility. Possible mechanisms of the IG-related HE as well as future strategy for mitigating the mechanical degradation is discussed based on the fractographic observation and post-mortem microstructural analyses.