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Recently, strengthened environmental regulations have required the downsizing of gasoline engines, and as a result, demand for gasoline turbochargers has rapidly increased. The vane of a turbocharger controls the flow of gases toward the turbine, and it is manufactured by powder metallurgy due to its complex shape. Gasoline engines have a high exhaust gas temperature (~1000℃), and thus, Alloy1.4957 (GX15CrNiCo21-20-20) containing large amounts of Cr, Ni, and Co is used. In this study, Alloy1.4957 powders were sintered by hot isostatic pressing (HIP), and then homogenized and thermally exposed to exhaust gas temperatures. Then, a microstructural analysis was conducted to observe the changes that occurred for each process. M₆X carbonitride containing Si, called Cr₃Ni₂SiX, was observed to be the main precipitate phase in this alloy. In general, it is known that Cr₃Ni₂SiX is only rarely observed in heat-resistant steel. However, in Alloy1.4957, a large amount of Cr₃Ni₂SiX was precipitated or dissolved depending on the process, and this is probably due to the high Si and N content of Alloy1.4957. In addition to Cr₃Ni₂SiX, Cr₂₃C₆ and NbX were observed. Cr₂₃C₆ was dissolved during the homogenization process, but NbX, which has high thermal stability, retained a fine size during the homogenization process and provided a nucleation site for Cr₃Ni₂SiX during thermal exposure. (Received 18 May, 2023; Accepted 5 July, 2023)

keyword : heat-resistant steel, precipitation, powder metallurgy, turbocharger, carbonitride

To analyze the effects of intercritical heat treatment on the temper embrittlement of SA508 Gr.4N steels, two model alloys with different phosphorus (P) contents were fabricated. Each sample was heat treated by applying a conventional heat treatment process (quenching-tempering) with/without an intercritical heat treatment process (IHT) and a step-cooling heat treatment for temper embrittlement. Then their microstructure and mechanical properties were evaluated. The microstructure of the SA508 Gr.4N model alloy was composed of tempered lower bainite and martensite, and nano-sized precipitates formed both inside and at boundaries. The grain size was reduced when IHT was applied. There was a small difference in tensile properties according to the heat-treatment conditions and P contents, but the difference in Charpy impact properties was large. The heat treatment for temper embrittlement (TE) increased the impact transition temperature, and a very significant increase was observed in steels with a high P content. The increase in transition temperature owing to TE was reduced when IHT was applied. The fractograph analysis of Charpy fractured specimens at transition temperatures showed that an increase in intergranular fracture was main reason for the TE, and that IHT reduced the formation of intergranular fracture. The AES results showed that P-Ni was segregated at grain boundaries, and the level of segregation was reduced by applying IHT. This occurred because the formation of prior austenite grain boundaries by IHT dispersed the P at grain boundaries, and reduced the amount of P segregation. (Received 11 May, 2023; Accepted 17 August, 2023)

keyword : Reactor pressure vessel, Ni-Cr-Mo low alloy steel, SA508 Gr.4N, temper embrittlement, Intercritical heat treatment

The addition of expensive elements to β titanium alloys, which are widely used in the aerospace and automobile industries, causes their price to increase. Low-cost, high-strength alloy (Ti-12.1Mo-1.0Fe) was designed in this study and the compressive behaviors of this alloy were evaluated for application to automobile parts, etc. As a result of ambient compression in the quasi-static and dynamic strain rate range (1×10-4/ s~6×103/s), twinning occurred at all strain rates, and in particular, adiabatic shear bands were observed, which cause cracks and fractures under dynamic strain rate conditions. In addition, when the relationship between these bands and mechanical characteristics was evaluated, an increase of compression strength and Vickers hardness was confirmed to occur, due to the strain rate hardening effect under compression loading. Some twins were formed by the deformation behavior during compression plastic deformation of the Ti- 12.1Mo-1.0Fe alloy, and it exhibited excellent compression strength characteristics, showing a very high strain rate hardening effect at a high strain rate. (Received 12 May, 2023; Accepted 5 July, 2023)

keyword : Beta Ti alloy, deformation behavior, strain rate, twinning, split hopkinson pressure bar (SHPB)

To suppress Ag dewetting from around 200 ℃ in Ag-coated Cu (Cu@Ag) flakes, Ag and Ni-coated Cu (Cu@Ni@Ag) flakes were fabricated by successive Ni and Ag electroless plating. The Ni bath type was an important consideration to induce differences in the Ag dewetting and resultant Cu oxidation. An acid Ni bath contained succinic acid as a complexing agent and sodium hypophosphite as a reductant, and an alkaline Ni bath contained sodium citrate as a complexing agent and sodium hydroxide as a pH adjuster as well as sodium hypophosphite. A hydrazine-based Ni bath contained sodium citrate, sodium hydroxide, and hydrazine hydrate as a reductant. The acid Ni bath provided amorphous coatings with a P content of approximately 10 wt%. The Cu@Ni@Ag flakes started the Ag dewetting and Cu oxidation at 350 ℃, together with the formation of the Ni₃P phase. Meanwhile, the alkaline Ni bath created Ni-5 wt% P amorphous Ni coatings, which transformed into a crystalline phase after heating at 350 ℃. The Ag shell was dewetted at 450 ℃, which caused oxidation of the flakes. Finally, the hydrazine-based Ni bath formed crystalline coatings without P, which induced rapid mixing with the core Cu. The Ag shells on the mixed Cu-Ni alloy showed repressed dewetting behavior, and thus the dewetting and oxidation temperature was the highest, such as 500 ℃. Enhancing the high oxidation resistance at approximately 300 ℃ will enable the use of Cu@Ni@Ag flakes as a low-cost filler material in conductive pastes, especially for long-time or hightemperature curing. (Received 3 February, 2023; Accepted 4 July 2023)

keyword : Cu flake, Ni plating, Ag plating, Ag dewetting, flakes oxidation

In this study, AISI 316L stainless steels were manufactured by the L-DED process with various laser scanning speeds and laser powers, in order to analyze how volumetric energy density (VED) can affect their final shape, microstructure and mechanical properties. Under various VED conditions, it was found that a higher VED decreases the formation of lack of fusion when the powder feed rate is adequate, and the ratio of VED and powder feed rate affect the dimensional accuracy. Furthermore, the microstructure and mechanical properties of the L-DED processed AISI 316L were not significantly influenced by the laser scanning speed when the same VED was used, indicating that the solidification rate and solidification cell size are mainly determined by the VED. (Received 25 May, 2023; Accepted 12 July, 2023)

keyword : Additive manufacturing, AISI 316L, L-DED, Dimensional accuracy

This study aimed to investigate the effect of process parameters on the microstructure and mechanical properties of Inconel 625 alloy manufactured by direct energy deposition process. The Inconel 625 samples were produced by varying the laser scanning speeds from 720 - 960 mm/min while maintaining the same the laser energy volume density. The microstructure and mechanical properties of the produced samples were evaluated, and their dimensional accuracy and mechanical properties were also analyzed in terms of the laser scanning strategy. Microstructural observations at the same energy density revealed a dendrite substructure near the laser melt pool boundaries, indicating that the dendritic microstructure was primarily formed at the beginning of the solidification of each laser bead. When the solidification further progressed into the melt pool, solidification cell substructures became dominant regardless of the laser scanning speed. The size of the solidification cell and the dendrite structure were nearly unchanged as laser scanning speed increased. This suggests that changing the laser scanning speed while maintaining the volumetric energy density does not significantly alter the solidification rates of the Inconel 625. As a consequence of the similar cell sizes, the samples produced with different laser scanning speed led to similar mechanical properties. When samples produced with two different scanning strategies, of unidirectional and 90o rotation, were compared, a better dimensional accuracy was obtained with the 90o rotation strategy, compared to that obtained with the unidirectional approach. Comparisons of mechanical properties obtained along different directions with the different laser scanning strategies revealed that the Inconel 625 produced by the laser direct energy deposition had pronounced anisotropic mechanical properties, and was the highest in strength but the lowest in maximum elongation along the laser scanning direction. (Received 25 May, 2023; Accepted 12 July, 2023)

keyword : Direct Energy Deposition, Inconel 625, Superalloy, Microstructure

Mg₃Sb₂-based materials are promising candidates to replace n-type Bi₂Te₃ for cooling and power generation at low temperatures. Generally, the thermoelectric performance of a material is sensitively affected by synthesis process parameters, and among them, sintering temperature (T_s) is a critical one. In this study, n-type Mg₃SbBi_0.99Te_0.01 polycrystalline samples were fabricated by mechanical alloying and spark plasma sintering (SPS), and the effects of varying T_s (923 - 1073 K) on the thermoelectric properties were investigated. Sintering Mg₃SbBi_0.99Te_0.01 at an elevated temperature of 1073 K resulted in a notable increase in electrical conductivity at low temperatures below about 423 K. This is ascribed to a sharp reduction in carrier scattering by ionized impurities. For the same reason, the carrier mobility increased sharply at a T_s of 1073 K, which is a critical temperature for sintering in this study. Moreover, the Seebeck coefficient increased and thermal conductivity decreased simultaneously by raising T_s, resulting in the maximum power factor (PF_max) of 2.2 × 10^-3 W m^-1K^-2 and the maximum dimensionless figure of merit (zT_max) of 1.20 in the sample sintered at 1073 K. Therefore, when T_s was raised from 923 K to 1073 K, the PF_max and zT_max increased by 29 % and 64 %, respectively. This improvement in performance is attributed to the annihilation of defects generated during the mechanical alloying process, which was confirmed by microstructure analysis by transmission electron microscopy (TEM). (Received 21 April, 2023; Accepted 28 June, 2023)

keyword : Mg₃SbBi_0.99Te_0.01, sintering temperature, mechanical alloying

Polymer composites enriched with conductive fillers hold immense potential for flexible pressure sensors, because of the remarkable piezoresistive effect they possess. Research on polymer composite pressure sensors has been actively ongoing because of their flexibility and high electrical resistance performance. This paper presents a comprehensive comparison of the piezoresistive characteristics of conductive composites for pressure sensors, focusing on the influence of carbon nanotube (CNT) content and aspect ratio. Polymer composites with conductive fillers, specifically CNTs, have demonstrated significant potential for pressure sensing applications based on the piezoresistive effect. By systematically varying the CNT concentration and aspect ratio, we investigated the impact of these parameters on the piezoresistive behavior of the composites. A pressure in the range of 0-200 kPa was applied to the conductive composite, and resistance change due to pressing was measured. The best performing samples were evaluated in 150 cycle tests to verify repeatability and durability. Experimental analysis and characterization revealed the intricate relationship between CNT content, aspect ratio, and the resulting piezoresistive properties. Through this study, we aim to enhance understanding of how CNT concentration and aspect ratio influence the performance of CNT/PDMS composites as pressure sensors, thereby facilitating the development of optimized sensing materials for various pressure sensing applications. (Received 8 June, 2023; Accepted 5 July, 2023)

keyword : carbon nanotube, piezoresistive, aspect ratio, content, polymer composite

Glass fiber-reinforced polymers (GFRPs) have attracted significant attention as structural materials because of their high fatigue resistance, corrosion resistance, strength, and stiffness. This study examined the effect of elevated temperatures (150, 250, 350, and 450℃) on the microstructural and mechanical properties of GFRP plates. The number of bubbles increased as the firing temperature increased, and the bubbles burst at 250℃ or higher, forming pores on the surface. A tensile test was conducted, and the maximum stress of the GFRP plates fired at 150, 250, and 350℃ was reduced from 54.2 to 52.2, 40.3, and 24.0 MPa, respectively, compared to that of the unfired GFRP plate. Meanwhile, the elastic moduli of the GFRP plates fired at 150, 250, and 350℃ reduced from 19.1 to 18.3, 16.1, and 12.1 GPa, respectively, compared to that of the unfired GFRP plate. This reduction in the mechanical properties of the GFRP plates at elevated temperatures was attributed to the degradation of the mechanical properties of the resin matrix due to glass transition and decomposition, debonding, and an increase in surface defects. The maximum strain decreased gradually with increasing firing temperature, suggesting that the brittleness of the GFRP plates increased at elevated temperatures. (Received 22 June, 2023; Accepted 21 July, 2023)

keyword : glass fiber reinforced polymers, degradation, tensile strength, glass fiber