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The phenomena of hydrogen induced cracking (HIC) in 304 stainless steels was considered in a hydrogen pressure and stress corrosive atmosphere. Microstructures with chloride pits and stress corrosion cracks around the HIC were analyzed by SEM/EDS. Abnormal phase transformations induced by the hydrogen were analyzed using TEM and diffraction. In the hydrogen pressure atmosphere, pits and pores were observed on the surface of the 304 stainless steels. In addition, it was determined that Cl, an etchant component, was concentrated at a high concentration in the pits. SCC (stress corrosion cracking) was induced in the Cl atmosphere by stress caused by the abrasive embedded in the pits. It was assumed that the SCC mechanism is similar to HIC in that it occurs in the surface tensile stress and Cl atmosphere and is accompanied by grain boundary cracks similar to IGSCC (inter-granular SCC). The deformation induced phase transformation accompanied by planar slip should be related to the main cause of HIC in the hydrogen pressured atmosphere. Abnormal forbidden spots between the main diffraction spots were induced by the HIC in the hydrogen attacked area, where the microstructure was hardened. Understanding the HIC mechanism related to chloride corrosion can be used to assess the fitness of austenitic stainless steels for uses where there is a possibility of various susceptible cracking in hydrogen and chloride atmospheres.

keyword : hydrogen induced crack, stainless steel, planar slip, forbidden diffraction, phase transformation, stress corrosion crack

Fe-Mn-Si based shape memory alloy is a new functional material that can be used to apply prestress to civil and structural components such as concrete structures. In this study, the effects of alloying elements Ni, C and Ti on the mechanical and shape memory properties of the alloy were investigated in detail using a base alloy composition of Fe-17Mn-5Si-5Cr (wt%). Enhanced shape recovery ratio and decreasing strength were observed in the alloy when 4 wt% Ni was added. With the further addition of 0.05 wt% C, the alloy showed even higher shape recovery behavior, although the mechanical strength was lowered by the C addition. Increasing the carbon content up to 0.1 wt% led to higher mechanical strength while the shape recovery ratio was decreased slightly. It was shown that the addition of 1 wt% Ti can significantly enhance the shape recovery behavior of the alloy. The recovery behavior of the alloy with the co-addition of 0.3 wt% C and 1 wt% Ti was similar to the alloy without Ti but containing 0.1 wt% C, even though the C content was significantly higher. The alloy with the co-addition of 0.3 wt% C and 1 wt% Ti has great potential as functional components in prestressed civil engineering structures, since the yield strength of the alloy is as high as 400 MPa in the solution heat treated condition and can be further improved by aging heat treatments, which precipitate TiC particles.

keyword : fe-shape memory alloy, Fe-Mn-Si, shape recovery, mechanical property, bending, microstructure

The effects of β phase decomposition on recrystallization and texture variation in Zr-2.5% Nb alloy pressure tube material were investigated. Isothermal annealing was conducted at 350 to 550 ℃ for 240 hours, and isothermal annealing was performed at 500 ℃ for 240 to 3,000 hours. The recrystallization and texture variation were analyzed by inverse pole figure variation using the XRD and EBSD methods. Annealing in α-Zr region at below 610 ℃ induced recrystallization and texture variation in the α-Zr. These results differ from those from a previous annealing study of the α+β region at 750-830 ℃. Annealing above 400 ℃ for 240 hours caused β-Zr decomposition into β-Nb. The decomposition of the β-phase by annealing above 475 ℃ caused a contraction of 7.5% in the d₍₁₁₀₎ spacing in the β-phase, and a reduction in the volume fraction of the β phase by about 80%. It seems that the stress internally formed by the lattice contraction of the β-phase provides the driving force for recrystallization. In addition, it suggests that the newly formed α-Zr produced by β phase decomposition provides new nucleation sites for recrystallization and causes texture variation in the α-Zr. The reason why the recrystallization and the texture variation occurs only in the α-Zr stable region at below 610 ℃ is discussed.

keyword : Zr-2, 5%Nb alloy, annealing, recrystallization, β phase decomposition, texture variation

This study aimed to investigate the wear performance of laser direct energy deposited Fe-8Cr- 3V-2Mo-2W alloy under various wear environments, in terms of different heat treatment conditions. Ball on disk tribology tests were performed using high-carbon steel and zirconia balls as counter materials. The wear rates of the alloy depended significantly on both the wear sliding speed and the wear load. Microstructural observations of the worn surface and the wear debris indicated intensive tribo-oxidative wear that was presumably responsible for the strong dependency of the wear rate on the wear sliding speed. Regardless of the type of counter materials, the alloy in the as-built state had better wear performance than the alloy with heat treatments. Therefore, the use of the alloy without post heat treatment would be favorable to obtain long-term durability of the alloy in severe wear environments. The wear tests with two different counter materials of high-carbon steel and zirconia showed the highcarbon steel counter material had a higher wear rate than the zirconia. This was thought to be due to that strong third-body abrasive actions of the high-carbon steel counter material, evidenced by the severe abrasive wear of the counter material.

keyword : direct energy deposition, ball on disk, metal additive manufacturing, high-wear resistance steel, wear, heat treatment

The thermal storage tank in a district heating system is a component that stores excess heat during normal operation and releases the stored heat to increase the efficiency of the system, when the heat source is stopped or additional demands occur. Recently, an obsolete thermal storage tank was dismantled for the first time since it began operation 30 years ago. In this work, the corrosion integrity of the obsolete thermal storage tank was evaluated by examining its appearance, thickness thinning, corrosion products, microstructure, and mechanical properties. Samples were taken at various locations (roof, shell, bottom) of the thermal storage tank, which enabled diagnosis of the respective environmental degradations. Severe corrosion was found in the roof edge plate due to corrosion under the insulation, and exhibited thinning exceeding ~49% of the designed thickness. In this location, the ferrite-pearlite band structure disappeared and deteriorated microstructures, such as decarburization and spheroidized pearlite, were measured, which resulted in a ~27% decrease in hardness. The inner surfaces of the bottom and shell plate were well covered with a magnetite film, and the degradation of the microstructure and mechanical properties showed a permissible limit in terms of ASTM A285/ A516. In addition, no particular drop in hardness was found in the weld zone of each plate.

keyword : district heating system, thermal storage tank, corrosion, degradation, structural integrity

Al-Mg-Si alloy was rolled asymmetrically at several temperatures to apply shear deformation, and the effects of the initial precipitate on shear deformation, texture evolution, formability, and plastic anisotropy were studied. Texture was analyzed using a EBSD, and the formability and plastic anisotropy of the specimen were evaluated using the □ value and Δr value calculated from the plastic strain ratio (r-value) which was determined from the change in the length of the specimen during tensile deformation. Asymmetric rolling induces a larger equivalent strain than symmetric rolling, and the equivalent strain increases as the asymmetric rolling temperature increases. When a specimen with peak-aged initial precipitates was asymmetrically rolled, less shear deformation occurred at room temperature than in a solution-treated specimen without initial precipitates. In contrast, a larger shear deformation occurred at high temperatures (500°C). With asymmetric rolling at room temperature, the specimens without initial precipitates had higher formability and lower plasticity, while for asymmetric rolling at high temperature, the specimens with initial precipitates had higher formability and lower plastic anisotropy. This is due to the <111>//ND texture, such as {111}<110> and {111}<112> orientation that has similar and high r-values at 0°, 45°, and 90° to the rolling direction, developed by the shear deformation that occurred during asymmetric rolling.

keyword : Al-Mg-Si alloy, asymmetric rolling, precipitate, shear deformation, texture, formability

In this study, the critical role of substrate bias during the sputter deposition of Al thin films is discussed. Two sets of Al thin films having a nominal thickness of 300 nm were deposited at sputtering pressures of 4.1 and 1.5 mTorr, respectively, with an applied negative substrate bias in the range of 0-200 V. It was found that the microstructure, surface roughness, film resistivity and grain size were greatly altered by the combination of bias magnitudes and sputtering pressures. The sputtering pressure of 4.1 mTorr resulted in greater changes in the film properties with the application of substrate bias, and a lesser but still significant degree was observed for the films deposited at 1.5 mTorr. The resistivity values for the films deposited at 1.5 mTorr were found to be significantly lower, with the lowest resistivity value of 3.1 μΩcm achieved at a substrate bias of 50 V. Based on grain size measured by the line intercept method and Mayadas- Shatzkes grain boundary scattering model, the resistivity contribution of grain boundary scattering for the lowest-resistivity film was found to be 0.37 μΩcm, which indicates that the film resistivity in the optimized condition is close to the known bulk resistivity of 2.65 μΩcm.

keyword : aluminum, sputter, thin films, substrate bias, grain size

In this study, we report the influence of Te vacancy formation on the thermoelectric properties of n-type Cu_0.008Bi₂Te_2.7Se_0.3 alloys, including their electronic and thermal transport properties. Te-deficient Cu_0.008Bi₂Te_2.7-xSe_0.3 (x = 0, 0.005, 0.01 and 0.02) samples were systematically synthesized and characterized. Regarding electronic transport properties, carrier concentration was increased with Te vacancies, while carrier mobility was maintained. As a result, the electrical conductivity significantly increased while the Seebeck coefficient reduced moderately, thus, the power factor was enhanced from 3.04 mW/mK² (pristine) to 3.22 mW/mK² (x = 0.02) at 300 K. Further analysis based on a single parabolic band model revealed that the weighted mobility of the conduction band increased, which is favorable for electron transport, as Te vacancies were generated. Regarding thermal transport properties, lattice thermal conductivity decreased with Te vacancies due to additional point defect phonon scattering, however, total thermal conductivity increased due to larger electronic contribution as Te vacancies increased. Analysis using the Debye-Callaway model suggests that the phonon scattering by the Te vacancies is as efficient as the substitution point defect scattering. Consequently, the thermoelectric figure of merit zT increased at all temperatures for x = 0.005 and 0.01. The maximum zT of 0.95 was achieved for Te-deficient Cu_0.008Bi₂Te_2.69Se_0.3 (x = 0.01) at 400 K.

keyword : thermoelectric, vacancy, phonon scattering, Bi_2Te3

We introduce the Korea Institute of Science and Technology-Novel Materials Discovery (KISTNOMAD) platform, a materials data repository. We describe its functionality and novel features from an academic viewpoint. It is a data repository designed for computational material science, especially focusing on managing and sharing the results of molecular dynamics simulation results as well as quantum mechanical computations. It consists of three main components: a database, file storage, and web-based front end. The database hosts material properties, which are extracted from the computational results. The front end has a graphical user interface and an open application programming interface, which allow researchers to interact with the system more easily. KIST-NOMAD’s panel displays the searched results on a well-organized and research-oriented web page. All the open access data and files are available for downloading in comma-separated value format as well as zipped archives. This automated extraction function was developed by utilizing database parsers and JSON scripts. KISTNOMAD also has an efficient option to download simulation and computation results on a large-scale. All of the above functions are designed to satisfy academic and research demands, and make highthroughput screening available, while incorporating machine learning for computational material engineering. We finally stress that the repository platform is user-driven and user-friendly. It is clearly designed to follow the modern big-data architecture and re-use principles for scientific data, such as being findable, accessible, and interoperable.

keyword : repository for computational material science, open-access data sharing, scientific data, data extraction