Key Engineering Materials Vol. 967

Abstract: Hydrogen technology can be one key for a transition to sustainable energy necessary to achieve climate targets and limit global warming to 1.5 °C since the beginning of the industrial revolution. Hydrogen as a CO₂ neutral energy carrier must replace fossil fuels from the existing natural gas grid and infrastructure to enable an environmentally friendly and circular economy in future societies. Batteries and e-fuels are practicable technologies for short term and quantitatively limited energy provision, with disadvantages including raw material demands and technologically complex transformation cycles. Utilizing advanced power-to-gas concepts, hydrogen will not only be most efficient technology in energy storage, but also allows adaption and reuse of existing energy transportation infrastructure.To provide volatile hydrogen gas in the required flow and energy densities, advanced compression technology needs to be developed inspired by conventional gas compression systems. Reciprocating piston compressors are developed for high-pressure hydrogen applications, providing high pressure levels and flow rates. Compression equipment must be designed for non-lubricated dry-running conditions, as high gas purity standards of hydrogen do not allow for oil-based lubricants to be introduced into the process gas. High-strength carbon fiber reinforced composites are developed as piston and packing ring materials to withstand extreme pressure differences under harsh thermo-mechanically loaded operation conditions.Promising candidates with high strength and wear resistance in the form of PPS-polymers, are developed with PTFE solid lubricants and different carbon fiber fractions to combine high strength, with low friction and wear, improve pressure operation range, and limit down times of hydrogen piston compressors. The current work describes tribological testing of advanced PPS-polymers with 10 to 30 wt.% carbon fibers in a high-velocity tribometer under hydrogen gas atmosphere. Supporting thermo-mechanical tests give new insights in deformation mechanisms of fiber reinforced polymer composites and allow conclusions on their applicability for hydrogen compression.

Abstract: Hydrogen embrittlement (HE) is increasingly becoming a critical issue for using high-strength steels in the automotive and infrastructure industries. To overcome the risk posed by HE of structural components under a hydrogen uptake environment in long-term service, it is necessary to clarify the mechanism of HE. In the present study, the presence of hydrogen-enhanced strain-induced vacancies (HESIVs)—one type of defect associated with proposed HE mechanisms—was validated by low-strain-rate tensile tests with in-situ electrochemical hydrogen charging for tempered martensitic steel showing quasi-cleavage fracture with a tensile strength. The effect HESIVs on the mechanical properties of tempered martensitic steel was also studied. The combined use of low-temperature thermal desorption spectroscopy and tensile tests led to the following observations: (i) hydrogen enhanced the accumulation of vacancy-type defects under plastic strain, (ii) accumulated vacancy-type defects adversely affected the ductility of the tempered martensitic steel after hydrogen release, and (iii) aging at 150 °C after applying a given plastic strain with hydrogen charging decreased the amount of newly formed vacancy-type defects and resulted in recovery of ductility.

Abstract: Hydrogen embrittlement (HE) is a well-known issue, especially with ultrahigh-strength steels (UHSS). Various testing methods are utilised to study HE, but they typically require tensile test equipment, or are impractical due to limited stress control with standard geometries. We have developed a novel Tuning-fork test (TFT) to study HE susceptibility of steels with a new specimen geometry, which can be stressed accurately without tensile test equipment. The test method utilises in situ electrochemical hydrogen charging and constant displacement for stressing of the notched specimens by bending. Crack initiation and propagation are controlled with an isolated tensile stress region, and the failure process is monitored with a loadcell. TFT is a simple and fast testing method, which allows ranking of UHSSs, and to investigate, e.g., microstructural effects on susceptibility to HE and H-induced fracture processes. Here in this study, we present the state-of-the-art with the improved more precise second-generation TFT setup, which benefits from a more sensitive loadcell and a more stable fine-tuneable differential screw adjustment. We extend TFT to testing of martensitic steels with nominal hardness from 400 HBW to 600 HBW with the Incremental step loading technique (ISLT). The results show that TFT with ISLT is well applicable for ranking ultrahigh-strength steels based on their susceptibility to HE. Force-time data from ISLT can also be used for the determination of a material-specific threshold stress level, and the last step for the calculation of a crack initiation-time and time-to-fracture. However, the current manual operation of the loading screw can still limit maximum duration of a test.

Abstract: Austenitic stainless steels are commonly used for hydrogen storage and transportation. These alloys have a high nickel (Ni) content, which increases alloy cost. In this study, high manganese (Mn) austenitic alloys were evaluated as potential lower cost alternatives. Two heats of high Mn alloys with different stacking fault energies (SFE) of ~29 mJ·m^-2 and 49 mJ·m^-2 were acquired. Additionally, a new vanadium (V)-microalloyed high Mn alloy was designed to achieve a SFE of 47 mJ·m^-2 to minimize planar slip deformation mechanisms. Post-processing via cold working in conjunction with aging was also performed on the V-microalloyed high Mn steel. Hydrogen embrittlement sensitivity was investigated using circumferential notch tensile specimens cathodically charged with hydrogen in a 0.05M NaOH electrolytic solution. The alloys were compared to a cold-worked 316L stainless steel, which exhibited no strength loss due to hydrogen. The high Mn alloys with SFE of ~29 mJ·m² and 49 mJ·m^-2 had notch strength losses of 11 and 6 pct, respectively. The V-microalloyed high Mn steel in the as-hot-rolled condition had a notch strength loss of 17 pct. The V-microalloyed high Mn steel in the cold worked and aged condition indicated no notch strength loss in hydrogen, which was comparable to the performance of the 316L stainless steel.

Abstract: This study investigated the electrochemical behavior of Ni–Cu alloy plating baths during plating and dealloying reactions. Furthermore, the potential and plating time effects on the shape of three-dimensional structural Ni–Cu alloy plating film and its adhesion to epoxy resin were investigated. Nodules were observed on the surface of the plating film generated at potentials from-0.5 V to-1.0 V. When the plating films formed at potentials of-1.0 V to-1.5 V were dealloyed, the formation of pores was observed. The shear test results showed that the average shear strength of the specimen joined with epoxy resin between two Cu plates with Ni–Cu alloy plating film formed at a potential of-1.0 V was the highest under all conditions.

Abstract: This study investigated the microstructure and melting point of the Zn–Al composite electroplating film. The cross-sectional microstructure and shear strength of the joints made from the plating films were also evaluated. Zn and Al were confirmed in the plating films from initial microstructure observation. The plating film prepared by a plating bath without cationic surfactant melted near the melting point of Zn and the eutectic point of Zn–Al. When jointed at a joining temperature of 443°C, joining pressure of 5 MPa, and holding time of 5 min, multiple intermetallic compounds, Zn–Al eutectic layers, and unreacted Al particles were observed in the joint layer. From quantitative analysis, the multiple intermetallic compounds were estimated to be Zn–Ni and Zn–Al–Ni intermetallic compounds. The shear strength of the joints increased with increasing joining pressure but was lower than that of Sn–5Sb solder. Fracture after the shear test was observed at the interface between unreacted Al particles and Zn–Al–Ni intermetallic compounds, and inside unreacted Al particles.

Abstract: The effect of a small amount of Sb addition to Sn-3.0Ag-0.5Cu (mass%) solder on microstructures and impact properties of the solder ball joint was investigated. Cross-sectional microstructural observation revealed that scalloped Cu₆Sn₅ is formed at the solder/Cu interface, and Cu₃Sn is formed at the interface between Cu₆Sn₅ and Cu with aging. It was confirmed that the growth of the reaction layer is suppressed by the addition of Sb. Moreover, the result of the impact ball shear test showed that the decrease in impact properties with aging can be suppressed in Sn-3.0Ag-0.5Cu-1.5Sb (mass%). It was also suggested that the suppression of linear crack propagation at the Cu₃Sn/Cu₆Sn₅ interface is effective to prevent the reduction of absorbed energy.

Abstract: To realize ion trajectory control in processing plasmas for nano-fabrication, we applied amplitude modulation (AM) discharges to control of ion trajectory in high aspect trenches. We investigated behavior of incident ions in AR25 (aspect ratio = 25) trench structure in AM discharges using data of Ar⁺ ion with ion energy and ion angular distribution functions (IEDF and IADF) on the substrate obtained by the PIC-MCC model. AM discharges have higher ion flux onto the trench sidewalls than the continuous waveform (CW) discharges, whereas AM discharges have almost the same ion energy as CW ones. SRIM simulation results suggest that AM discharges can desorb more hydrogen atoms from TEOS-PECVD SiO₂ films on the trench sidewall than CW ones, which explains the previous results of improved SiO₂ film quality on trench sidewall by AM discharges.

Abstract: GaN-based nanopillar crystals are directly grown on multicrystalline Si and amorphous-carbon-coated graphite substrates whose surfaces are not mirror-polished. Light-emitting diodes (LEDs) of a double-hetero structure are prepared from the nanopillar crystals, and their optical–emission properties are investigated. Despite the substrate type and surface conditions, moderate light emissions are obtained from nanopillar LEDs though the light emissions are not always homogeneous, especially in the LEDs prepared on the graphite-based substrate. Nevertheless, these results will lead to realizations of novel large-area light-emitting devices.