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To improve automobile fuel efficiency and reduce emissions, there has been a growing demand for more durable alloys for engine components with improved high temperature fatigue resistance. This study examined the effect of alloying elements on the high temperature mechanical behavior of Al-Si-Mg-(Cu) casting alloys for diesel cylinder heads. Depending on the effect of the alloying elements on the strength of the matrix, hardness at high temperatures increased with increasing Cu and Ti content with a concomitant increase in tensile strength. The LCF (low cycle fatigue) lives decreased with increasing alloy content according to the Coffin-Manson relation, due to smaller elongation. On the other hand, an analysis of fatigue lives with hysteresis loop energy, which consists of both strength and elongation, showed that the fatigue lives were normalized with an alloy of the same strengthening mechanisms, regardless of the test temperature. In the case of the alloy without Cu, an obvious cyclic softening behavior was observed, whereas the cyclic softening decreased significantly and the peak stress was maintained at 250 ℃ for the alloy with Cu. Dynamic mechanical analysis results showed that the mechanical properties of the Al2Cu precipitate hardened alloy were maintained at temperatures over 250 ℃, whereas the degradation of mechanical properties of the Mg containing alloy occurred at 170 ℃ due to coarsening of the Mg2Si precipitation phase.(Received November 19, 2015; Accepted April 5, 2016)

keyword : alloys, casting, fatigue, hardness test, dynamic mechanical analysis

In this study, the effect of microstructural factors on the impact toughness of hypoeutectoid steels with ferrite-pearlite structure was quantitatively investigated using multiple regression analysis. Microstructural analysis results showed that the pearlite fraction increased with increasing austenitizing temperature and decreasing transformation temperature which substantially decreased the pearlite interlamellar spacing and cementite thickness depending on carbon content. The impact toughness of hypoeutectoid steels usually increased as interlamellar spacing or cementite thickness decreased, although the impact toughness was largely associated with pearlite fraction. Based on these results, multiple regression analysis was performed to understand the individual effect of pearlite fraction, interlamellar spacing, and cementite thickness on the impact toughness. The regression analysis results revealed that pearlite fraction significantly affected impact toughness at room temperature, while cementite thickness did at low temperature.(Received March 2, 2016; Accepted April 8, 2016)

keyword : impact toughness, microstructural factor, multiple regression analysis, ferrite-pearlite structure, hypoeutectoid steel

Pure Mg, Mg-5wt%Ca, and Mg-5wt%Ca-1wt%Zn alloys were cast, and their high-temperature oxidation behavior was investigated to determine the roles of Ca and Zn in oxidation. Ca and Zn additions refined the microstructure by formation of Mg₂Ca and Ca₂Mg₆Zn₃, respectively, at grain boundaries. Ca increased the oxidation resistance by forming a thin, compact CaO scale on the surface. However, Zn decreased the oxidation resistance because of the formation of Ca₂Mg₆Zn₃, which increased the MgO-forming tendency. (Received October 14, 2015; Accepted April 9, 2016)

keyword : alloys, casting, oxidation, scanning electron microscopy (SEM), Mg-Ca-Zn alloys

In this study, the effect of potassium persulfate on current efficiency for gold plating was investigated using electrochemical techniques. The current efficiency in the low-current-density range greatly decreased when potassium persulfate was added to the gold plating solution. However, the current efficiency in the high-current-density range did not change with the addition of potassium persulfate. The measured contact resistances of the resulting gold-plated layers showed almost similar values, regardless of the addition of potassium persulfate, suggesting that potassium persulfate does not affect the property of the gold layers. Linear sweep voltammetry and partial polarization curves showed that the persulfate ion was reduced when the noble potential was more than that of the gold ion. Therefore, the current efficiency for gold plating decreased when potassium persulfate was added in the low-current-density range because at such a current-density the reduction of persulfate preferentially occurs. (Received November 18, 2015; Accepted April 5, 2016)

keyword : electroplating, gold plating, current efficiency, potassium persulfate, connector

As a first step to optimizing the homogenization heat treatment following high temperature upset forging, the solidification segregation and the homogenization behaviors of solute elements were quantitatively analyzed for 1Cr-1.25Mo-0.25V steel ingot by electron probe micro-analysis (EPMA). The random sampling approach, which was designed to generate continuous compositional profiles of each solute element, was employed to clarify the segregation and homogenization behaviors. In addition, ingot castings of lab-scale and a 16-ton-sized 1Cr-1.25Mo-0.25V steel were simulated using the finite element method in three dimensions to understand the size effect of the ingot on the microsegregation and its reduction during the homogenization heat treatment. It was found that the microsegregation in a large-sized ingot was significantly reduced by the promotion of solid state diffusion due to the extremely low cooling rate. On the other hand, from the homogenization point of view, increasing the ingot size causes a dramatic increase in the dendrite arm spacing, and hence the homogenization of microsegregation in a large-sized ingot appears to be practically difficult.(Received November 18, 2015; Accepted March 17, 2016)

keyword : 1Cr-1.25Mo-0.25V steel, solidification segregation, homogenization, secondary dendrite arm spacing, residual segregation index

The surface microstructure and crack formation of an aluminum die-casting mold were investigated. The mold was made of a modified STD61 steel, and was used for more than 165,000 cycles. The mold surface consisted of four layers; an oxidized layer, a decarburized layer, a network carbide layer and a tempered martensite matrix. The depth down to the hardest network carbide layer was ~200 μm. Inside the matrix, M3C transition carbides were dissolved so that solute C joined pre-existing M23C6 and MC carbides to make them coarse. About 60% of thermal fatigue cracks had a depth less than 200 μm due to the hard network carbide layer, which obstructed the propagation of cracks. Cracks of over 200 μm were filled with oxide wedges of Al and Si which was which flown from the molten alloy as well as the Fe oxide.(Received March 2, 2016; Accepted April 7, 2016)

keyword : alloys, casting, microstructure, scanning electron microscopy (SEM), thermal fatigue crack

In this study the effects of Bis(3-Sulfo-Propyl)di-Sulfide (SPS) on the elongation, surface morphology and crystal structure of electrolytic copper foil were investigated. Additives HEC (hydroxyethyl cellulose) (A), SPS(B), Coll-A(C) were used in the study, and the additive used as D was a Cl- ion. The study results show that there are large differences in surface roughness and elongation depending on the amount of SPS added, from 0 ppm ~ 40 ppm. In this study, it was verified that the SPS additive plays the role of an accelerating agent, and makes it possible to control grain sizes. Also, the elongation rate increased, with increasing amounts of SPS. In addition, it was determined that SPS improves both the grain size and elongation rate due to its effect on the crystal structure. The addition of 40 ppm SPS resulted in a low surface roughness of less than 1 μm, and the sample showed both low tensile strength and large grain size. Here, the elongation was 9.0%, (Received October 13, 2015; Accepted March 29, 2016)

keyword : electroplating, copper, organic additive, elongation, surface morphology.

The liners of shaped charges, a type of weapon, should possess good penetration ability and explosive power. Conventionally this is achieved by fabricating reactive Al-Ni composite layers on the Cu liners, which by themselves have fine penetration ability but poor explosive power. However, there are no studies related to the deposition behavior and interfacial state of a mixed Al-Ni composite applied by kinetic spraying, despite its importance in terms of the effect on reactivity. Thus, this study investigated pure Al and mixed Al-Ni composites that were deposited by kinetic spray process and the effect of Ni on the deposition and interfacial bonding state. The deposition efficiency was degraded by addition of Ni, but the density and bond strength were enhanced with the increase in Ni fraction. The primary cause of the lowered deposition efficiency was insufficient particle velocity, with the intense effect of strain hardening of Ni. However, the tamping effect induced by harder Ni particles contributed to a reduction in pores, and improvement of the interfacial bonding state. (Received March 14, 2016; Accepted April 7, 2016)

keyword : Al-Ni composite, kinetic spray process, deposition behavior, interfacial state

Experimental techniques for serial sectioning and reconstruction were investigated in the three-dimensional imaging of microstructures in metallic materials. Mechanical polishing equipped with a light optical microscope and a digital image processor was used for the serial sectioning and reconstruction. Different processing conditions were investigated for three different alloys: Al-Si alloy, Al-SiC composite, and AZ31 magnesium alloy. Proper selection of polishing cloths and polishing abrasives was pre-requisite for obtaining good slice images. The thicknesses of the slices were precisely controlled for each alloy by changing the mechanical polishing and chemical etching conditions. Two-step polishing was a very effective method for serial sectioning of multi-phase alloys with a soft matrix and hard particles such as Al-SiC composites. Image filtering with a delineate and median filter was very useful for successful image segmentation of multi-phase alloys.(Received October 8, 2015; Accepted January 23, 2016)

keyword : metals, casting, microstructure, optical microscopy, 3d imaging

Micro-twinning has been investigated in some state-of-the-art materials by transmission electron microscopy, but such characterizations have not been clear so far. Micro-twins must be characterized by identifying their spots in the electron diffraction pattern, which is very unique compared to normal defects such as dislocations or stacking faults. In this study the electron diffraction pattern which should be developed by micro-twins was derived theoretically for the [011] beam direction assuming that the symmetrical mirror plane was {112}. The angles between the diffraction spots of the (200) and (111) planes were concluded to be 15.7°. This conclusion could be utilized as an indicator of the likely offset of crystal rotations or stress relaxations due to micro-twinning formation. The presence of micro-twinning should also be confirmed, by making sure that twin spots appear in the diffraction patterns and micro-twin images in dark field.(Received December 29, 2015; Accepted March 20, 2016)

keyword : nanostructured materials, deformation, twinning, transmission electron microscopy(TEM), fcc crystal