Materials Science Forum Vols. 539-543

Abstract: Embossing or imprint lithography is the key-technology for the mass production of nanosized structures with low cost. Currently Si or quartz template which is produced by e-beam or DUV lithography and reactive ion etching, is used. However they are very expensive and easily damaged due to their brittleness. On the other hand, Ni template has high mechanical durability and can be fabricated with low cost by electroplating. However, one of the key obstacles of Ni template is poor antistiction property, when it is used with sticky thermoset polymer. Due to its poor antistiction property, detachment of Ni template from epoxy substrate is one of the key obstacles. In this experiment, quartz template with 150nm to 1μm sized surface protrusion was fabricated and used to emboss the PMMA coated Si wafer. Then the imprinted PMMA layer was coated with metal seed layer and electroplating of Ni was followed to fabricate Ni template with 150nm to 1μm sized patterns. In order to form antistiction layer on Ni template, SAM antistiction layer was formed on SiO2 coated Ni template. As a result, nano patterns could be successfully transferred to sticky thermoset polymer using Ni template without any degradation of antistiction property.

Abstract: The paper explores texture in the titanium alloys Ti-6-4 and Ti 550. It illustrates how texture evolves under plane strain compression in Ti-6-4. This evolution is dependent on temperature, degree of reduction (strain) and strain rate. Rolled (Ti-6-4) and forged (Ti 550) variants with different textures are then examined under tension and torsion loading in relation to their monotonic and fatigue response. Correlation of the observations with regard to orientation of the basal plane is demonstrated.

Abstract: Samples of Ti-15Cr and Ti-15V-3Sn-3Al-3Cr (wt%) containing controlled additions of carbon up to 0.2wt% and different oxygen contents have been quenched and aged at temperatures between 400 and 600°C. Optical, scanning and analytical transmission electron microscopy have been used to characterise the microstructures of the quenched and aged samples. Hardness testing has been used to follow the kinetics and extent of age hardening, which are accelerated in Ccontaining samples. The addition of carbon results in the formation of Ti(CxOy) precipitates which pin grain boundaries in forged samples so that the grain size in the quenched C-containing samples is about a factor of ten less than that in the C-free samples. In the C-free samples coarse grain boundary alpha tends to form, but in the C-containing samples alpha precipitation is more uniform throughout the beta grains. The extent of omega precipitation is very different in the two alloys; the Ti-15Cr alloy forms athermal omega in the as-quenched samples and large omega precipitates are formed on ageing at 400°C. No evidence for omega has been obtained in the Ti-15-3. The hardening responses and microstructural observations are interpreted in terms of the different grain boundary oxygen contents in the C-containing and C-free samples and the different roles of omega and of carbon in the two alloys.

Abstract: Over the past decades, a large number of researchers have been trying titanium alloys in an attempt to combine most of their advantages, such as high specific yield strength, good corrosion resistance, excellent fatigue property and biocompatibility by casting route. However, the wide use of titanium alloys casting has been limited, since it is considered as only a near net shape forming process in titanium alloys due to the catastrophic reactivity of molten states, the alpha-case formations and the casting defects. In order to maximize the unique property of titanium alloys casting which are comparable to wrought products and quite often superior, it is necessary to take a close examination of titanium alloys casting procedure. Therefore, the merits and demerits of various melting devices, pouring methods and mold materials will be addressed with regard to improving titanium alloys casting.

Abstract: The high-temperature deformation behavior of the single-phase α (Ti-7.0Al-1.5V) and α + β (Ti-6Al-4V) alloy were determined and compared within the framework of self-consistent scheme at various temperature ranges. For this purpose, isothermal hot compression tests were conducted at temperatures between 650°C ~ 950°C to determine the effect of α/β phase volume fraction on average flow stress under hot-working condition. The flow behavior of α phase was estimated from the compression test results of single-phase α alloy whose chemical composition is close to that of α phase of Ti-6Al-4V alloy. On the other hand, the flow stress of β phase in Ti-6Al-4V was predicted by using self-consistent method. The flow stress of α phase was higher than that of β phase above 750°C, while the β phase revealed higher flow stress than α phase at 650°C. Also, at temperature above 750°C, the predicted strain rate of β phase was higher than that of α phase. It was found that the relative strength between α and β phase significantly varied with temperature.

Abstract: To develop new shape memory and super-elastic alloys for medical applications, titanium alloys using non-toxic metallic elements, such as Ta and Nb, are being actively investigated. In this study, aimed at developing new shape memory Ti alloys, we investigate the effect of oxygen, a powerful alpha stabilizing interstitial element, on the heat treatment behavior of Ti-50mass%Ta-5mass%Zr through electrical resistivity and Vickers hardness measurements and shape-recovery tests. Ti-50Ta-5Zr-0.14Ox and 0.33Ox alloys, and the β and α” bi-phase was confirmed by XRD. Only the β phase was identified in the Ti-50Ta-5Zr-0.62Ox alloy. Upon isochronal heat treatment, the resistivity at LN and resistivity ratio of Zr-0.33 and 0.62Ox alloys decreased up to around 523 K. In the 5Zr-0.62Ox alloy, orthorhombic martensite and the α” and β phases were identified in the specimens heat-treated at 473 and 523 K. The decreases in resistivity at liquid nitrogen temperature and resistivity ratio are due to the formation of α” during isochronal heat treatment. The formation of α” was confirmed by X-ray diffraction in the 5Zr-0.62Ox alloy. The shape memory effect was observed in 5Zr-0.14 and 0.33Ox alloys and the shape recovery ratio of both alloys was about 40% at 673 K.

Abstract: Since titanium alloys are the most promising structural materials for the high velocity vehicles, the impact tensile strength of the materials is presently investigated. Three kinds of aging treatments on the beta-titanium alloy were performed, and the tensile deformation behaviors were identified in the wide range of the temperature and the strain rate. The stress－strain relations of the titanium alloy significantly depend on the temperature and the strain rate investigated. Thermally activated process concept was applied to explain the experimental results, and the stress-strain relations at high strain rates were well understood with taking account of adiabatic heating effect. It has been found that the stress-strain curves depend on the microstructures, while the temperature and the strain rate effects are almost independent of the different aging treatments.

Abstract: Beta phase decomposition in Ti-44Al-4Nb-4Hf-0.1Si during continuous cooling from β phase field has been investigated. A wide cooling rate range (0.3-1000°Cs-1) was provided by mainly using Jominy end quenching, which has been introduced into TiAl research recently, together with iced brine quench (IBQ) and furnace cooling (FC). At different cooling rates beta phase decomposes via different paths through diffusion or diffusionless mechanisms and lamellar transformation may occur after β decomposition at certain cooling rates.