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Recent emphasis on development of light-weight/ high-temperature structural coatings for aerospace applications, e.g. hypersonic vehicles, has generated renewed interest in ordered intermetallics, especially titanium aluminides. Of the three binary intermetallics under consideration in this system (Ti3Al, TiAl and TiAl3) the first two have been more extensively studied. These intermetallics can be produced by sputter deposition from single target of the intermetallic compound of chosen composition. The room temperature sputter deposited thin films usually consist of an amorphous mixture of Ti and Al. In the present study, the thin film of Ti-Al intermetallic compound was deposited at room temperature and elevated substrate temperature by using a two-facing-targets-type DC magnetron sputtering system with Al-rich(25Ti75Al (atm%)) composition of binary Ti-Al alloy target. The crystallization process of Ti-Al thin films have been studied by X-ray diffraction(XRD), field-emission scanning electron microscope(FE-SEM) and transmission electron microscope(TEM). The results of the phase transformations of thin films by heating substrate are Al + (Ti) → TiAl2 → Ti2Al5. In case of thin films by annealing treatment, phase transformation is Al + (Ti)/TiAl2/ Ti2Al5 →αTiAl3/ Ti2Al/TiAl3. The results of the selected area electron diffraction patterns analyses are in accord with XRD results. Dynamic hardness of films, dynamic hardness increased with increasing annealed temperature.

keyword : Ti-Al intermetallic compound thin films, TFT type DC sputtering system, Heating substrate, The effect of film thickness

Effects of the stacking layer number(two and six) on microstructure and mechanical property of a ARB-processed commercially pure aluminum were studied. Two-layer stack ARB was performed up to eight cycles at ambient temperature without lubricant according to the conventional procedure. For six-layer stack ARB, the six aluminum sheets 0.5 mm thick were degreased and wire-brushed. The sheets were first stacked together and roll-bonded by 50%-reduction rolling, followed by four-pass rolling so that the final thickness was 0.5 mm. The sheet was then cut to the six pieces of same length and the same procedure was repeated up to six cycles. The tensile strength increased with actual equivalent strain up to a strain of ~5 at the same increasing rate in both two-layer and six-layer ARB, however above a strain of ~5 it was larger in two-layer stack ARBed sample than in six-layer stack ARBed one. The grain formed by the two-layer stack ARB was finer than that by the two-layer stack ARB. The effects of the layers in stacking were explained by the redundant shear deformation.

keyword : Accumulative roll-bonding, ARB, Pure aluminum, Ultrafine grain, Mechanical property, Shear strain

Correlation between microstructure and wear resistance of Al2O3-8wt.%TiO2 coatings plasm-sprayed using nanopowders was investigated in this study. Four types of nanostructured Al2O3-8wt.%TiO2 powders were plasma-sprayed on a low-carbon steel substrate by using different critical plasma spray parameters (CPSP). The coatings consisted of completely melted region of γ-Al2O3 phase and partially melted region of α-Al2O3. The fraction of the partially melted regions and pores decreased with increasing CPSP. The hardness and wear test results revealed that the hardness of the coatings increased with increasing CPSP, while the wear resistance was the highest for the coating sprayed with the lowest CPSP. The main wear mechanism was a delamination mode in the coating sprayed with the high CPSP, but was changed to an abrasive mode in the coating sprayed with the low CPSP. According to this change in the wear mechanism, the wear resistance was the best in the coating sprayed with lowest CPSP.

keyword : Plasma spray coating, Al2O3-TiO2 Nanopowder, Critical plasma spray parameter, Wear resistance

Methanol decomposition (CH3OH→2H2+CO) was performed over thin foils of intermetallic compound Ni3Al. It was found that the methanol decomposition was effectively catalyzed over the Ni3Al foils in the temperature range of 440-520℃. The rate of H2 production rapidly increased during the initial several hours of reaction at 520℃, and then stabilized at a high value (42 mol/hr-㎡) during the subsequent reaction. Furthermore, the Ni3Al foils showed almost no loss of activity through the whole range of reaction time, displaying an excellent catalytic stability. Surface analyses by BET, SEM, XRD, and TEM revealed that porous carbon nanofibers containing fine Ni particles formed spontaneously on the NisAl foils during the reaction at 440-520℃. These surface products, i.e., fine Ni particles and carbon nanofibers, are supposed to serve as a catalyst and a catalyst support for the methanol decomposition, respectively.

keyword : Intermetallic compounds and alloys, Fuel cell, Microreactor, Hydrogen production, Methanol decomposition, Carbon nanofibers

Al-doped ZnO (AZO) thin films were deposited on glass substrates by RF-magnetron sputtering and effects of RF-power and O2/Ar gas flow ratio on the electrical and optical properties were investigated. According to XRD analysis results, the FWHM of the ZnO (002) peak tends to increase substantially with increasing the ratio of the RF power for the AZO target to that for the ZnO target for R>1. On the other hand, FWHM is found to be minimal for r = 0.5. AFM analysis results show that the surface roughness increases with R. The lowest roughness is obtained for r = 0.5. According to the Hall measurement results both the carrier concentration and mobility are highest for R = 1.5 and the resistivity is lowest for R = 2.0. In the case of r both the carrier concentration and mobility tends to decrease and the resistivity tends to increase as r increases, which maybe caused by the recombination of electrons due to Al doping and holes due to oxygen atoms. Also, the transmittance of the ZnO film tends to decrease for R<1.5 but to increase for R>1.5. Judging from the above analysis results on the crystallinity, surface roughness resistivity and transmittance of AZO thin films, optimum R and r values are 1.0 and 0.5, respectively.

keyword : AZO, RF magnetron sputtering, Al doping, O2/Ar flow ratio, Transparent conducting oxides

W-rich particle reinforced Ti-based bulk metallic glass (BMG) matrix composites with a compressive strength approaching 3 GPa and a fracture strain of approximately 12% were developed. A composite microstructure consisted of um-scale W-rich solid solution particles distributed in the BMG matrix was successfully obtained by injection casting. In contrast to most existing BMG matrix composites, in which the improved ductility was only obtained at the expense of the strength, the composites developed in this study exhibited a significant enhancement in their strength, as well as an improvement in the plasticity. This improvement in the plasticity was attributed to the blocking and circumscription of the shear band propagation, leading to the formation of a large number of shear bands.

keyword : Ti-based alloy, Metallic glass matrix composite, Mechanical property, Eshelby`s equivalent inclusion model

Delayed hydride crack (DHC) tests were conducted on Zr-2.5Nb tubes with different distributions of the β-Zr at temperatures ranging from 125 to 300℃ to determine temperature dependences of their DHC velocity (DHCV) and striation spacing. The Zr-2.5Nb tube with a semi-continuous β-Zr and a higher yield strength had a higher activation energy for the DHCV and for the striation spacing, corresponding to 48.9 kJ/mol and 12.4 kJ/mol, respectively, compared to the Zr-2.5Nb tube with a fully discontinuous β-Zr and a lower yield strength. It is found that the activation energy for the DHCV is the sum of the activation energy for hydrogen diffusion and the striation spacing representing the hydrogen concentration gradient at the crack tip. This study provides supportive evidence to the feasibility of Kim`s DHC model.

keyword : Zr-2.5Nb, delayed hydride crack velocity, activation energy, hydrogen diffusion, striation spacing

A series of hot rolling experiments was conducted on a Ti-6Al-4V alloy by varying the rolling temperature from 800 to 920℃ in both the unidirectional and cross rolling methods, to investigate the effect of processing variables on the texture and mechanical properties in this study. In the case of cross rolling, the cross rolling ration (CR) was varied from 0.3 to 1.0. Textures of α phase of hot rolled specimens were analyzed on the plane normal to the rolling direction. The (0002), (10(-)10) and (10(-)11) pole figures were measured by reflection method up to 75° using CuKα radiation. The anisotropy of mechanical properties were closely related to crystallographic texture. With the decrease of rolling temperature, in the unidirectional rolling, anisotropy of the yield stress decreased, which could be explained by the evaluation of orientation distribution function value and texture components. In the cross rolling experiments, anisotropy was negligible and ND//(11(-)20) fiber texture was developed. With the cross rolling ratio decreased from 1.0 to 0.3, the texture components changed from those similar to fully cross rolled specimen to those similar to unidirectionally rolled ones and the anisotropy was found to increase.

keyword : Ti-6Al-4V Alloy, Hot Rolling, Cross Rolling Ratio, Texture, Pole Figure, ODF, Mechanical Properties, Anisotropy

The mechanical properties of A356 alloy has not been well understood in terms of microstructural aspects even though A356 is one of the most widely used alloys for the industrial purpose. Specially, quantitative relationship between properties like ductility and fracture toughness with microstructural features is still lacking. In the present study, different processing routes were used to fabricate A356 samples for different microstructures such as the size and distribution of primary alpha and eutectic phases. Also, compressive deformation was used to eliminate porosity from casting. With those samples with different microstructure the characteristics of A356 alloy on tensile behavior was investigated in the present study. Strength as well as ductility of A356 alloy were shown to be strongly dependent on the distribution of Si particles since dislocation motions are hindered mainly by the hard Si particles. Also, it was found to be possible to enhance the ductility by modifying the Si distribution when deformation was applied to cast sample.

keyword : A356 alloy, Casting, Cast-Forging Process Porosity, Ductility

Creep properties and carbide reaction of a nickel base superalloy Mar-M247 at above 900℃ were investigated in this study. Creep and thermal exposure tests were carried out. Activation energy for creep deformation of Mar-M247 was calculated at 530.2 kJ/mol and the stress exponent was calculated at five (5). Two morphological types of MC carbide were observed in heat treated specimen. Tungsten was abundant in the script type MC carbide and hafnium in the blocky type one. Film-like M6C carbide as well as globular-like M23C6 carbide precipitated along the grain boundary during thermal exposure at 982℃/1000 hours. Globular M23C6 carbide also precipitated near blocky type MC carbide and needle-like M6C carbide precipitated near script type MC carbide in the interdendritic region. Thermo-Calc predicted that M23C6 carbide was stable and M6C carbide was unstable at 982℃. However, M6C carbide precipitation was accelerated during creep deformation.

keyword : Ni-base superalloy, Mar-M247, Creep, Fhermal exposure, Carbide reaction