Keesam Shin

Microstructure and mechanical properties of a thixocast Mg–Cu–Y alloy

Abstract The possibility of producing high strength Mg–Cu–Y alloys by the thixocasting process was investigated. A microstructure, consisting of globular primary Mg particles surrounded by quenched liquid (partly amorphized), could be obtained and gave significantly high tensile strength compared to conventional Mg castings.

Property improvement of stainless-steel-base surface composites fabricated by high-energy electron-beam irradiation

This is a study on the fabrication of surface composites of SiC, TiC particulates, and AISI 304 substrate by high voltage electron beam irradiation. Using CaF2 powders as flux, two kinds of surface composites were fabricated for a comparative analysis of the microstructural modification and mechanical properties. Through the employed process, the powders and substrate surface were melted and surface composite layers were successfully formed in both cases. In the specimen fabricated with SiC powders, a volume fraction of Cr23C6 particles (−22 vol.%) were homogeneously distributed along solidification cell boundaries. The large amount of Cr23C6 particles in combination with solid solution hardening of Si in the matrix resulted in the improved hardness and wear resistance of the surface composite layer, that are 2 to 3 times those of the substrate. In the specimen fabricated with SiC and Ti+SiC powders, TiC and Cr23C6 particles were precipitated without precipitation of SiC.

Effect of tempering on hardness improvement in a VC/steel surface-alloyed material fabricated by high-energy electron-beam irradiation

Abstract The present study is concerned with the tempering effect in improving the hardness of a vanadium carbide (VC)/carbon steel surface-alloyed material fabricated by high-energy electron-beam irradiation. The mixture of VC powders and flux (50%MgO–50%CaO) was placed on a plain carbon steel substrate, and then electron beam was irradiated. The surface-alloyed layer of 1.8 mm in thickness was homogeneously formed without defects. The microstructural analysis indicated that coarse VC particles were formed along solidification cell boundaries, and the matrix inside cells was mostly composed of lath-type martensite and fine cuboidal VC particles. A large amount of these VC particles in the lath-type martensitic matrix provided hardness four times greater than that of the substrate. When the VC/steel surface-alloyed material was tempered, fine VC particles precipitated in the tempered martensitic matrix, thereby leading to additional hardness increase. In addition, reduction of residual stress and an increase in fracture toughness could be expected.

Simple phenomenological determination of contact stiffness and elastic modulus of Ce-based bulk metallic glasses through nanoindentation

The viscoelastic deformation of Ce-based bulk metallic glasses (BMGs) with low glass transition temperature is investigated at room temperature. Contact stiffness and elastic modulus of Ce-based BMGs cannot be derived using the conventional Oliver-Pharr method [W. C. Oliver and G. M. Pharr, J. Mater. Res. 7, 1564 (1992)]. The present work shows that the time dependent displacement of unloading segments can be described well by a generalized Kelvin model. Thus, a modified Oliver-Pharr method is proposed to evaluate the contact stiffness and elastic modulus, which does, in fact, reproduce the values obtained via uniaxial compression tests. (c) 2007 American Institute of Physics.

Effect of Ultrasonic Nanocrytalline Surface Modification on the Microstructural Evolution of Inconel 690 Alloy

Specimens of Inconel 690 were investigated after ultrasonic nanocrystalline surface modification (UNSM) using microhardness tests, electron backscattered diffraction, and transmission electron microscopy (TEM). After UNSM treatment, a 60% increase of hardness up to ∼310 µm in depth was observed. Layer-by-layer TEM analysis showed well-refined grains and twins in addition to the high dislocations density. The mechanism of the microstructure refinement was attributed to the development of nano-grains, twin structures, and dislocations.

Alloying effect of copper on the corrosion properties of low-alloy steel for flue gas desulfurization system

The alloying effect of Cu for a flue gas desulfurization materials was investigated using the electrochemical methods in the modified green death solution and the surface analyses. The test results demonstrated that the densely formed rust layer with high metallic Cu content improves the corrosion resistance of Cu-containing steel in the flue gas desulfurization (FGD) environment. The rust layer on the surface of the 0.02 wt% Cu steel, which has an insufficient Cu content, was less protective than others. The 0.05 wt% Cu steel represented the highest corrosion resistance due to the formation of the densely formed rust layer with optimum Cu content. Because the free standing Cu2S precipitates had the insoluble characteristic in highly acidic solution, it produced the relatively porous Cu-enriched layer on the 0.08 wt% Cu steel surface. From these phenomena, the corrosion resistance of specimen decreased as the Cu content of specimen increased from 0.05 wt% to 0.08 wt%.

Microstructural Characterization of SS304 upon Various Shot Peening Treatments

In the majority of cases, material failure initiates from the surface because of fatigue, creep, wear, corrosion, etc. Thus, optimizing the surface microstructure enhances the general behavior of a material in terms of fatigue life, friction, wear, corrosion and even tomography, and also its lifetime. Thanks to intensive and extensive studies on nanostructured metals in the past several decades, the beneficial effects of optimizing nanostructure of metals and alloys have become more and more obvious (Gleiter, 2000; Meyers et al., 2006), e.g., ultra-high hardness and strength (Liu et al., 2013), enhanced physical properties (Lu et al., 2004), enhanced corrosion resistance (Lee et al., 2009). Fabrication of a nanostructure on the surface of a bulk metal can involve either coating it with a dense, hard film creating a surface alloy, or inducing a phase transformation via plastic deformation such as a strain induced surface grain refinement (Mayer et al., 2012), or also nano crystallization (Lu & Lu, 2004; Zhong et al., 2010). Of course, even the combination of the methods can be used for some specific applications (Du et al., 2009; Hong et al., 2011). Shot peening technologies have been used for a long time and are proven to be an effective method for introducing severe plastic deformation to the surface region, thereby refining and possibly nanocrystallizing the surface microstructure (Tong et al., 2003; Umemoto et al., 2003; Suh et al., 2007). One such method, air blast shot peening (ABSP), utilizes hard balls (tungsten carbide) for the energy transfer from compressed air to the surface of the work pieces. ABSP is a very flexible technology and is normally used in a cast factory to clear and strengthen the surface of the castings. However, little is known

Development of the Hybrid Conjugated Polymer Solar Cell Based on GaN Quantum Dots

We report the hybrid p–n junction based on GaN quantum dots (QDs) as an electron transport layer and poly(3-hexylthiophene-2,5-diyl) (P3HT) as a hole transport layer, which has not been tried for the solar cell until now. The growth of GaN QDs was achieved by the hydride vapor phase epitaxy (HVPE) technique and P3HT film sequentially was coated on the top of QDs. The overall performance of P3HT/GaN QDs hybrid heterojunction was analyzed by current density–voltage (J–V) characteristics and finally exhibited an open-circuit voltage, short-circuit current density, and fill factor of 160 mV, 3.6 mA/cm2, and 0.25, respectively. Also, its efficiency was shown up to 0.14% in an active area of 0.04 cm2 under AM1.5G illumination with an intensity of 100 mW cm-2. In this paper, we discuss the factors which affect the power conversion efficiency for future works.

Distribution of NiO/Al2O3/NiAl2O4 in the Fabrication of Spray-Dry Oxygen Carrier Particles for Chemical-Looping Combustion

NiO/Al2O3, known as one of the most efficient oxygen carrier, has been fabricated by spray-drying method and calcinated at 1100 °C and 1300°C, and the structural characteristics are investigated using XRD, SEM, TEM and XPS. For the characterization of surface and bulk microstructure of the fabricated NiO/Al2O3 oxygen carrier particle, investigated were 1) as-fabricated powders, 2) internal structure of the crumbled particles, and 3) cross-sectional specimens. The results showed that the fabricated oxygen carrier formed well distributed NiAl2O4 with NiO particles of 100~500 nm via reaction keeping on the mole ratio. The oxygen carriers developed in this study showed pertinent characteristics for chemical-looping combustion, and good effect on the strength, indicating a potential for wide application in the future. The calcination at 1100 °C was good enough and as efficient as that at 1300 °C.

Development of High-Temperature Solders: Contribution of Transmission Electron Microscopy

This article briefly reviews the results of recently reported research on high-temperature Pb-free solder alloys and the research trend for characterization of the interfacial reaction layer. To improve the product reliability of high-temperature Pb-free solder alloys, thorough research is necessary not only to enhance the alloy properties but also to characterize and understand the interfacial reaction occurring during and after the bonding process. Transmission electron microscopy analysis is expected to play an important role in the development of high-temperature solders by providing accurate and reliable data with a high spatial resolution and facilitating understanding of the interfacial reaction at the solder joint.