발간논문

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Here we show multiple abnormal grain growth phenomena in nanocrystalline pure Ni with temperature change, based on the microstructural analysis. Abnormally large grains are observed in the nanocrystalline Ni thermally exposed in air for 1 hr at 200℃, 500℃, and 800℃, respectively. Such large grains formed at 500℃ for 1 hr having facet grain boundary structure, show kinetically fast growth rate rather than others exposed to 200℃ and 800℃ for the same time period. The facet grain boundary appears to be related to the abnormal grain growth rate, suppressing the fine matrix grain growth. The mechanism underlying abnormal grain growth is examined for each temperature range.

keyword : Multiple abnormal grain growth, Nanocrystalline pure Ni, Facet grain boundary structure

The high temperature deformation behavior and the effect of hot compression on the thermal expansion behavior of Fe-29Ni-17Co low thermal expansion Kovar alloy were investigated in the compressive temperature range of 900~1300℃ at the strain rate range of 25~0.01 sec.^(-1). The temperature (T) and strain rate (ε˙) dependence of the flow stress (σ_(0.7)) could be well described by hyperbolic sine law as ε˙= A [sinh(0.0084 σ)^(5.2018)] exp(-330.41/RT). Surface hot cracking and significant voids around grain boundary could be found at low temperature below 1000℃ and low strain rate (0.01 sec.^(-1)). Thermal expansion coefficient α_(30~400) generally increased with increasing compressive temperature. Moreover, α_(30~400) remarkably increased as strain rate decreased at high temperature above 1100℃. However the thermal expansion coefficient α_(30~400) of low compressive temperatures (900℃ and 1000℃) anomalously increased especially at high strain rates. Experimental results indicated that not only α phase appearance but also grain size had definite influence on the thermal expansion behavior with hot deformation. The correlation between microstructure and thermal expansion behavior of Kovar alloy were also discussed.

keyword : Kovar, Hot compression, Zener-Holloman parameter, Low thermal expansion, Grain size, α phase

The effect of rotating angle on creep rupture properties of transient liquid phase (TLP) bonded joints at elevated temperature of Ni base single crystal superalloy CMSX-2 was investigated using MBF-80 as a insert metal. The rotating angle was defined by <100> twist angle at bonded interface. The post-bonded heat treatment, consisting of the solution treatment and sequential two-step aging, was carried out in the Ar or vacuum atmosphere prior to creep rupture testing. Creep rupture properties of joints were comparable to those of base metal in case of the rotating angles less than 3 degree while drastically fell down with increasing the rotating angle more than 5 degree. The grain boundary energy and oxygen content at bonded interface increased with increasing the rotating angle. Creep rupture properties of joints could be improved with lowering the oxygen partial pressure during post bonded heat treatment attributed to the restraint of grain boundary oxidation of bonded interface.

keyword : Ni base single crystal, TLP bonding, Creep rupture properties, Rotating angle, Grain boundary energy, Grain boundary oxidation

The aim of the present study is to investigate the alpha-case formation mechanism between titanium melts and Al₂O₃mold during investment casting of titanium alloys. The metal/mold reaction products of titanium castings were investigated by optical microscope, microhardness profile, EPMA and XRD analysis. In this study, the mechanism of alpha-case formation was verified with thermodynamic calculations. The reaction of only TiO₂interstitial alpha-case formation could not occur spontaneously, on the basis of thermodynamic calculations. From the results of the experimental and thermodynamic calculations, the alpha-case is the reaction products of interstitial oxygen and the substitutional atoms dissolved from the mold materials with titanium, and then this reaction is spontaneous process.

keyword : Titanium casting, Investment casting, Metal/mold reaction, Alpha-case

In this study, we investigated the segregation behavior of Terfenol-D in vertical Bridgman growth with steady ampoule rotation. In Bridgman growth of Terfenol-D, the axial macrosegregation occurs due to the compositional inhomogeneity of initial melt and microsegregation due to the Tb-Dy-Fe ternary phase diagram. Because the melt segregation which causes axial macrosegregation was hard to diminish by buoyant or solutal convection, we introduced the steady ampoule rotation technique to change the flow pattern and investigated the influence of the intensity of melt stirring on segregation behavior. The results showed that as the rotation speed was increased, axial segregation and microsegregation were both diminished. Besides, both segregation were also affected by the growth rate so that its effect was discussed.

keyword : Bridgman method, Terfenol-D, Segregation, Steady ampoule rotation, Convection

This paper reports an investigation of the characteristics of plasma duplex treated ductile cast irons during thermal fatigue tests on a thermal cycling rig. The thermal fatigue cracking behaviors and microstructure of untreated and plasma surface treated ductile cast irons have been investigated. Both ferrite and pearlite matrix ductile cast irons were evaluated. During the tests, 60 mm long cylinders with 20 mm diameter were subjected to 1,000 high frequency induction heating and water cooling cycles, lasting 7 and 5 sec, respectively. The thermal fatigue damage was evaluated by analyzing different crack dimensions and distribution by light microscopy. The results showed that the plasma surface treatment increased thermal fatigue cracking resistance. When thermal fatigue cracking occurs, the cracks always initiate at the surface of the specimen. The major path of crack propagation is generally along the ferrite grain boundaries. The hardened layer had an effect on the thermal fatigue property. In addition, the ductile cast iron with an pearlite matrix has a better resistance to thermal fatigue than that of ferrite matrix.

keyword : Thermal fatigue, Plasma diffusion treatment, Cast iron, Oxidation, Duplex treatment, Plasma nitriding

Multilayered WC-Ti_(1-x)Al_(x)N coatings are characterized by periodically repeated structures of lamellae of WC-Ti/WC-Ti_(1-x)Al_(x)N. WC-Ti_(1-x)Al_(x)N coatings of changing Al concentration were deposited on AISI D2 steel by high-ionization sputtered plasma vapor deposition (PVD). The Al concentration could be controlled by using evaporation source for Al and fixing the evaporating rate of other metals. Four kinds of WC-Ti_(1-x)Al_(x)N coatings were prepared: WC-Ti_(0.6)Al_(0.4)N, WC-Ti_(0.53)Al_(0.47)N, WC-Ti_(0.5)Al_(0.5)N and WC-Ti_(0.43)Al_(0.57)N. The corrosion behavior of WC-Ti_(1-x)Al_(x)N coatings was investigated using electrochemical techniques in deaerated 3.5% NaCl electrolyte (galvanic corrosion test, potentiodynamic polarization test, electrochemical impedance spectroscopy and Mott-Schottky method) and surface analyses (GDOES, SEM and EDS). Particular attention was paid to the effect of growth defects on the coating properties related to the corrosion behavior. The results of the coating and substrate galvanic corrosion test showed low galvanic current densities. In the potentiodynamic polarization test and EIS measurement, the corrosion current density of WC-Ti_(0.5)Al_(0.5)N was lower than others and presented higher R_(ct) values than others after 240 h immersion time. This could be attributed to the formation of a Ti oxide, which promotes partial closing of the defects. Capacitance values obtained from EIS as a function of different potentials (C^(-2) vs E) showed that the oxide layer exhibited an n-type semiconductor behavior.

keyword : GDOES, Galvanic corrosion test, EIS, Mott-Schottky method

This paper presents the corrosion behavior of the low alloy steel contained alloying elements such as Cr, Co, W, Cu, Ni and Ca in synthetic tap water using potentiodynamic polarization tests, electrochemical impedance spectroscopy (EIS), XPS and EPMA. Potentiodynamic poalrization curves indicated that all the specimens had active corrosion behavior. Corrosion rate tended to reduce as the result of the alloying. Also, EIS test results showed that the more the content of element, the larger the polarization resistance. It can be seen that these results were caused from the formation of a coherent, protective film contained alloying elements. Actually, XPS results showed that Cr, Cu, and Ca were formed as protective metal oxides at the surface of rust film under the stagnant condition. The results of EPMA indicated that Cr, Cu, and Ca were distributed densely at the inner layer of the rust film, while Co, W, and Ni were distributed at the whole layer under the flow condition. Consequently, alloying elements improved corrosion resistance of low alloy steel in synthetic tap water. Especially, Cr, Cu, Ni, and Ca-containing specimen had the lowest corrosion rate due to the inner rust film formation during the initial stage of corrosion, which suggested a potential substitute for carbon steel in potable water system.

keyword : Carbon steel, Tap water, Alloying elements, Rust layer, Corrosion behavior

Thick Pb(Zr_(0.52), Ti_(0.48))O₃(briefly, PZT) films are required for the cases of micro actuators and sensors with high driving force, high breakdown voltage and high sensitivity, and so on. In this work, thick PZT films were fabricated by chemical solution deposition process. Total 16 types of samples using thick PZT films with thicknesses, about 1 ㎛ and 2 ㎛, and Pt top electrodes shapes for measuring effective transverse piezoelectric coefficients (e_(31,f)) were prepared using MEMS processes. All samples were characterized by fabricated e_(31,f) measurement system before and after poling process at 125 kV/cm and 150℃ for 10 min. |e_(31,f)| values of samples after poling were higher than samples before poling. Those of 2 ㎛ thick PZT films were also higher than 1 ㎛ thick PZT films. We found that |e_(31,f)| was directly related with {111} to {h00} crystal structures of thick PZT films. And those with narrow electrodes (x₁- x_(0)) as top Pt electrodes were also higher than cases with wide top Pt electrodes in agreement with given equation.

keyword : Thick PZT film, Transverse piezoelectric coefficient, Sol-Gel, Poling process

SiC nanorod has been synthesized directly on Si using CVD method. Ni was used for liquid catalyst during growth, and HMDS was used for source materials. Growth temperature and time effected grown nanorod’s diameter by growth rate. Deactivation was occurred at high source gas concentration. From the cross-sectional SEM image, SiC nanorod growth was divided two regions because of lateral VS growth effect. SiC nanorod was grown with uniaxial direction and highly crystalline phase.

keyword : SiC nanorod, HMDS, CVD, Ni