Eui-Pak Yoon

Micromechanical fracture process of SiC-particle-reinforced aluminium alloy 6061-T6 metal matrix composites

Abstract To investigate the role of microstructural parameters on the fracture mechanism, SiC-particle-reinforced aluminium alloy 6061-T6 composites with 5, 10, 20 and 30% volume fraction were tested to measure the fracture toughness on a precracked compact tension specimen. The interparticle spacing of two adjacent particles with the critical size was related to the fracture toughness. The critical particle diameter was experimentally determined from the plastic zone size and the crack-tip-opening displacement using the Hutchinson-Rice-Rosengren solution and the Hahn-Rosenfield model. It has been found that the critical particle diameter decreased as the volume fraction of SiC particles increased.

Liquid miscibility gap in the Al–Pb–Sn system

Abstract The liquid miscibility gap boundary of two immiscible liquids, Al- and Pb-rich, has been measured in a temperature range between 1073 and 1293 K and a ternary liquid thermodynamic parameter has been evaluated based on experimental data measured in the present work together with phase equilibrium data available in the literature. From these, the phase diagram of the Al–Pb–Sn system including the liquid miscibility gap has been calculated using the CALPHAD method. The calculated results are in good agreement with the experimental data.

The effect of electromagnetic stirring on the microstructure of Al-7 wt %Si alloy

Abstracts are not published in this journal

Microstructure and properties of laser remelted chromium carbide layer

This study evaluated the feasibility of laser consolidation for improving the properties of the plasma-coated layer. Furthermore, the mechanism of the degradation sequence of the chrome carbide layer applied on the turbine blades was postulated. The laser consolidation could be successfully applied for improving the surface properties of the plasma-coated blade if the proper conditions were carefully chosen. The consolidated layer had erosion and corrosion resistance and a bond strength superior to those of the as-plasma-coated layer. The properties of the consolidated layer were strongly dependent upon the degree of dilution, especially on the Fe pick-up from the substrate. The degradation of the plasma coating layer was thought to be a result of the repeating action of the solid particle erosion, corrosion penetration through the pores and oxide films formed along the interlayer surface and impact spalling.

A study on the variation of solidification contraction of A356 aluminum alloy with Sr addition

A356 aluminum alloys are very useful engineering materials that have been widely applied in the automotive, machinery, aerospace and defense industries. However, modification treatment of the alloys is needed to change the alloy structure, particularly to change the form of the eutectic silicon from flake like in order to improve mechanical properties [1, 2]. The modifying agents usually used in A356 alloys are sodium and strontium. Sodium is subject to loss from the melt, as it disappears by evaporation or oxidation. In addition, its use presents undesirable environmental problems. Strontium, on the other hand, offers a semi permanent modifying effect and is often used to modify hypoeutectic Al-Si casting alloy [3–5]. From previous research, thermal analysis results indicate that modification depresses the eutectic transformation temperature [6–8]. As a result, the freezing range of the alloy is increased and so is the mushy zone [1]. The mushy zone may be divided into two parts, the semi-liquid zone and semi-solid zone, which are distinguished by physical characteristics. The alloy melt has good fluidity and has no strength in semi-liquid zone, while it has poor fluidity and some strength in semi-solid zone [9]. When the semi-liquid temperature range is increased, the fluidity of melts is improved. On the other hand, as the semi-solid temperature range is decreased, contraction stress during solidification is decreased so that solidification cracking can be restrained. Therefore, the range of the semi-liquid zone or semi-solid zone is one of the most important factors in determining the castability of the alloy [9]. The present study attempts to investigate the effects of Sr addition on the variation of the range of the semiliquid zone and to compare the results with microstructures of each specimen. All melts produced in this work were made with an A356.2 alloy with the composition given in Table I. Melts were prepared in 2 kg quantities using SiC crucibles in an electric furnace and degassed with pure argon which was bubbled into the melt at a rate of 0.2 ml/min for 20 min. Strontium was added to the melt in the form of A1-10wt%Sr mother alloy and the melt was held at temperatures in the range of 740–750 ◦C for 30 min.

Fracture toughness and the master curve for modified 9Cr−1Mo steel

Modified 9Cr−1Mo steel is a primary candidate material for the reactor pressure vessel of a Very High Temperature Gas-Cooled Reactor (VHTR) in the Korean Nuclear Hydrogen Development and Demonstration (NHDD) program. In this study, the T0 reference temperature, J-R fracture resistance and Charpy impact properties were evaluated for commercial Grade 91 steel as part of the preliminary testing for a selection of the RPV material for the VHTR. The fracture toughness of the modified 9Cr−1Mo steel was compared with that of SA508−Gr.3. The objective of this study was to obtain the pre-irradiation fracture toughness properties of the modified 9Cr-1Mo steel as reference data for an investigation of radiation effects. Charpy impact properties of the modified 9Cr-1Mo steel were similar to those of SA508−Gr.3. T0 reference temperatures were measured as −67.7 and −72.4°C from the tests with standard PCVN (pre-cracked Charpy V-notch) and half-sized PCVN specimens respectively, which were similar to the results for SA508−Gr.3. The KJc values of the modified 9Cr-1Mo steel with the test temperatures are successfully expressed by the Master Curve. The J-R fracture resistance of the modified 9Cr−1Mo steel at room temperature was nearly identical to that of SA508−Gr.3; in contrast, it was slightly higher at an elevated temperature.

NCG Reinforced MMC Fabricated by the Squeeze Casting Method

In order to increase the wettability between carbon fiber and aluminum matrix, pure nickel was applied as coating on carbon fiber. And scanning electron microscopy was used to characterize the microstructure of the coating, fiber and matrix. The tensile tests of carbon fiber and aluminum matrix were done to examine how the behavior of nickel layers with variation of applied pressure make an effect on the mechanical properties of composites. As the applied pressure increases, nickel layers were resolved into the aluminum matrix and ultimate tensile strength of the composite decreased. This was due to premature fracture of the reaction layer acting as a surface notch.

Effect of microstructures on magnetic Barkhausen noise level in the weld HAZ of an RPV steel

Microstructural state and magnetic Barkhausen noise (BN) level have been correlated in the weld heat-affected zone (HAZ) of a pressure vessel steel. The BN level increased with increasing size of carbide, and the tempered bainite structure showed higher BN signal than the tempered martensite. The results indicated that heat-treated materials may result in microstructurally different domain wall pinning obstacles at different thermal cycles.

A study on elimination of alumina particles in molten aluminum using direct electromagnetic force

From the viewpoint of energy reduction, the recovery of metal scrap and its recycling have been rising as an important global subject. However, it is difficult to remove deleterious impurities, which have detrimental effects on the mechanical properties. In particular, it is difficult to eliminate nonmetallic inclusions such as alumina(Al2O3) in aluminum alloys when they are subject to recycling. In this study, an experiment on the elimination of the impurities in the melt by imposing electromagnetic force to molten aluminum was conducted. The principle behind these electromagnetic methods is that as the electromagnetic force induced in metal scarcely acts on non-metallic inclusions due to low electric conductivity, the non-metallic inclusions are moved to the direction opposite the electromagnetic force and can be separated from the melt. The prediction based on the theoretical analyses was confirmed through the visualization of the polystyrene particle motion in an NaCl aqueous solution. We also proposed optimum electromagnetic conditions such as current density, magnetic field intensity and particle size for eliminating the inclusion particles continuously found through numerical analysis and experiments.