In previous pilot plant studies, the authors developed a method for producing permanent mold (PM) spheroidal graphite iron (SGI) castings that prevents chill formation under as-cast conditions. Despite the advancement, chill structures (ledeburitic cementite, Fe₃C) are still commonly from PM-SGI castings produced through current methods. Although heat treatment can decompose Fe₃C and induce graphitization, concerns remain that the distribution of tempered graphite may adversely impact mechanical properties.

This study examines the graphite distribution in both the developed and conventional PM-SGI castings. Additionally, it identifies factors contributing to undesirable graphite distribution following heat treatment. Results demonstrate that the newly developed PM casting method offers advantages not only in eliminating the need for heat treatment but also in enhancing mechanical design safety.

Solder alloys are susceptible to creep deformation when subjected to prolonged stress during practical applications, with the creep resistance playing a crucial role in determining the life of alloy. In this study, In was introduced as a modification element in Bi-based solder alloys to enhance creep resistance. The addition of In resulted in a noticeable reduction in grain size and the formation of the strengthening phase Ag₂In within the microstructure. Nanoindentation analysis revealed that while the elastic modulus exhibited no sensitivity to strain rate, the hardness showed a positive correlation with strain rate. The load-depth curves displayed displacement bursts, indicating the formation of dislocations at the initial stages of deformation. The stress exponents for pure Bi and the modified Bi-2Ag-0.5Cu-0.5In alloy were measured at 10.12 and 10.75, respectively, suggesting a dislocation slip mechanism. Furthermore, as the strain rate increased, the stress exponent of the Bi-2Ag-0.5Cu-1.5In alloy ranged from 2.5 to 13.78, leading to a transition in the creep mechanism from grain boundary slipping (GBS) to dislocation slipping. The enhanced elastic modulus, hardness, and resistance to creep in the alloy can be ascribed to the precipitation strengthening of Ag₂In and the refined microstructure.

The microstructure and properties of Fe17Mn and Fe17Mn0.1C0.4Ti (i.e. FM and CTi) alloys were studied comparatively. Some fine TiC particles are precipitated within CTi alloy and the sizes of γ-austenite and ε-martensite phases are reduced by adding trace C and Ti. The γ → ε transformation temperature decreases while the content of ε phase increases with C and Ti additions. In addition, the CTi alloy possesses higher strain energy storage, lower stacking faults probability, more strong and weak pinning points, resulting in a lower damping capacity compared to FM alloy. Although CTi alloy exhibits lower damping capacity than FM alloy, the addition of C and Ti reduces the annealing temperature and time to achieve the optimal damping capacity. The additions of C and Ti significantly enhance the tensile strengths, with the highest values achieved for both alloys after annealing at 900°C for 1 h, reaching 699.5 MPa for FM alloy and 819.1 MPa for CTi alloy.

Steel sheets with bake hardening (BH) are widely used for automotive outer panels because of their high dent resistance due to a remarkable increase in yield strength induced by a paint baking process. In the dent resistance test, a panel is subjected to (i) forming and springback, (ii) BH, and (iii) concentrated loading that induces a local plastic deformation and springback. Corresponding to the above BH dent process, in this work, the effect of BH on the elastic-plastic behavior of 270D, 340BH, and 440BH steel sheets was investigated by performing uniaxial tension and compression experiments for tensile prestrained and bake-hardened specimens. Furthermore, the degradation of the elastic modulus of bake-hardened specimens with increasing plastic deformation was measured. The important finding was that the elastic modulus decreased by the plastic strain was recovered to the initial Young’s modulus by BH. Consequently, the plastic-strain-dependent elastic modulus including the BH effect was successfully expressed by the extended Yoshida–Uemori elastic model.

This Paper was Originally Published in Japanese in J. JSTP 65 (2024) 139–145.

Copper tubes used in heat exchangers were joined by brazing using copper phosphorus brazing filler metals. Leakage due to the progression of groove-like corrosion near the joints of copper tubes using copper phosphorus brazing filler metals became a problem. In addition to copper, copper phosphorus brazing filler metals also contained phosphorous and silver. In this study, we investigated the effects of silver on the corrosion resistance of copper phosphorus brazing filler metals. Observation and analysis of the metallographic structure of the copper phosphorus brazing filler metals revealed uneven distributions of phosphorus and silver components. Electrochemical measurements of anodic polarization curves showed that the current density of the copper phosphorus brazing filler metals with silver at high potentials was higher than that without silver. Furthermore, on the surface of the brazing material after the test, part of the structure of the silver-containing brazing material was observed to be corroded in the form of black dots. Therefore, the silver contained in the brazing material was considered to have effect on corrosion resistance of copper phosphorus brazing filler metals.

This Paper was Originally Published in Japanese in J. Japan Inst. Copper 63 (2024) 144–148. Figure 1, 3 and 5 were slightly modified.

In this study, we explored a method for preparing evaluation samples for hydrogen analysis, aiming to advance hydrogen analysis technology in metallic materials. Titanium-titanium hydride sintered specimens with varying hydrogen concentrations were prepared via spark plasma sintering (SPS) using titanium powder and titanium hydride powder, and the hydrogen concentration and distribution in the sintered specimens were evaluated. The obtained results are summarized below. SPS proved to be an effective technique for producing dense sintered compacts in a short time, allowing for easy control of the hydrogen concentration. Furthermore, most hydrogen existed as titanium hydride in the titanium-titanium hydride sintered compact, and high densification with a relative density of 98% or higher was confirmed. In addition, the hydrogen distribution in the upper, inner, and lower parts of the sample was measured using glow discharge optical emission spectrometry (GD-OES). The relative standard deviation of the hydrogen emission intensity in the titanium-titanium hydride sintered body was less than 5%, indicating that the hydrogen was uniformly distributed throughout the sintered body. These results suggest that titanium-titanium hydride sintered by SPS is useful as an evaluation sample for hydrogen analysis and is expected to be particularly effective for hydrogen analysis in GD-OES.

The effect of sand material type on the heat absorbing properties of molds in the expendable pattern casting (EPC) process of aluminum alloy castings was investigated experimentally. The bulk densities and thermal conductivities of dry packed beds composed of several types of sand were measured. Because the true density of sands used in this study was higher than that of natural silica sand, the bulk density of the sand packed beds was also higher. The thermal conductivities of the packed beds of zircon sand and steel shot used in this study were almost the same as that of natural silica sand. The thermal conductivities of the dry packed beds composed of artificial sand were slightly smaller than that of the natural silica sand. Using test sands used in this study, an aluminum alloy plate was cast by the EPC process. The solidification time was obtained from the cooling curve. Compared to the solidification time for natural silica sand, the times for artificial sands were longer, while those for zircon sand and steel shot were shorter. Considering the interfacial resistance at the molten metal – sand mold interface, the solidification time and heat diffusivity showed relatively good correlation.

This Paper was Originally Published in J. JFS 94 (2022) 733–739. Some spelling errors were modified.

The microstructural change and fracture by repeated gas nitriding and gas nitrocarburizing of SUS304 jig were investigated in this study. It was found that an oxide layer which was formed owing to high-temperature oxidation of the microstructure near the surface, was easily detached from the surface layer. Also, many micro voids in the oxide layer were observed. Beneath the oxide layer, a compound layer containing mainly nitrides was identified. A carbon diffusion region was observed in the matrix adjacent to the interface between the compound layer and the matrix. It was considered that the fine voids and uneven surfaces induced under loading stress during both treatments were the crack initiation points, which coalesced for the development of cracking. The fractured surface exhibits a sequence of brittle and ductile fracture patterns as the microstructure changes from the surface to the interior.

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 88 (2024) 16–25.

The impact properties at a low temperature of 213 K were improved by statically heat treatment using the cast steel containing 0.13 C (corresponding to JIS standard SCPL11) at low material and manufacturing costs. The objective of this study was to satisfy the mean value of 18 J on the absorbed energy for low-temperature cast steels showing the JIS standard for SCPL11, by employing conventional static heat treatment equipment for low manufacturing costs. Heat treatments consisting of quenching at 1203 K for 14.4 ks followed by tempering at 953 K and 983 K for 14.4 ks followed by air cooling, LQ-LTA, and LQ-HTA, were selected as candidates at the points of the primary α phase size and absorbed energy at 213 K. For both the LQ-LTA and LQ-HTA specimens, the mean values of the absorbed energy at 213 K were 18.5 and 29.3 J, respectively, indicating that the objective was 18 J. The DBTTs of LQ-LTA and -HTA were observed at approximately 255 K and 235 K, indicating a promising heat treatment condition, HTA, because of further equilibrium and morphological changes in tempered microstructures, including the primary α phase. According to the nanoindentation results, the purpose of tempering, which is to adjust the hardness and toughness, was fulfilled in both treatments according to the conversion Hv and Young’s modulus. For the tensile properties at 288 K, both the specimens satisfied the objectives of the JIS standard for SCPL11. For the absorbed energy and tensile behaviors, both candidate treatments are useful for satisfaction of the JIS standard for SCPL11. LQ-HTA with higher upper shelf energy and lower DBTT can be suggested as a promising treatment in terms of microstructural stability.

Severe plastic deformation (SPD) has emerged as a transformative tool in materials science, enabling the development of ultrafine-grained, nanostructured and heterostructured materials with exceptional mechanical and functional properties. Initially gaining prominence in the early 2000s for microstructure control and mechanical property enhancement, SPD is now increasingly applied to improve functional properties, particularly in biomedical, energy, and hydrogen-related applications. The scope of SPD has expanded from metallic materials to encompass a wide range of non-metallic materials, including ceramics and polymers. Additionally, SPD methods have provided insights into natural phenomena involving high strain and pressure, such as phase transformations and certain geological and astronomical processes. This article reviews recent research trends, as highlighted in the 2023 special issue of Materials Transactions entitled “Superfunctional Nanomaterials by Severe Plastic Deformation”, focusing on recent advancements and interdisciplinary applications of SPD.