Sintered Ag nanoparticles display brittle fatigue crack propagation behavior at 298 K because of their submicron-size crystals. On the other hand, at elevated test temperatures above 413 K, ductile fatigue crack propagation characteristics similar to those of soft metals such as solder appear owing to viscous creep behavior at the grain boundaries, and no temperature dependence of the fatigue crack propagation properties was observed. The fatigue crack initiation lives observed experimentally and the fatigue lives derived by a pseudo-fracture mechanics approach using numerical fatigue tests with the fatigue crack propagation law mostly coincided with each other. However, the fatigue crack initiation life was calculated on the excessively safe side because of the long incubation period for crack initiation due to the influence of continued sintering during the tests performed near the sintering temperature. The application of the pseudo-fracture mechanics approach made it possible to derive the fatigue crack initiation law for smooth specimens of sintered Ag nanoparticles with satisfactory accuracy if the test temperature is lower than the sintering temperature.

Based on electronic structure calculations and the strategic selection of commonly available transition metal elements, this study proposes Co-Ni-Al shape memory alloys (SMAs) with transformation temperatures exceeding 373 K, guided by two electronic parameters derived from fundamental electronic concepts. Nine Co-Ni-Al alloys with γ+β dual-phase structures were synthesized via cold crucible levitation melting: Co-40Ni-18Al, Co-23Ni-18Al, Co-6Ni-18Al, Co-40Ni-23Al, Co-23Ni-23Al, Co-6Ni-23Al, Co-40Ni-28Al, Co-23Ni-28Al and Co-6Ni-28Al. They showed γ+β dual phase and volume fraction of each phase could be evaluated by two electronic parameters bond order and d-orbital. Alloys exhibiting both high shape recovery rates and high phase transformation temperatures were concentrated within specific regions of the Bo–Md diagram. Furthermore, manganese (Mn) was selected as a fourth element taking into consideration the effect on the enhancement of the mechanical properties and phase transition temperature. The Co-29Ni-27Al-3Mn alloy emerged as a promising quaternary alloy, demonstrating excellent shape memory behavior characterized by high recovery strain and transformation temperatures. The promising alloy showed an excellent shape memory behavior. This provides a theoretical basis for composition optimization and enhancement of shape memory properties in Co-Ni-Al alloys.

The effects of extrusion and dispersed particles (SiC or SiO₂) on the mechanical properties are examined on aluminum (Al) based composites prepared from powder metallurgy. Extrusion is effective for i) grain refinement of the α-Al matrix and ii) producing high quality bulk specimens on a large scale. This is because of a high applied stress during hot-extrusion contributes to the degradation of oxide films covering the powder particles, leading to the creation of new real surfaces. Microstructural observations show that powder-based extruded Al and its composites have fine-grained structures, i.e., an average grain size of less than 5 µm in the α-Al matrix. Accordingly, associated to these microstructures, they show higher strength (∼30 MPa) and hardness (∼10 Hv) than those of cast Al and its composite. In addition to beneficial mechanical properties, the extrusion process does not give a negative impression as for wear property, i.e., the wear rate. Plasticity-controlled void growth mechanism is focused to consider the impact of extrusion on bonding quality. The time required to shrink voids is estimated, and this value is consistent with the actual processing duration.

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.