Papers by Keyword: TiAl Alloys

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Authors: Li Mei Cha, Helmut Clemens, Gerhard Dehm, Zao Li Zhang
Abstract: In-situ heating transmission electron microscopy (TEM) was employed to investigate the initial stage of lamellae formation in a high Nb containing γ-TiAl based alloy. A Ti-45Al-7.5Nb alloy (at %), which was heat treated and quenched in a non-equilibrium state such that the matrix consists of ordered a2 grains, was annealed inside a TEM up to 750 °C. The in-situ TEM study reveals that g laths precipitate in the a2 matrix at ~ 750 °C possessing the classical Blackburn orientation relationship, i.e. (0001)a2 // (111)g and [11-20]a2 // <110]g. The microstructure of the in-situ TEM experiment is compared to results from ex-situ heating and subsequent TEM studies.
Authors: J. Zhang, Anja Kutzsche, K. Rosenberg, Christoph Leyens, Bernd Viehweger
Abstract: A novel canning technology to forge gamma-TiAl alloys was developed at the BTU Cottbus. A TiAl specimen was encapsulated with multilayer stainless steel foil and glass. The steel foil layers prevented the heat loss through radiation and the glass layer reduced the temperature decrease through conduction. First, the effect of steel foil on the cooling rate was investigated. Cooling curves were recorded for TiAl specimens without steel foil layer, with 1, 2, 3 and 4 layers of steel foil, as well as with 3 coated steel foil layers, respectively. While the unprotected specimen cooled from 1200 °C to 1100 °C within 12 s, the specimen with 3 coated steel foil layers needed 52 s for the same temperature decrease. The efficiency of the glass layer was examined with forging of steel specimens. The cooling rate during forging of the specimen with a glass layer was only half of that without a glass layer. Based on the results, Ti-45Al-0.5Mo-0.5Cu-0.2Si specimens, canned with steel foil and glass, were successfully forged at strain rates of 0.1 s-1 and 0.04 s-1 with warm dies which were heated to 500 °C. Visual and metallographic examinations revealed no cracks, pores or micropores. The microstructures are fine-equiaxed grains.
Authors: Si Young Sung, Beom Suck Han, Young Jig Kim
Authors: Guang Yang, Hong Chao Kou, Jin Shan Li, Heng Zhi Fu
Abstract: An experimental platform for controlled solidification and heat treatment and its applications in TiAl alloys are reported. The controlled solidification facility can be used to study the microstructure evolution of metals and alloys during solidification process and the controlled heat treatment device can realize the complex heat treatments and high temperature quenching conveniently. Compared with the traditional experimental equipments, this platform has three advantages. First, it can precisely control heating and cooling speed as well as isothermal holding time during solidification and heat treatment. Second, the high temperature microstructure can be obtained by quenching with high accuracy. Third, these two devices are cheaper and suitable to use in laboratory.
Authors: Guo Tao Zhang, Zhi Yuan Rui, Rui Cheng Feng, Chang Feng Yan
Abstract: The motion of interstitial atoms (one of the point defects) in material is random under certain temperature. However, the diffusion of interstitial atoms has priority along some certain directions to the others, so it is necessary to reveal the phenomenon (the phenomenon also can be called anisotropic). In order to illustrate the anisotropic and reveal the magnitude of probability of the micro crack formation along crystal axes, some formulae derived from former literatures have been adopted, and the values of diffusion probability and diffusivity of interstitial atoms-Ti or Al along 〈110〉and〈100〉have been calculated. The conclusion that the diffusion of interstitial atoms along different crystal axis has different diffusivity is verified by the comparison of the results of calculations.
Authors: Eisaku Sakurada, Takashi Matsuo
Abstract: The superiority of creep in Ti-48at%Al alloy with fully transformed lamellar structure to that in Ti-50at%Al alloy with γ single phase is characterized by the extension of transient stage. This extension of the transient stage derives by the retarding effect of α2 plate on the onset of the accelerating stage, through suppressing the dynamically recrystallization which is the main reason of the accelerating stage. This superiority in Ti-48at%Al alloy will become more clear by investigating the creep of the single crystal designated as the PST crystal, because of removing the grain boundaries which is the formation site of dynamic recrystallization. By using the PST crystal, the predominant deformation using primary slip plane of γ plate will continue, because the α2 plate restricts the operation of other slip planes. In PST crystals with the angle between the stress axis and the lamellar plates, designated as φ, less than 45°, the uniform deformation will proceed, because of the decrease in creep rate due to the decreasing in Schmid factor through the monotonous decrease in φ. But these suppositions have not confirmed. In this study, the deformation manner of the PST crystals with φ of less than 45° is investigated by the analyzing of creep curve, macrostructure and inverse pole figure of the PST crystals interrupted the creep tests at 1148K/68.6MPa at the strains of 0.20 and 0.65. Inverse pole figures of PST crystal are obtained using SEM-EBSD method. By accepting the creep deformation, the stress axes of the PST crystals move for [001]-[1, – 11] line with slip system of (111)<1, – 01>, and after reaching at this line, the stress axis turn to [1, – 11] pole position with (111)<1, – 10> slip system. The change in stress axis is not homogeneous in gauge portion accepting small strain, by subjecting the further creep deformation up to the onset of the accelerating stage, this heterogeneous deformation in gauge portion disappeared.
Authors: Jie Ren Yang, Rui Run Chen, Hong Sheng Ding, Jing Jie Guo, Heng Zhi Fu
Abstract: This paper has systematically studied the evolving process of column crystals of Ti46Al0.5W0.5Si alloys that is directionally solidified by rectangular cold crucible. The results showed that: the solid/liquid interface presented convex to the liquid metal due to the complicated influences by cold crucible. Overheating temperature and new crystal nucleus had great effects on the growth continuity and the stability of column crystals. Furthermore, the precipitation phases have been studied. As a result, the methods for controlling growing process of columnar crystals have been submitted, which are playing a critical role in optimizing experimental parameters and devising the construction-elements of cold crucible.
Authors: Mohammad Rizviul Kabir, Marion Bartsch, Liudmila Chernova, Janine Schneider, Klemens Kelm
Abstract: At room temperature the macroscopic tensile behavior of TiAl alloys is extremely microstructure sensitive. In general the microstructures of TiAl alloys are heterogeneous at micro and meso scale. The materials micromechanisms that occur at different length scale have to be linked for a proper understanding of the macroscopic response. In order to explore those micromechanisms, methodologies combining advanced experimental and computational analysis have been proposed. Linking microstructure and properties using a two-scale numerical model we are able to explain the stress-strain and hardening behavior of this alloy.
Authors: Gen Zong Song, Lin Zhang
Abstract: In an effort to develop into a practical application of TiAl alloys for aerospace materials, Researchers at home and abroad in the use of alloying and thermal processing methods to improve the brittleness and other areas a lot of work.The existence of cracks which led to the kinds of material degradation of the major causative factor. However, it is difficult for us to observed the change of the local atom structure of the material by experiments in the limitation from our experimental conditions, nevertheless, computational research provide us the possibility that we can observe the evolution of the structure. Molecular dynamics calculation is considered to be well suited to describe the change of potential energy of the atoms in such a system. Embedded atom method (EAM) and canonical ensemble (NVT) molecular dynamics simulations have been carried out to obtain pre-cracks of different lengths of the structure of TiAl alloy. According to the average energy of atoms and the radial density distribution function, with increasing temperature the changes, analysis of TiAl alloy films of micro-cracks in the heating process, the structure changes with temperature.
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