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Computational approaches such as first-principles calculations, atomistic simulations, phase field simulations, computational thermodynamics and finite element method simulations are widely used in the metals and materials community. Even with their successful applications for better understanding of material phenomena, and the design of new materials or processes, a gap remains between the results of computational approaches and experimental data. This gap originates with differences in the computational and experimental conditions, and limits the wider application of computational approaches. In this review article, some successful examples of computational materials and process design are outlined, focusing on ways to utilize the results of computational approaches. It will be emphasized that it is more important to clarify the governing mechanism of materials phenomena from the effects of individual experimental variables on simulation results rather than make an effort to obtain good agreement between simulations and experiments with phenomenological issues. It will be also emphasized that research efforts to extend the applicability of the computational approaches are continuously required.

keyword : computational materials science, alloy and process design, atomistic simulation, computational, thermodynamics, phase field method

This study was carried out to investigate the effect of thermo-mechanical treatment on the formation behavior of martensite in 316L stainless steel. Dislocation, α and ε-martensite were formed by thermo-mechanical treatment. Martensite with surface relief and specific direction was formed by thermomechanical treatment. Martensite formed by thermo-mechanical treatment were reversed to austenite with an ultra-fine grain size of less than 1 μm by annealing treatment at 700℃ for 20 min. α -martensite with a K-S orientation relationship and ε-martensite with a [110] γ [100]ε orientation relationship to the matrix was formed by thermo-mechanical treatment. The volume fraction of dislocation, α and ε-martensite were increased with the cycle number of thermo-mechanical treatment. In 5-cycle number thermo-mechanical treated specimens, more than 25% of the volume fraction of α -martensite and less than 5% of the volume fraction of ε-martensite were attained. The austenite grains were fairly deformed by thermo-mechanical treatment and the grain size of austenite with various orientation relationship were changed from micrometer to sub-micrometer by the 5-cycle thermo-mechanical treatment.

keyword : thermo-mechanical treatment, 316L stainless steel, α -martensite, ε-martensite, TEM, EBSD

Cr_25.2Al_19.5Si_4.7N_50.5 thin films were deposited on steel substrates by cathodic arc plasma deposition. They consisted of alternating fcc-Cr(Al)N/hcp-Al(Cr,Si)N nanolayers. They were corroded at 900 and 1000 ℃ for 5-100 h in N₂/0.1%H₂S gas atmosphere. Their corrosion mechanism, the structure of the formed scales, and the role of film-constituting elements during corrosion were studied. Despite the presence of hydrogen and sulfur in the corrosion gas, the films displayed good corrosion resistance because Cr, Al, Fe preferentially reacted with impurity oxygen in the corrosion gas to form Cr₂O₃, Fe₃O₄, and amorphous Al₂O₃ owing to the thermodynamic stability of these oxides. The films were corroded by the outward diffusion of Cr, Al, Fe and nitrogen as well as the inward transport of sulfur and then oxygen. Silicon in the film was primarily oxidized by the inwardly transported oxygen. This study aimed to investigate the high-temperature corrosion behavior of nano-multilayered Cr_25.2Al_19.5Si_4.7N_50.5 thin films in a serious H₂S-containing environment.

keyword : CrAlSiN thin film, H₂S corrosion, oxidation, sulfidation

The effects of heat treatment and thermal cycle conditions on the bonding strength and interfacial characteristics of electroplated Cr/electroplated Ni-P coatings were systematically evaluated. With step1 samples, heat treatment was performed at 750 ℃ for 6 hours after successive electroplating of Ni-P and Cr layers. With step2 samples, 1st heat treatment was performed at 500 ℃ for 3 hours after electroplating Ni- P, and then 2nd heat treatment was performed at 750 ℃ for 6 hours after electroplating Cr. The bonding strengths measured by ASTM C633 were around 54.6 MPa before thermal cycling, and decreased to 35 MPa and 29.9 MPa for the step1 and the step2, respectively, after 1,000 cycles between 150~600 ℃. Increasing the number of thermal cycles led to an increasing adhesive/Cr interface delaminated mode, which seemed to be closely related to the decreaed bonding strength possibly due to severe surface Cr oxidation. Also, the higher residual stress was investigated in the step1 than step2 samples. It is thought to be responsible for the higher density of channeling cracks in the Cr layer, which led to slightly stronger bonding strength due to the mechanical interlocking effects of the adhesive inside the channeling cracks.

keyword : metals, plating, microstructure, tensile test, thermal cycle

Oxi-nitrocarburizing treatment was performed to improve the anti-corrosion and anti-wear properties of low carbon steels. The oxi-nitrocarburizing was carried out in 2 steps: a gas nitrocarburizing step at 560 ℃ and an oxidation step at 550 ℃ for 30 min with steamed water. The mechanical and chemical properties of the samples were then measured using nanoindentaion test with a load of 10 mN and salt spray test. Investigation on the surface hardening layer with scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis and electron prove microanalysis (EPMA) were also carried out. The thickness and nanohardness of the oxide layer were about 4 μm and 470 Hv, respectively. The oxi-nitrocarburized sample fabricated by gas nitrocarburizing for 50 min followed by oxidation for 30 min showed much better anticorrosion property than the sample prepared by the conventional gas nitrocarburizing method for 100 min. The improved anti-corrosion property of the oxi-nitrocarburized samples was due to the formation of a dense magnetite (Fe₃O₄) layer during the oxidation process on the porous nitride layer. These results indicate that oxi-nitrocarburizing is effective for surface treatment of low carbon steels with high anti-corrosion and high wear resistance.

keyword : oxi-nitrocarburizing, low carbon steel, nanoindentation, nitride layer, oxide layer

347 austenitic stainless steel weld (ASSW) was thermally aged at 343, 400 and 450 °C up to 20,000 h in this study. Effects of thermal aging induced microstructure evolution on the mechanical properties and corrosion resistance of delta-ferrite in the 347 ASSW were qualitatively and quantitatively assessed by high resolution transmission electron microscopy (0.07 nm) with energy-dispersive spectroscope (EDS) and Fast Fourier Transform (FFT). After thermal aging at 343 °C for 20,000 h and 400 °C over 5,000 h, fluctuation of major alloying elements such as Fe, Cr, and Ni was observed by spinodal decomposition in the delta-ferrite. Meanwhile, a Ni+Si-rich G phase was developed as thermal aging progressed at 400 °C for 20,000 h and 450 °C for 5,000 h in the delta-ferrite. Such microstructure evolutions tended to be accelerated with increasing aging temperature and exposure period, while the G phase was formed at a higher exposure temperature and/ or period compared to spinidal decomposition. These effects increased the tensile strength and decreased the elongation of 347 ASSW at room temperature, compared to the as-welded condition. Moreover, when spinodal decomposition and G phase were observed, the degree of sensitization values of the 347 ASSW in the double loop-electrochemical potentiodynamic reactivation tests were significantly increased, due to localized Cr depletion in the delta-ferrite.

keyword : thermal aging, austenitic stainless steel weld, delta-ferrite, spinodal decomposition, sensitization

We investigated the electrical properties and gate bias stress stability of solution-processed amorphous oxide thin film transistors (TFTs) with multi-stacked active layers. With the multi-layered InZnO (In:Zn = 1:1), mobility was increased from 4.6 to 21.2 cm²V^-1s^-1 and the subthreshold swing (SS) was improved from 0.71 to 0.54 V/decade compared to the single-layered InZnO TFT. The tri-layered InZnO TFT showed a reduced threshold voltage shift (ΔV_th) under positive bias stress (PBS) from +4.4 to + 0.9 V, whereas ΔV_th under negative bias stress (NBS) deteriorated from -0.03 to -1.5 V. We also fabricated bi-layered (bottom/top layer) TFTs using different oxide compositions with InZnO (In:Zn = 7:3) and InGaZnO (In:Zn:Ga = 3:3:1) which showed high mobility and small ΔV_th under PBS and NBS. The InZnO/InGaZnO TFT showed a high mobility of 17 cm²V^-1s^-1, SS of 0.65 V/decade, and good stability, with ΔV_th under PBS and NBS of +1.2 and -1.2 V, respectively. Both the electrical properties and gate bias stress (GBS) stability were better with the InZnO/InGaZnO TFT than the single-layered InZnO TFT.

keyword : oxide thin film transistor, multi-active layers, solution-process

Encapsulation of the flexible OLED devices requires a low temperature process and great barrier characteristics against the permeation of water vapor and oxygen. We suggest a new OLED encapsulation method for the flexible OLED device using a sealing line instead of the TFE encapsulation. For the experiments, polyimide substrate with Cu layer was used. First, 0.5 mm width Cu pattern was formed on the polyimide substrate and sealing material was printed on it, followed by the heating. For the sealing line, two different designs are compared. First, a mixture of LMPA and epoxy was investigated to develope the LMPA-epoxy double line structure. In this case, control of the spreadability of the epoxy was a key issue, as the epoxy spread out on the polyimide substrate when the viscosity was low, therefore, various ways were tried to control the spreadability of the polymer. Secondly, LMPA single line was manufactured and the reliability characteristics such as adhesion, bending characteristics and WVTR(water vapor transmission ratio) were investigated and compared with the double line structure.

keyword : flexible, OLED(Organic light emitting diode), encapsulation, sealing, barrier, LMPA(Low melting point alloy)

Changes in the properties of the hole transport layer (HTL) and the perovskite layer in a perovskite solar sell (PSC) were observed under various Au thermal evaporation conditions. We employed a conventional thermal evaporator with a distance of 30 cm between the source and substrate in a vacuum of 6 × 10^-6 torr. We prepared 70 nm-thick Au layers on PSCs by varying the evaporation condition: 60 W-90 min, 70 W-60 min, 80 W-25 min, 90 W-10 min, and 100 W-7 min. Subsequently, the photovoltaic properties of the PSC were measured for each deposition condition. Also, the microstructure of the HTL/perovskite was observed using a scanning electron microscope (SEM) and electron probe xray micro-analyzer (EPMA) after the Au layer was selectively removed. The most outstanding energy conversion efficiency (ECE) was measured for the 80 W-25 min condition. The ECE was extremely low at relatively low power-long time (60 W-90 min, 70 W-60 min) conditions, due to contamination of the HTL and the perovskite layers during the Au evaporation. The contamination was confirmed from the SEM and EPMA investigations. Moreover, even with high powered-short time (90 W-10 min, 100 W-7 min) conditions, the HTL underlayer of the electrode was damaged due to the high deposition energy.

keyword : evaporation, deposition rate, deposition time, perovskite solar cells

Traditional organic binders used by the foundry industry produce problems such as generation of volatile organic compounds(VOCs), smoke and etc. A water glass binder system has been developed to improve the casting strength and to solve the environmental problems. However the water glass binder has a serious problem of sand burning. In this study, a commercial water glass binder was modified to reduce sand burning during aluminium casting by the addition of Calcium carbonate, activated carbon, and monosaccharide. The modified water glass binder was characterized by XRF, viscometer and TGA-DTA. We then evaluated the water glass binder core strength. Casting tests were conducted using Al-Si7Mg alloy to compare the ability to prevent sand burning in a sand mold. The XRF results of the prepared modified water glass binder showed a similar molar ratio of SiO₂/Na₂O to the commercial one. From the TGA-DTA analysis and gas emission calculation, it was considered that gas generation and corresponding inhibition of sand burning would be maximized with the monosaccharide-added water glass binder. Casting evaluation showed reduced sand burning for the water glass binder with mono saccharide. Also, core strength was found to be similar to the commercial water glass binder. These results prove that the monosaccharide-added water glass binder could replace the traditional organic binder.

keyword : inorganic binder, sand burning, Al-Si7Mg alloy casting, sand core