Advanced Materials Research Vols. 29-30

Abstract: The nanostructured titanium was fabricated via the surface mechanical attrition treatment (SMAT) process and the effect of nano-crystalline structure on the biomechanical and biocorrosion properties were studied. It was found that the Young’s modulus of nanostructured Ti decreased significantly and thus the biomechanical property was improved. The electrochemical results revealed that the corrosion resistance of Ti became worse after SMAT process, which is contributed to the higher activation of the nanostructured surface.

Abstract: Nanoindentation technique has been widely used for measuring mechanical properties from a very small volume of material. The hardness measured using the depth sensing nanoindentation technique often decreases with increasing indentation size, the so called indentation size effect (ISE)[1, 2]. It has been generally acknowledged that the ISE in crystalline materials originates from the density change of geometrically necessary dislocations (GND) needed to accommodate a permanent indentation imprint. Conventionally, to characterize an ISE often requires a series measurement of hardness values at different indentation size. Based on the celebrated Oliver-Pharr scheme[3]. We propose a method to derive the ISE from the loading curve of one single indentation test. The application and limitation of the proposed method will be discussed.

Abstract: The effect of surface states of substrate and additives on copper electrodeposition for thin film applications was investigated. Titanium substrate states were mechanically and chemically moderated and several additives such as Arabic gum, hydroxyl ethyl cellulose, and chlorine were used during electrodeposition process under a constant current condition with current density of 500 mA/cm2. Results obtained using SEM, X-ray, and AFM for early stage of copper nucleation and growth revealed that substrate conditions and additives appear to be effective in producing uniformly-distributed copper nuclei and their subsequent growth in a regulated manner of surface leveling. The shape of copper nuclei was clearly affected by the surface state of the substrate. It seems to be related with dislocations produced on the titanium cathode during surface moderating. Hardness, growth direction, and resistivity of copper deposits changed with the kind of additives.

Abstract: This study examined the effect of the current density, anodizing voltage and time on the surface characteristics of titanium during anodic spark oxidation. Anodic spark oxidation was performed at different concentrations of electrolyte composed of DL-α-glycerophosphate (DL-α-GP) and calcium acetate (CA). The specimens were anodized at various conditions (current density, anodizing voltage, and duration) in a fixed electrolyte concentration. Anodized specimens were treated hydrothermally under high pressure steam using an autoclave. Homogeneous anodic oxide films were produced by anodic spark oxidation on the titanium surface with 1~4 μm diameter pores, and the breakdown voltage was changed by the electrolyte concentration. After the hydrothermal treatment, hydroxyapatite crystals formed as an enlarged polygonal shape in the higher concentration of DL-a-GP group and as thin needles in higher concentration of CA group. The bioactivity and osteoblast growth was good for the anodized surface at 340V compared with the groups applied other parameters.

Abstract: The surface characteristics of an extruded 6060 aluminium alloy were investigated with X-ray Photoelectron Spectroscopy (XPS). The results revealed that the extruded surface was covered by oxides of aluminium and magnesium. The thickness of aluminium oxide was found to change along the extrusion direction with the thinnest and thickest oxide at the beginning and end of the extrudate, respectively. Magnesium segregation was found on the surface of the extrusion with the highest and lowest Mg concentration at the beginning and end of the extrudate, respectively. This is the inverse result of that expected where increasing Mg content was believed to be associated with film instability and thicker films.

Abstract: Ni3Al shows the unique feature of increasing strength with increasing temperature. However, it is too brittle to use as a structural material due to grain boundary weakness. Ductility could be enhanced by controlling grains using directional solidification. In order to increase the ductility or strength of Ni3Al alloys, a ductile γ (Ni-rich) phase of dendrite fibers or a strong β (NiAl) phase of dendrite fibers were arrayed in the γ´ (Ni3Al) matrix by directional solidification. The dendrite spacing could be controlled by varying the solidification rates, and the volume fraction of the γ or β phase could be changed by using alloy compositions, from 23 to 27 at. % Al-Ni alloy. With increasing solidification rates, the dendrite spacing decreased, which caused the tensile strength to be enhanced and the elongation to decrease, evidently due to the phase boundary augmentation. With increasing Al content, the γ dendritic microstructure changed to β dendrites in the γ´ matrix, which resulted in a decrease in elongation as a result of an increase in the volume fraction of the brittle β dendrites in the γ´ matrix.

Abstract: Micro-porous nickel foams with an open cell structure were fabricated by the space-holding sintering. The average pore size of the micro-porous nickel specimens ranged from 30 μm to 150 μm, and the porosity ranged from 60 % to 80 %. The porous characteristics of the nickel specimens were observed using scanning electron microscopy (SEM). The mechanical properties were studied using compressive tests. For comparison, macro-porous nickel foams prepared by the chemical vapour deposition method with pore sizes of 800 μm and 1300 μm and porosity of 95 % were also presented. Results indicated that the ratio value of 6 and higher for the specimen length to cell size (L/d) is satisfying for obtaining stable compressive properties. The micro-porous nickel specimens exhibited different deformation behaviour and dramatically increased mechanical properties, compared to those of the macro-porous nickel specimens.

Abstract: Cu2ZnSnSe4 (CZTSe) thin films were grown on Corning glass 1737 by sequential methods of sputtering deposition and selenization process. As-grown films showed that elemental Cu, Zn, and Sn were in the nearly CZTSe stoichiometric ratio with Se-deficiency as detected by Energy Dispersive X-Ray spectrometry (EDX). In order to attain film stoichiometry, as-deposited films were subjected to selenization process in tube furnace under Ar ambient at different selenization temperatures for 10-60 min. It was found that compositions of binary compound in the sputtering target as well as selenization are critical for the growth of the CZTSe films. The structural characteristics of the selenized CZTSe films revealed a highly oriented stannite CZTSe phase with (112), (220/204) and (312/116) growth orientations and a CuSe secondary phase. By using 0.5% KCN solution, CuSe secondary phase could be totally etched from the CZTSe film surface.

Abstract: Al-Mg alloy sheets with high Mg contents (3~10wt%Mg) were fabricated by twin roll strip casting. The optimum process conditions to get a good surface quality of Al-Mg strip have been suggested in this experiment. Controlling the cooling rate of cast roll was important to improve the surface quality of the strip and a compositional homogeneity through the thickness. The size of intermetallic particle like Al-Fe compound was reduced down to 1~2μm due to a high cooling rate of Al melt during strip casting. In addition, the dendrite structure was fine and the segregation of Al8Mg5 phase between grains was remarkably reduced. Therefore, the strips with a thickness of 3mm have good workability during additional hot/warm rolling processes. The hot/warm rolled Al-5wt%Mg sheets show high strength and elongation. When the rolled sheets were annealed at 300 oC for 1hr., the tensile strength and total elongation of the sheets reached at 290 MPa and 30%, respectively.

Abstract: Warm hydroformability and mechanical properties of pre- and post- heat treated Al6061 tubes were investigated in this study. For the investigation, as-extruded, fully annealed and T6- treated Al 6061 seamless tubes were prepared. To evaluate the hydroformability, uni-axial tensile test and free bulge test were performed at room temperature and 200ÓC. Also mechanical properties of hydroformed part at various pre- and post-heat treatments were evaluated by tensile test. The tensile test specimens were obtained from hexagonal shaped tube hydroformed at 200ÓC forming temperature. As a result, hydroformability of fully annealed tube is 25% higher than that of extruded tube. The tensile strength and elongation were more than 330MPa and 12%, respectively, when hydroformed part was T6 treated after warm hydroforming. However, hydroformed part using T6 pre treated tube represents low elongation, 8%. Therefore, the T6 treatment after hydroforming for as-extruded tube is proved to be the most cost-effective among various processing conditions.