발간논문

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The interrelationship between new parameter σ and maximum diameter, D(max) (or critical cooling rate, Rc) is elaborated and discussed in comparison with four other glass forming ability (GFA) parameters, i.e. (1) super-cooled liquid region ΔTx (= Tx - Tg), (2) reduced glass transition temperature Trg (= Tg/Tl), (3) K parameter K (= [Tx -Tg]/[Tl -Tx]), and (4) gamma parameter y (= [Tx]/[Tl +Tg]) in Mg-based bulk metallic glass (BMG) systems. The new parameter σ, defined as, ΔT* × P`, has a solid correlation with Dmax and Rc in Mgbased BMG systems. The σ parameter has a much better interrelationship with GFA in Mg-based BMG systems than the parameters for GFA suggested so far.

keyword : Mg-based alloy, Bulk metallic glass, Glass forming ability, σ parameter

The coherent interface energies and misfit strain energies of Fe/XN (X = V, Nb, Ta) systems were calculated by first principles method. The interface energies at relaxed interfaces Fe/VN, Fe/NbN and Fe/TaN were -0.051, -0.226 and -0.643 J/㎡, respectively. It was found that the dependency of interface energy on the type of nitride was closely related to changes of the bond energies between Fe, X and N atoms before and after formation of the interfaces Fe/XN by the discrete lattice plane/nearest neighbor broken bond(DLP/NNBB) model. The misfit strain energies in Fe/VN, Fe/NbN and Fe/TaN systems were -0.052, 0.178 and 0.005 eV per 16 atoms (Fe; 8 atoms and XN 8 atoms). More misfit strain energy was generated as the difference of lattice parameters between the bulk Fe and the bulk XNs increased.

keyword : Coherent interface energy, Misfit strain energy, Transition metal nitrides, bcc iron, First principles calculation

High-temperature deformation behavior of extra-low interstitial Ti-6Al-4V alloy with initially equiaxed microstructure (d = 5 μm) was investigated with the variations of temperature and strain rate in this study. Thin striped beta-phase of the starting material was uniformly distributed throughout the microstructure by imposing beta-annealing followed by hot cross-rolling processes (ε = 1). A series of uni-axial tension tests was carried out at temperatures of 800℃ to 950℃ and strain rates of 10(-4) s(-1) to 10(-2) S(-1). The tension tests evidenced that the present samples showed significantly higher elongation as compared to conventional ELI grade Ti-6Al-4V alloy at the same temperature and strain rate. It was mainly attributed to increased alpha/beta interface area resulting from the breakdown of beta-phase. A total elongation-to-failure of 1898% was obtained at temperature of 850℃ and strain rate of 10(-3) S(-1). In addition, a predominant deformation mechanism operating at high-temperature was suggested by using both microstructure and activation energy.

keyword : ELI grade Ti-6Al-4V, Beta-phase, Superplasticity, Strain rate sensitivity, Activation energy

Using a mechano-luminescent (ML) paint enabling the visualization of fast propagating crack as well as bridging stress distribution under a conventional loading condition, this paper presents a catastrophic fracture mechanism in association with the crack wake bridging in alumina ceramics. The mechanism appears to be inconsistent with currently accepted R-curve concepts. Fracture arises in two steps; the bridged crack propagation (=wake formation) and the breakage of bridge. The magnitude and shape of the bridging stress distribution changes with the advancing crack. It continues to change as the applied load change, even after the cessation of crack propagation.

keyword : Mechano-Luminescence (M-L) paint, Cracking propagation, Bridging (Shielding) stress, Alumina

Present study investigated the observation of intermetallic phase formation, in Al-7Si-0.9Fe and Al-7Si-0.9Fe-trace (0.3% Mn and 0.3% Cr) alloys, as a function of growth rate in Bridgman solidification. The Bridgman QDS technique allows close control of the solidification conditions and resulting sample can provide detail information on inter metallic phase formation in these alloys. These alloys were solidified unidirectionally with different growth rates (1 ~30 mm/min). Based on these results, solidification of these alloys consists of the growth of primary aluminium and multiple second phase reactions. In the Al-Si-Fe alloys, two type of iron-rich phase form during the directional solidification. When small amounts of Mn and Cr are present, another phase with different morphology appears. The morphology of all types of intermetallic was dependent on cooling rate.

keyword : Directional solidification, Al-Si alloy, Birdgman solidification, β-AlSiFe

CrSiN coatings of stepwise changing Si concentration were deposited on stainless steel by unbalanced magnetron sputtering (UBMS) system. Microstructure of the films due to the Si concentration is measured by XRD. The corrosion behavior of CrSiN coatings in deaerated 3.5% XaCl solution was investigated by electrochemical impedance spectroscopy (EIS). EIS measurements showed that the corrosion resistance of Si-bearing CrN was improved by phase transformation of the film, which leads to increase of charge transfer resistance. The microstructure of CrSiN film depends on the Si concentration. At the Si/ (Cr + Si) ratio of 9.8, the Si-bearing CrN possesses the best corrosion resistance due to the phase transformation and the decrease in grain size.

keyword : EIS, CrN, CrSiN, Si

Effects of boron content on electrochemical properties of the bulk amorphous alloys Fe(73.8-x)C(7.0)Si(3.5)BxP(9.6)Cr(2.1)Mo(2.0)Al(2.0)(X = 3, 5, 7) that can be fabricated using hot metal and industrial raw materials have been investigated in the 3.5% NaCl solution, IN H2SO4 solution and IN HCl solution. Bulk amorphous alloys tend to readily passivated in the solutions exhibiting wide passive regions and low passive current densities. The increase of Boron content improves the corrosion resistance of bulk amorphous alloys in the IN HCl solution. The origin of improvement of corrosion resistance of these alloys is attributed to the formation of Cr-rich passive film.

keyword : Corrosion resistance, Metallic glass, Amorphous alloy, XPS, Passive film

Effects of annealing on the electrical resistivity and transmittance properties of Ga-doped ZnO (GZO) thin films deposited on glass by rf magnetron sputtering were investigated. The electrical resistivity of a GZO thin film is effectively decreased by annealing in a reducing atmosphere such as N2+5%H2. This is attributed to passivation of grain boundaries and zinc ions by hydrogen atoms resulting in increases in carrier concentration and mobility. However, annealing at a temperature higher than 400℃ is less effective. The lowest resistivity of 2.3 × 10(-4)Ωcm is obtained by annealing at 400℃ in an N2+5%H2 atmosphere. The optical transmittance of the GZO film is improved by annealing regardless of the annealing atmosphere. Annealing in N2+5%H2 atmosphere widens the optical band gap, while annealing in an O2 atmosphere makes the band gap narrower, which can be explained as a blue shift phenomenon.

keyword : GZO, RF magnetron sputtering, Ga doping, Transparent conducting oxides

Effects of substrate temperature and atmosphere on the electrical and optical properties of Ga-doped ZnO thin films deposited by rf magnetron sputtering were investigated. The electrical resistivity of Ga-doped ZnO (GZO) films decrease as the substrate temperature increases from room temperature to 300℃. A minimum resistivity of 3.3 × 10(-4) Ωcm is obtained at 300℃ and then the resistivity increases with a further increase in the substrate temperature to 400℃. This change in resistivity with the substrate temperature is related to the crystallinity of the GZO film. The resistivity nearly does not change with the O2/(O2 + Ar) flow ratio, R for R < 0.33 but increases rapidly with R for R>0.33. This change in resistivity with R is also related to crystallinity. The crystallinity is enhanced as R increases, but if the oxygen partial pressure is higher than a certain level (R = 0.33 ± 0.10) gallium oxides precipitate at grain boundaries, which decrease both carrier concentration and mobility. Optical transmittance increases as R increases for R < 0.67. This change in transmittance with R is related to changes in oxygen vacancy concentration and surface roughness with R.

keyword : Ga-doped ZnO, resistivity, Transmittance, Gas flow ratio, Substrate temperature