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The permeation behavior of hydrogen through the oxide layer on the modified Zircaloy-4 (Zry-4) alloy was investigated. The modified Zry-4 was prepared by changing the chemical composition of standard Zry-4. The tin content of Zry-4 (1.5 wt%) reduced to 0.5 wt% and alloying elements such as Si, 0, and Nb were newly added from 0.01 wt% to 0.2 wt%. The oxide layers were grown in a static autoclave at 360℃ under the pressure of 2660 psi for 150 days. Permeation behaviors were characterized by measuring the time lag (t_L) of hydrogen permeation in the oxide. After the steady state of permeation flux, Fick`s law was used to calculate the diffusivity of hydrogen in the modified Zry-4 specimens. The diffusivity of hydrogen in 0.5Sn-0.1Nb-0.1Fe-0.2Cr-0.2O-Zr specimen (1.26×10^-8 ㎠/s) turned out to be lower than that in 1.5Sn-0.2Fe-0.1Cr-0.1O-0.01Si-Zr and Zry-4 specimens. It was shown from the oxide structural investigation that the hydrogen diffusivity is dependant on the amount of tetragonal phase in the zirconium oxide; the phase transition of the oxide from tetragonal to monoclinic was found in 150 days exposure and the 0.5Sn-0.1Nb-0.1Fe-0.2Cr-0.2O-Zr specimen showed higher amount of tetragonal phase than the 1.5Sn-0.2Fe0.1Cr-0.1O-0.01Si-Zr specimen and Zry-4. In addition to the oxide structural study, effects of the microstructure of zirconium alloy such as precipitates and grain boundaries on the diffusivity and solubility of hydrogen were studied.

keyword : Modified Zircaloy - 4 , Hydrogen behavior , Oxide layer , Diffusivity , Solubility , Tetragonal phase , Grain size

Electrostatically actuated testing devices were used to estimate the elastic modulus of single-crystalline Si for microeletromechanical systems. The devices were moved laterally by alternating electrostatic force at a series of frequencies, and then a resonance frequency was determined, with temperature cycling, by detecting the maximum displacement. The elastic modulus was calculated from the detected resonance frequency by Rayleigh`s energy method. The elastic modulus decreased with heating and then increased with cooling, but after thermal cycle the specimen showed a permanent change to a value lower than the initial one. This phenomenon can be explained, to some extent, by the temperature-dependent variation in interatomic force of Si atoms, but the permanent change in elastic modulus implies that it may also be caused by the formation of a thermal oxide layer on the Si with increasing temperature. Composite bending theory was invoked to analyze the oxide effect and the thickness of the oxide layer formed during thermal cycling was estimated from the permanent change in the resonance frequency. The existence of the oxide layer was confirmed by field emission scanning electron microscopy. Finally, the method for compensating the decreasing elastic modulus with increasing temperature were proposed by using the temperature coefficient of elastic modulus.

keyword : Microelectromechanical system , Single - crystalline Si , Resonance frequency , Elastic modulus , Temperature effect , Thermal oxide

A study has been made of the grain coarsening in IF steel after strain annealing. A 92.5% cold rolled and annealed IF steel sheet was temper rolled by 0.4% reduction in thickness and subsequently annealed at 750℃, 850℃ and 950℃ for various time periods. The grains occasionally started to grow abnormally near the surface and grew into the center during strain annealing. The pole figure and EBSD measurements showed that the abnormally grown grains did not have any orientation relation with the primary recrystallization texture before temper rolling. Grains which underwent less strains due to their orientations during temper rolling are believed to grow at the expense of surrounding grains from the calculated Taylor factors. Thus, the major coarsening mechanism was regarded to be the strain induced grain boundary migration rather than grain boundary characteristics.

keyword : Texture , Temper rolling , Strain annealing , Grain coarsening , SIGM , EBSD , Taylor factor

Mechanical properties of metal foams depend on cell type, shape, size, homogeneity, surface area and relative density, and particularly are proportional to its density. Deformation of metal foams includes the plastic deformation and collapse and shows a plateau area followed by yielding point. Metal foams follow the morphological rules that are about cell type, size and homogeneity. Metal foams with homogeneous wall thickness and linear cell boundary show the similar mechanical properties to bulk metal and thickness averaged by diameter affect the mechanical properties more seriously than cell size. In this paper, the effects of holding furnace temperature on macrostructure of Al foams were studied, sound absorption and thermal conductivity tests were carried out and impact test did with different cell size. To avoid the disappearing of bubbles by coalescence or burst, foaming agent(TiH_2) containing gas releasing component was used to fabricate the closed-cell Al foams. Melting was carried out near at 680℃ considering the melting point of recycled Al alloy and gas releasing point of TiH_2. The velocity of decomposition decreased with decrease of the time and temperature. Impact absorption energy of Al foam has 27 ㎏-㎝ at about cell size 2.7 ㎜. The acoustical performance of Al foam is considerably improved: its absorption coefficient shows over NRC 0.7 from 2000 to 4000 ㎐. Thermal conductivity of Al has about 2 W/mk.

keyword : Al foam , Sound absorption , Foaming agent , Viscosity , Closed cell

In the process of metal foaming, hydride metal often is used as foaming agent. In our experiment TiH_2 was chosen as foaming agent to produce the Al foams. Ti scraps were usually produced in the machining process of panels, wire and pipes. To recycle Ti scraps, Ti scraps of various shape as like plate, chip, pipe etc., were hydrogentreated in self-made hydride furnace. The samples were milled in a ball mill, and then morphologies of milled particles were observed by SEM. Laser and SEM were used to analyze the size distribution, and XRF, XRD, TGDTA analysis were performed to investigate the composition and reaction conditions to prove the foaming possibility of forming agent in the production of Al foams.

keyword : Metal hydride , Recycle , TiH2 , Titanium scrap , Metal foam , Foaming agent