Materials Science Forum Vols. 654-656

Abstract: In this study, we focused on the sintering performance of Ti-4.5Al-3V-2Mo-2Fe (SP-700) and mechanical properties of SP-700 reinforced with titanium boride (TiB/SP-700) fabricated by spark plasma sintering (SPS). TiB whiskers formed in titanium by a solid-state reaction of titanium and TiB2 particles were analyzed with scanning electron microscopy and X-ray diffraction. The TiB/SP-700 was sintered at temperatures of 1073, 1173, and 1273 K and a pressure of 70 MPa for 10, 30, and 50 min. The volume fraction of TiB ranged from 1.7 vol.% to 19.9 vol.%. Tensile tests of TiB/SP-700 were conducted at room temperature, and the effect of TiB volume fraction on the tensile properties was investigated.

Abstract: Development of microstructure in chips during machining of Grade 2 titanium at different cutting speeds has been investigated. The morphology of the chip changes from continuous chip to irregular and regular segmented chip with increasing cutting speed. The deformation in continuous and segmented chips is characterized as continuous and localized shear respectively. The deformation mechanism in the irregular segmented chip is the dislocation slip in the continuous region and twinning around the localized shear. Deformation twinning was observed inside the segment between the shear bands in the regular segmented chip. These deformation twins are responsible for the hardening inside the segment.

Abstract: In the study, both experimental work and numerical modeling are performed to investigate the pore size effects on the mechanical properties and deformation behaviours of titanium foams. Cylindrical titanium foam samples with different pore sizes are fabricated through powder metallurgy. Scanning electron microscope (SEM) is used to determine the pore size, pore distribution and the ratios of the length to width of pores. Compressive tests are carried out to determine the mechanical properties of the titanium foams with different pore sizes. Finally, finite element modeling is attempted to simulate the deformation behaviour and the mechanical properties of the titanium foams. Results indicate that titanium foams with different pore sizes have different geometrical characteristics, which lead to different deformation behaviours of cell walls during compression, resulting in different mechanical properties of titanium foams.

Abstract: The superplastic forming and diffusion bonding (SPF/DB) of hydrogenated Ti-6Al-2.8Sn-4Zr-0.5Mo-0.4Si-0.1Y (Ti600) alloys were carried out in the temperature range of 1073-1213K under 1.5MPa gas pressure. The effects of hydrogen contents and diffusion temperature on welding-on ratio of SPF/DB and microstructure of interface and matrix in Ti600 alloy were investigated by OM and SEM. According to the experimental investigation, when the parameters of SPF/DB were as follows: T=860°C, P=1.5MPa and t=70min, the welding-on ratio of Ti600 alloy with hydrogen 0.5wt% was 100 percent. However, the physical contact of Ti600 alloy without hydrogen which was related to plastic forming could not occur. Moreover, the size and amount of voids at the diffusion bonding interface decreased and diffusion bonding quality improved gradually with the increase of hydrogen content and diffusion temperature, which was attributed to the decrease of phase transformation temperature and flow stress of plastic forming as well as the release of hydrogen. After SPF/DB, the recrystallization of joint grains through the interface was formed, and the matrix of hydrogenated Ti600 alloy changed.

Abstract: Ti-6Al-4V matrix composite (TMC) reinforced with TiB plus TiC was prepared and hydrogenated. Isothermal compression tests and high temperature tensile tests were carried out to study the effect of the hydrogen on hot deformation and superplastic deformation. The flow behaviour and microstructure evaluation of hot deformation was investigated. The results show hydrogen can reduce the flow stress and decrease the deformation temperature or increase the strain rate at the same flow stress level in hot deformation. Hydrogen increasing β phase and promoting dynamic recrystallizaiton (DRX) was considered as the main reasons for hydrogen-induced plasticity in hot deformation. The results of superplastic deformation indicate hydrogen can decrease the superplastic temperature 100°C or increase strain rate one order of magnitude at the same elongation level in superplastic deformation. Hydrogen promoting DRX were considered as the main reason for improvement of superplastic elongation.

Abstract: The titanium reduction from titanium tetrachloride (TiCl4) by molten magnesium pool, called the Kroll process, is regarded as a well-known process for the commercial-scale production of titanium sponge. Purified titanium tetrachloride vapor reduced by magnesium, forms sponge titanium with generating excessive heal. The heal transfer phenomena in a Kroll reactor should be thoroughly understood for productivity and quality enhancement. In this work, a computational modeling method to describe the thermal behavior in the TiCl4 reduction reactor was investigated and validated with the measured temperature distribution in a 500kg titanium sponge-capacity pilot-scale reactor in terms of various reduction ratios. The approach model for heat flow phenomenon in a reduction reactor could be utilized as a tool to predict the influence of operating process parameters on the optimization of Kroll process.

Abstract: A good combination of high strength and hardenability makes the alloy Ti 10V-2Fe-3Al a prime candidate for applications in the aerospace arena. However, these properties are very dependent on a post-forming heat treatment. The overall objective of this work is to determine the effect of prior deformation on the aging behaviour. In this particular study, the influence of the heat treatment, either solution and/or aging, on the microstructures, and consequently on the mechanical properties, without introduced strain is reported. Various solution heat treatments have been conducted, either in the β phase or in the (α+β) phase field, followed by rapid quenching or slow cooling, and aging treatments at different temperatures (250, 350, 400, 500C°) above and below the ω-transus temperature. Vickers hardness indentations were used to follow the precipitation hardening behaviour, and mechanical properties were determined using a shear punch test. The aging response is dependent not only on the presence of the athermal ω phase but also on the proximity of the aging temperature to the ω-transus. Most treatments showed an unusual initial softening behaviour prior to age hardening, however this appears to be related again to the composition and fraction of the β phase retained after solution treatment.

Abstract: A metastable β titanium alloy, BTi-6554 (Ti-6Cr-5Mo-5V-4Al) has been developed for structural applications in aircraft because of its high strength, high toughness, and good fatigue properties. This paper reports on the effect of heat treatment on microstructure and microhardness of the alloy. It has been shown that in the as hot rolled condition, the alloy consists of a single β phase. Heat treatment between 450-750°C results in the precipitation of α laths, while exposure to temperatures between 700-800°C results in the gradual transformation of the α phase back to β phase with larger grain sizes resulting from higher heat treatment temperatures.

Abstract: In present work, microstructure of Ti-6Al-2Mo-1.5Cr-2Sn-2Zr-1V-0.15Si-0.4Fe alloy as a function of aging temperature was investigated using optical microscopy, scanning electron microscopy (SEM), x-ray diffraction (XRD) and transmission electron microscopy (TEM).Three types of precipitates were found in this alloy, they were α2 phase (Ti3Al), silicides, and Sn rich phase. Ti3Al were observed in all aged specimens covering a range 500-740°C. A method of quantitative analysis toward α2 based on high resolution images and Flourier transformation is used. The results showed that the ordering in α phase was highly related to the property of fracture toughness. Silicides were first found at 580°C ageing and later determined in the formulate (Ti5+xZr3-x)Si3 in the specimens aged at 740°C, and they were found to influent the fracture toughness significantly. The Sn rich phase was emerged with large area during the 780°C aging. And its direct correlation to property was not found. The density of acicular α phase was found almost invariable by calculating through the modification XRD quantitative analysis during the elevated aging temperature.

Abstract: Titanium exhibits many attractive properties. It is considered to be ubiquitous since it has the 9th-highest Clarke number of all the elements. However, the principal beta-stabilizing elements for titanium can be very expensive, making many titanium alloys expensive. Manganese is a beta stabilizer for titanium alloys and it is also considered to be ubiquitous since it has the 11th-highest Clarke number of all the elements. The behavior of Ti-Mn alloys during heat treatment has been investigated and it was found that in some alloys the isothermal omega phase is precipitated. Because this phase can lead to brittleness, it is very important to suppress its precipitation. Since it is well-known that aluminum suppresses isothermal omega precipitation, we investigated the effect of adding aluminum using Ti-7mass% Mn-0, 1.5, 3.0 and 4.5mass% Al alloys by performing electrical resistivity, Vickers hardness, and X-ray diffraction measurements. In solution-treated and water-quenched 0 and 1.5 alloys, only beta phase was identified, while hcp martensite and bate phase were identified in 3.0 and 4.5Al alloys. The resistivities at room and liquid-nitrogen temperatures were found to increase monotonically with increasing Al content. Isothermal  precipitation was suppressed by aluminum addition, while alpha precipitation was accelerated by Al addition.