Materials Science Forum Vols. 544-545

Abstract: An AZ31 (Mg-3Al-1Zn-0.35Mn in mass%) alloy sheet exhibiting the inclination of the basal plane from the rolling plane at about 15º, was fabricated by a DSR processing with a roll speed ratio of 1.167. Compared with the normal rolled sheets, the DSR processed sheets showed a lower 0.2% proof stress, a larger elongation, a smaller r-value and a larger n-value. The Erichsen value at room temperature significantly increased from 2.2 to 3.1, and the deep drawability also improved.

Abstract: Through more than three decades of development, a semi-solid metal processing has been successfully established as a unique casting technique to produce a structural component for an automobile industry with high integrity and improved mechanical properties. A slurry-on-demand process to make the semi-solid slurry having a fine and globular microstructure has been very important in the semi-solid metal process. In the present study, the orientation distribution functions (ODFs) calculated from the pole figure data were examined and tried to characterize the bulky morphology of primary solid phase of the semi-solid slurry of Al-Cu alloy produced with various magnetic flux density of 100 to 300Gauss in the specially designed electromagnetic (EM) stirrer. Columnar dendritic structure of primary α phase was turned into a rosette and globular structure by EM stirring during solidification. The primary α phase was refined and globularized with increasing a magnetic flux density of EM stirring. Also, due to the EM stirring the tendency to random orientation was appeared. In the case of unstirred Al-Cu alloy the <110>//ND texture was developed strongly and <100>//ND and <111>//ND texture was weakly developed. But with the increase of the EM stirring strength, <100>//ND and <111>//ND texture were more strongly advanced. Due to EM stirring the texture was almost completely randomized.

Abstract: The currently commercialized cathode material for Li ion batteries such as LiCoO2 exhibited limit to further improve the performance of the batteries, since the employed screen printing method for cathode fabrication is difficult to reduce the thickness and control the microstructure of the oxide layer. In this studies, we have synthesized Li(Ni1-xCox)O2 thin film by utilizing Li-diffusion reaction on the surface of Ni-Co alloy substrates. For the preparation of Ni-Co alloy rod, Ni and 20at.%Co powder were mixed for 24hrs by ball milling, and then pressed into rod-shape by cold-isostatic pressing. The Ni-Co rods were sintered at 1100°C for 6hrs in the reducing atmosphere of Ar 96% and H2 4%. The sintered Ni-20at%Co rod was cold-rolled into tape at 5% reduction ratio with the final thickness of 100㎛, and the recrystallization heat treatment for the development of the cube texture of the rolled Ni alloy tape was carried out at 1000°C in Ar 96% and H2 4%. After thin layer of metallic Li was deposited on the surface of Ni-Co template using thermal evaporation method in the glove box, the Li/Ni-Co composite tape were heat-treated at 700~850°C for 1~2hrs in oxidizing atmosphere to induce Li-diffusion into Ni-Co substrate and Li(Ni0.8Co0.2)O2 phase formation. The phase evolution of Li(Ni0.8Co0.2)O2 was confirmed by X-ray diffraction and the grain size and morphology of the surface were analyzed by scanning electron microscopy and atomic force microscopy. Also the charge and discharge test were conducted to confirm the electrical characteristics of Li(Ni1-xCox)O2/Ni-Co thin film for the cathode application.

Abstract: The aims of this study ares to investigate the microstructure evolution of AZ31 Mg alloys with normal rolling and different speeds rolling during hot rolling affects microstructure, texture and mechanical properties of AZ31 Mg alloy. In the microstructures of as-rolled both samples, twins are clearly apparent, small and recrystallized grains are visible along some grain boundary and twinned regions. The tensile strength and yield strength of DSR sample were slightly higher than that of NR sample. Also, in the case of the NR sample, tensile strength indicated different values to the rolling directions. From this result, NR sample compared to DSR sample strongly indicated to the plastic anisotropy tendency. Therefore, it is noted that DSR sample could be presented to the good formability, comparing to the NR sample. DSR samples deformed at 473K and 523K could be perfectly formed, indicating the potential application of the DSR process to improve formability of the Mg alloys at warm temperatures.

Abstract: Inconel 617 is a candidate tube material for high temperature gas-cooled reactors (HTGR). The microstructure and mechanical properties of Inconel 617 were studied after exposure at high temperature of 1050oC. The dominant oxide layer was Cr-oxide. The internal oxide and Crdepleted region were observed below the Cr-oxide layer. The major second phases are M23C6 and M6C types of carbides. The composition of M23C6 and M6C were determined to be Cr21Mo2C6 and Mo3Cr2(Ni,Co)1C, respectively, by EDS. These carbides are coarsened during exposure. M6C carbide is more stable than M23C6 at high temperature. There was not much change in mechanical properties after exposure at 1050oC for 1000 h.

Abstract: In order to produce Si3N4/hBN composite with low cost, it seems necessary to use nitrided Si3N4 powders since the cost of Si powder is much cheaper than that of Si3N4 powder. The purpose of the present work is to investigate the nitride conditions, and in particular, we focused on the relationship between microstructures and mechanical properties of hot-pressed Si3N4/hBN ceramic composite using nitrided Si3N4 powders. The mixed powders of Si3N4 and hBN were prepared by nitriding Si powders at 1380oC for 24 h, and subsequently sintered by hot-pressing at 1800oC for 2 h in N2 atmosphere. The microstructure and mechanical properties of the Si3N4/hBN composites were investigated. Flexural strength, Young’s modulus, and hardness decreased by the addition of 20 vol% hBN. The addition of BN resulted in a decrease in the modulus as well as an increase in the size of fracture source, both contribute to the observed decrease in mechanical properties. The Si3N4/BN based ceramic composites revealed enhanced crack deflection. The Vickers indentation crack paths in specimens are sinusoidal due to pull-out of grains during crack propagation.

Abstract: Noncombustible Mg-8Al-2Ca rods were processed by RD-ECAP. The magnesium alloy rod had Mg matrix and Al2Ca second phase. Grains with about 20 μm in diameter were observed in the matrix of the raw materials. The grains in matrix had no anisotropy. On the other hand, positions of second phase particles had anisotropy and the second phase particles formed lines. The samples processed by RD-ECAP had no crack and the samples had 20mm diameter. Grains in matrix of the 4 pass RD-ECAP sample had no anisotropy and the grains had under about 5 μm in diameter. The second phase particles had round shapes and were uniformly distributed as compared with the raw material rod. Therefore, the RD-ECAP is useful for forming noncombustible Mg-8Al-2Ca alloy with fine-grains.

Abstract: The passive-active transition behavior of a 304 stainless steel was investigated by observing the self-activation behavior and nano-scale galvanostatic cathodic reduction experiment. The self-activation time, τ0 was dependent remarkably on concentration of sulfuric acid. It was appeared that applied nano-scale cathodic current density dissolved the passive film on a 304 stainless steel surface and shortened the activation time in galvanostatic cathodic reduction experiments. The applied cathodic current density was proportional to the reciprocal of activation time. From this linear relationship, the rate of the self-activation process, i0 was obtained. The i0 increased with increasing H2SO4 concentration. And i0 also increased with increasing passivation potential and passivation time. The stability of passive film increased in accordance with increasing τ0 and decreasing i0. Therefore, it was concluded that the stability of passive film on austenitic stainless steel is evaluated by the kinetic parameters of the self-activation rate i0 and the self-activation time, τ0.

Abstract: A 3-D transient heat transfer finite element (FE) analysis was performed to simulate the curing process of thick thermoset composites. The simulated temperature was compared with the available experimental data to check the validity of the analysis. The influence of thickness of composite laminates on the temperature distribution was investigated, and how the size of laminates affects the inside temperature was also discussed. The results indicate that the laminate thickness rather than the laminate size has a significant influence on temperature distribution, and the 3-D analysis offers more accurate predictions than the 1-D analysis.

Abstract: Three types of composite materials, Al-10Ni-6Ce (at%)/pure Al (Vf=0.3), Al-10Ni- 6Ce/Al-3.6Mn (Vf=0.3) and Al-10Ni-6Ce/Al-5.5Mg (Vf=0.3), and monolithic Al-10Ni-6Ce alloy were successfully fabricated to a fully dense rod-shaped bulk form having a diameter of about 10mm by adopting a powder forging or extrusion process using amorphous Al-Ni-Ce powder together with crystalline pure Al, Al-Mn and Al-Mg powders. The monolithic Al-Ni-Ce specimen forged at 648K showed a very high compressive strength of 1.3GPa without exhibiting any compressive plastic strain. All of the composite specimens forged at 648K gained a compressive plastic strain with the considerable sacrifice of strength. In contrast, Al-Ni-Ce/Al-Mg composite specimen extruded at 648K showed a noticeably high compressive strength of 1.2GPa with the compressive plastic strain of 0.5%. The extruded Al-Ni-Ce/Al-Mn composite specimen also exhibited a considerably high compressive strength (1.1GPa) accompanied with plastic strain (0.2%).