The monitoring of invasion/permeation hydrogen on entry/exit surfaces of cathodically charged SUS316 columnar crystals was conducted with a scanning Kelvin probe force microscope (SKPFM) under atmospheric pressure. Columnar crystal specimens covered with oxide films on their surfaces under room conditions were prepared for cathodic charging tests and subsequent SKPFM measurements. The invaded hydrogen on the entry surface was detected at the δ-ferrite phases for 7 d after charging, and the segregation of invaded hydrogen at the boundaries between the δ-ferrite and austenite matrix was prolonged for >10 d after charging. The permeated hydrogen on the exit surface was detected at the δ-ferrite phases for 3 d after charging, but was not substantial at some of the δ-ferrite phases regardless of the charging. Segregation of permeated hydrogen at the boundaries between the δ-ferrite and some of the intermetallic precipitates was prolonged for 7 d after charging. The behaviors of invaded/permeated hydrogen based on heterogeneous microstructures are discussed to improve understanding of the hydrogen embrittlement mechanism in weld metals.

Recent research has shown that some intermetallic compound particles with high interfacial hydrogen trap energies (e.g., Mg₂Si) are prone to damage at high hydrogen concentrations. In this study, the acceleration of particle damage in an A6061 alloy was observed in-situ via X-ray CT. The damage behavior of the particles that are located in the crack tip stress field, where high stress triaxiality causes a local increase in the hydrogen concentration, was analyzed. The influence of hydrogen on the damage behavior of the dispersed Mg₂Si particles was investigated by preparing a material charged with hydrogen to achieve extremely high hydrogen concentration, and further hydrogen enrichment in a crack tip region was also utilized. Interfacial debonding of Mg₂Si particles was frequently observed in the vicinity of a crack tip immediately prior to tensile fracture. Even though the fracture is typical of ductile fracture, hydrogen accelerates particle damage and reduces the macroscopic ductility of the aluminum alloy. This can be considered as a form of hydrogen embrittlement of aluminum alloys. Even in materials with relatively low hydrogen concentrations (0.85 mass ppm), interfacial debonding occurred in the hydrogen-enriched crack tip regions. A higher hydrogen concentration promoted interfacial debonding over a wider range of particle sizes and particle shapes. It can be inferred that localized hydrogen enrichment, which is expected to occur due to external hydrogen exposure, stress corrosion cracking, corrosion or crack tips, can directly contribute to debonding at the Mg₂Si particle/aluminum matrix interface. According to the analysis, reduction of the diameter and simplification of the shape of Mg₂Si particles are effective method for suppressing such hydrogen-induced debonding.

Y₂O₃/Hf co-doping has been used to design novel alumina-forming Co-Cr-Al-Ni oxide dispersion strengthened (ODS) superalloys. In order to gain a deep understanding of the effect of Y₂O₃/Hf co-doping on the microstructure and mechanical properties of the superalloys, a comparative study of Co-20Cr-10Ni-15Al (at%) superalloys with and without the Y₂O₃/Hf co-doping fabricated by mechanical alloying (MA) and spark plasma sintering (SPS) has been carried out. The microstructure and the elemental distribution of the constituent phases of the two superalloys, in particular the distribution of fine oxide particles, were studied in detail by electron microscopy. Compared with a large grain size and coarse Al₂O₃ dispersoids in Y₂O₃/Hf-free alloy, Y₂O₃/Hf co-doping effectively refined the grains and inhibited the generation of Al₂O₃ particles through the preferential formation of fine Y₂Hf₂O₇ oxide particles. Y₂O₃/Hf co-doped alloy exhibited an ultrahigh ultimate tensile strength (UTS) of 1816 MPa, which is 240 MPa higher than that of the undoped Y₂O₃/Hf one owing to the dispersion strengthening of the dense Y₂Hf₂O₇ oxide particles as well as fine-grain strengthening. The fracture mechanism of Y₂O₃/Hf doped alloy during tensile loading was illustrated through transmission electron microscopy (TEM) observations and the optimization based on Y₂O₃/Hf doping content was proposed.

The strain-rate dependence of slip persistence in TiZrNbHfTa was investigated using microcantilever bending tests instead of conventional single-crystal tensile tests. These tests were performed using micrometre-sized cantilevers fabricated within a grain of the polycrystalline material, and the ψ–χ relationship was obtained in detail. Bending tests were conducted in two strain rates to determine the ψ–χ relationship. The results showed that the slip plane macroscopically persisted on the {112} plane in the high-strain-rate test, while in the low-strain-rate test, the apparent slip plane roughly coincided with the plane where the maximum shear stress was applied. Detailed tracing of the slip bands, using atomic force microscopy, on the specimen surface showed that the slip plane was microscopically persisted on the {112} plane in high-strain-rate tests, while in low-strain-rate tests, the slip plane often cross-slipped between the {112} and {110} planes.

The influence of the volume fraction of the long-period stacking ordered (LPSO) phase on the strengthening mechanisms acting in Mg/LPSO two-phase extruded alloys is discussed by focusing on compression tests of Mg₉₄Zn₂Y₄ and Mg₉₂Zn₃Y₅ alloys. An increase in the LPSO phase volume fraction increases the yield stress of these alloys, but the magnitude of the increase is not monotonic with the volume fraction. For deformation parallel to the extrusion direction, the rate of increase in the yield stress shows two large gaps between the Mg_99.2Zn_0.2Y_0.6/Mg₉₇Zn₁Y₂ and Mg₉₂Zn₃Y₅/Mg₈₉Zn₄Y₇ alloys. This is derived from the change in strengthening mechanisms. The upper gap between Mg₉₂Zn₃Y₅/Mg₈₉Zn₄Y₇ is derived from the change in the strengthening mechanism between short-fiber reinforcement and the simple rule of mixtures. The lower gap between Mg_99.2Zn_0.2Y_0.6/Mg₉₇Zn₁Y₂ corresponds to the existence of a short-fiber strengthening mechanism or not. As the volume fraction of the LPSO phase decreases, the magnitude of kink-band strengthening to the yield stress decreases; however, it is still effective even in alloys with a low volume fraction of the LPSO phase.