Papers by Keyword: TiAl

Abstract: TiAl intermetallic alloy was prepared by gelcasting. The alloy powders were synthesized through reaction diffusion. The microstructure was investigated by XRD and SEM. The results showed that titanium aluminide powders were synthesized from elemental powders under the condition of 600°C for 3h. The solid loading of metal slurry was 45%. After gelcasting, drying and sintering at 1480°C for 2h, TiAl with the relative density of 93% was successfully prepared.

Abstract: The influence of laser remelting on the mechanical property and fracture behavior of the superplastic diffusion bonding between TiAl intermetallics and TC4 alloy was investigated. The joint bonded by the pre-remelted samples displayed well diffusion, but the mechanical properties of the joint should be further enhanced. The mechanical properties of the joint pre-remelted under the diffusion bonding conditions of 915/80MPa/1h are lower than that of the joint without remelting. After the heat treatment on pre-remelted joint sample at 860, the mechanical properties have been enhanced greatly.

Abstract: Melting TiAl based alloys in ceramic crucibles often leads to chemical contamination, alloy heterogeneity and non-metallic inclusions. The severity of such phenomena usually depends on the nature of crucible materials, the melting stock composition and the melting parameters, namely superheating time and temperature and melting pressure. Among the referred drawbacks, Al loss during melting is a critical aspect, as its concentration in TiAl based alloys has a very strong effect in their mechanical properties. Although a few studies of critical factors affecting the evaporation behaviour of Al during electron beam and induction skull melting of Ti-Al alloys had been carried out, until now no information was released on this subject for the ceramic crucible induction melting process. In this work a Ti-48Al alloy was induction melted in a zircon crucible with Y₂O₃ inner layer, using 50 and 100 °C superheating temperatures and 0, 60 and 90 second holding times, and poured into a graphite mould. The effect of different temperature/time combinations in the alloy composition, Al loss by evaporation and extent of the metal/crucible interaction was studied for different melting pressures. Al loss was found to increase significantly for melting pressures below around 10^-1 mbar, at a rate that increases as melting pressure decreases, until a maximum rate is reached, remaining constant for lower pressure levels. Metal/crucible interaction increased directly with the melting pressure and superheating time, leading to alloy contamination with yttrium and oxygen. For the experimental set-up and conditions used on this work, optimal superheating time/pressure combinations that lead to acceptable alloy composition and sanity have been identified.

Abstract: Post-weld heat treatments were carried out in Ti-6Al-4V (Ti-6-4) weldments after a gas tungsten arc welding (GTAW) process. The α₂-Ti3Al compound formed in the fusion zone (FZ) following the annealing process, whereas the γ-TiAl, α₂-Ti₃Al compounds formed in the FZ and heat affected zone (HAZ) respectively following a solution plus aging treatment. These γ-TiAl and α₂-Ti₃Al that formed in the FZ and HAZ respectively are associated with a low ductility of the Ti-6-4 weldment to 2.8%. In contrast, the only presence of α₂-Ti₃Al in the FZ still maintains good ductility to 10.8%. Possible mechanisms for γ-TiAl and α₂-Ti₃Al compound formation in related regions of weldment are discussed.

Abstract: In this paper, the relationship between the tensile ductility loss and the conditions including the exposure process and microstructures for the hot exposed TiAl alloys was researched. The critical temperature corresponding to the ambient tensile ductility loss to 0.8% was sensitive to the microstructure. It was 800°C/100h exposure for the near fully lamella microstructure. Due to the oriented lamella, the near fully lamella microstructure was benefit for forming the oxide layer rich in Al to present further oxidation of the matrix.

Abstract: Diffusion treatments of TiAl-based alloys (49.1 at% Al) aluminum coated by thermal spray were carried out at the temperature range of 700°C-1100°C. The influence of the diffusion condition for the formation of intermetallic phases in the coating has been investigated. In the initial stage of diffusion treatment, TiAl₃ was formed on the outermost surface by the diffusion between liquid aluminum and the substrate. In addition, an intermediate layer comprised of Ti₂Al₅ (at 1100°C), TiAl₂ and Al diffused layer (Al-rich TiAl) was confirmed under the outermost layer. The maximum thickness of TiAl₃ during the initial stage increases as the diffusion temperature decreases. In addition, the shape of TiAl₃ layer was dependent on the diffusion temperature; the outermost layer without pores was confirmed at the temperature of 700°C. TiAl₂ and Al-rich TiAl developed by solid-state diffusion from TiAl₃ layer following a parabolic low. The activation energies for growth have been calculated to be 194 kJ/mol for TiAl₂ and 292 kJ/mol for Al-rich TiAl.

Abstract: The morphology of titanium boride in as-cast and as-forged Ti-45Al-8.5Nb-(W, B, Y) alloy containing 0.2at. % boron and the effect of borides on tensile and creep properties of the alloy are investigated. The results show that in as-cast alloy the morphology of boride appears mainly convoluted ribbons with some flakes and particles. With the extent of forging increases, the length of the ribbons decreases and their distribution is more uniform. The long ribbon in as-cast alloy is detrimental to tensile properties at both room temperature (RT) and high temperatures. The short ribbon in as-forged alloy is not harmful to RT tensile properties, but is harmful to tensile and creep properties at high temperatures. The harmful effect of the boride is due to the strain incompatibility of boride and matrix, which causes many cavities at boride/matrix boundaries and results in ultimate fracture.

Abstract: Al-rich aluminide coating on Ti-49.1at%Al alloys has been performed by two-step process of thermal aluminum spray and diffusion treatment. Effect of the diffusion time on the oxidation resistance, and the change of microstructures in the coating during the oxidation test in air at 900°C for 100 h were investigated. In the aluminized coating before the oxidation tests, formations of TiAl₃ on the outer layer and an intermediate layer consisting of Ti₂Al₅, TiAl₂, and Al-rich TiAl were observed. The intermediate layer developed by the diffusion treatment for 3600s. In the oxidation test over 50 h, the specimen diffusion treated for 3600 s exhibited the lower oxidation rate than that diffusion treated for 900 s. Multi-oxide scales of TiO₂ and Al₂O₃ were observed on the both surfaces of aluminized specimens. Large TiO₂ particles were observed on the specimen of t_D = 900 s. It was found that the TiAl₂ layer developed during the oxidized specimen, while the thicknesses of Al-rich TiAl and TiAl₃ layer decreased by the growth of TiAl₂.

Abstract: A Nb-C alloyed layer formed on TiAl by plasma Nb alloying followed by carburization was investigated. The alloyed layer was characterized using SEM, TEM, XRD and GDS. Effects of Nb alloying and the carburizing atmosphere on microstructure of Nb-C alloyed layers were analyzed. The forming mechanism of the Nb-C alloyed layer was verified using first-principle calculation. The results reveal that the Nb-C alloyed layer is composed of NbC and Nb2C. An addition of niobium in TiAl accelerates the diffusion of carbon atoms in the Nb-alloyed layer, leading to a thicker alloyed layer with higher carbon content. The carbon concentration and diffusing depth increase with methane gas. First-principle calculation shows that the forming energy of Nb-C duplex-treated TiAl is lower than those of single carburized TiAl and Nb-alloyed TiAl.

Abstract: Surface treatment of TiAl alloys by the small amounts of halogens leads to the formation of the protective alumina scale. The halogens can be applied by ion techniques as well as by spraying or dipping with halogen-containing compounds. In this work the results of the oxidation in air of a Ti-47Al-6Nb alloy dipped in a fluorine-containing resin were presented. The thermal cycling runs were carried out in the temperature 900°C.