Papers by Keyword: Titanium Aluminide

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Authors: W.Joe Quadakkers, N. Zheng, A. Gil, E. Wallura, H. Hoven
Authors: Gorgees Adam, De Liang Zhang, Jing Liang, I. Macrae
Abstract: Titanium and titanium alloys are the materials of choice for many industrial applications because of their attractive combination of low density, good mechanical properties, and high corrosion resistance, and titanium is the fourth most abundant metal in the earth crust (0.86 % by weight) behind aluminum, iron, and magnesium. However, titanium and titanium alloys are not widely. The reason for this is the high cost of titanium and titanium alloys! The cost gap for titanium and titanium alloys widens when they are used for fabricating components and structures. Consequently, much effort has been expended to reduce the cost of titanium and titanium alloys. In conjunction with the University of Waikato, Titanox Development Limited-New Zealand has been successful in creating a modified novel process to produce TiAl based alloy powders with different particle sizes and compositions at low cost. The process offers several benefits to the titanium industry the most significant one of which is that it displays the potential to significantly reduce the commercial production costs of Ti-Al based alloys. This paper describes the Titanox Development Limited technology in brief, and shows how it can economically produce titanium alloy powders for different industrial applications and making titanium alloys affordable. The process has been disclosed in a PCT (Patent Corporation Treaty) application which was approved in 2004 [1], and the related patent applications either have been approved or are being filed in different countries.
Authors: Alain Jacot, Amin Rostamian
Abstract: A phenomenological modeling approach has been developed to describe the massive transformation and the formation of lamellar microstructures during cooling in gamma titanium aluminides. The modeling approach is based on a combination of nucleation and growth laws which take into account the specific mechanisms of each phase transformation. Nucleation of both massive and lamellar γ is described with classical nucleation theory, accounting for the fact that nuclei are formed predominantly at α/α grain boundaries. Growth of the massive γ grains is calculated with a mathematical expression for interface-controlled reactions. A modified Zener model is used to calculate the thickening rate of the γ lamellar precipitates. The model incorporates the effect of particle impingement and rapid coverage of the nucleation sites due to growth. The driving pressures of the phase transformations are obtained form Thermo-Calc based on actual temperature and matrix composition. The model permitted investigating the influence of alloy chemistry, cooling rate and average α grain size upon the amount of massive γ and the average thickness and spacing of the lamellae. Calculated CCT diagrams were compared with experimental data collected from the literature and showed good agreement.
Authors: Klaus Dieter Liss, A. Bartels, Helmut Clemens, S. Bystrzanowski, A. Stark, Thomas Buslaps, Frank Peter Schimansky, Rainer Gerling, Andreas Schreyer
Abstract: High-energy synchrotron X-ray diffraction is a novel and powerful tool for bulk studies of materials. In this study, it is applied for the investigation of an intermetallic γ-TiAl based alloy. Not only the diffraction angles, but also the morphology of reflections on the Debye-Scherrer rings are evaluated in order to approach lattice parameters and grain sizes as well as crystallographic relationships. An in-situ heating cycle from room temperature to 1362 °C has been conducted starting from massively transformed γ-TiAl which exhibits high internal stresses. With increasing temperature the occurrence of strain relaxation, chemical and phase separation, domain orientations, phase transitions, recrystallization processes, and subsequent grain growth can be observed. The data obtained by high-energy synchrotron X-ray diffraction, extremely rich in information, are interpreted step by step.
Authors: Guang Cai Su, Jia Ying Huang, Long Fei Qin, Hong Wei Liu
Abstract: A Detailed Transmission Electron Microscopy (TEM) Study Has Been Made of the Ageing Hardening Behavior of the γ1-Ti4Nb3Al9 Precipitation Phase in the γ-TiAl Matrix in Ti-Al-Nb System. it Is Revealed that the γ1-Ti4Nb3Al9 Precipitate, which Is Formed by a Normal Quench at 1473k and Ageing Treatment at 1073K, Takes Needle-Like Morphology with a Growing Axis Parallel to [001] Direction of the Matrix. the Needle-Like Precipitates Act as Barrier Grid to Inhibit Dislocation Moving on the Slipping Plane {111} of the γ-TiAl Matrix, which Helps to Improve the Strength of the Alloy. the Effects of the Third Element Niobium to the Strengthen of High Nb Containing TiAl Alloys Lie in Not only Solid Solution, but Also Ageing Hardening. the Interfaces between Precipitate and Matrix Are Also Investigated.
Authors: Tohru Takahashi, Na Liu, Yusuke Yazawa, Takuya Nunome
Abstract: Compression and compressive creep behavior was studied on Al-Ti-V ternary alloys containing gamma+beta dual phase microstructures; the gamma phase was based on an L10 face centered tetragonal lattice and the beta phase on a disordered body centered cubic lattice. Yield strength and its temperature dependence have been compared with those in the gamma and/or beta single phase materials. The ternary alloy compositions were located on one assumed conjugate line across the gamma+beta dula phase field: the terminal compositions for the gamma and beta phase constituents were Al51Ti40V9 and Al35Ti20V45, respectively (numbers in atomic %). Three other alloys were prepared that contained different fractions of the constituent gamma and beta phases. The Al47Ti35V18, Al43Ti30V27, and Al39Ti25V36 alloys contained beta phase by about 22, 57, and 76 % in their area fractions. All these alloys showed limited deformability at temperature below 900K. The 0.2% proof stresses of the alloys were described in a similar way as a combination law at the room temperature; the 0.2% proof stress increased from about 500 to 1000 MPa with increasing the vanadium content. The high strength of the alloys containing high level of vanadium retained up to 900K, but the proof stress drastically diminished as the temperature was raised above 900K. Under compressive creep tests performed at temperatures ranging from 1100 to 1200 K, the minimum creep rates were smaller in the alloys containing less vanadium, and this could be ascribed to the fact that the beta phase was much softer than the gamma phase at higher temperatures than about 1000K.
Authors: Olivier Berteaux, Roger Valle, Monique Raffestin, Marc Thomas, G. Henaff
Abstract: The fatigue behaviour of a powder-metallurgy (PM) + heat-treated Ti49Al47Cr2Nb2 alloy is investigated using detailed SEM characterization. Based on the results of comparative static and cyclic loadings at RT, the fracture origins for the different test specimens is investigated. Conventional fractographic analyses revealed that internal structural defects inherent to powder metallurgy such as compaction defects, porosities and non-metallic inclusions can promote crack initiation. However, the fracture surface of test specimens is markedly affected by the microstructure, which is indicative of the microstructure dependence on crack initiation and propagation. In sub-transus heat treatment conditions, the detrimental effect of structural defects is illustrated by clear crack initiation sites onto the fracture surfaces. In super-transus conditions, crack propagation from defects can be blunted due to crack deflection, branching through lamellar interfaces, thus leading to lower defect sensitivity. Correlation of the results of these microfractographic examinations with the stress-strain curves corresponding to the various specimens allows identifying the role of such structural defects on the static and cyclic deformation behaviours. Finally, implications of such dependencies will be assessed relative to the requirements for aerospace gas turbine applications.
Authors: Fritz Appel, Michael Oehring, Jonathan H.D. Paul
Abstract: Intermetallic titanium aluminide alloys are multiphase assemblies with complex microstructure and constitution, involving the phases γ(TiAl), α2(Ti3Al), β, and B2. The earlier stages of phase transformation and dynamic recrystallization occurring upon hot-working of such an alloy were investigated at the atomic scale by high-resolution electron microscopy. Accordingly, the conversion of the microstructure is triggered by heterogeneities in the deformation state and non-equilibrium phase composition. The β/B2 phase is apparently unstable under tetragonal distortion, which gives rise to the formation of the B19 phase via distinct shuffle displacements. These processes lead to a modulated microstructure, which is comprised of several stable and metastable phases. The phase transformations are accomplished by the propagation and coalescence of ledges. Large and broad ledges can apparently easily be rearranged into intermediate metastable structures, which serve as precursor for the nucleation of new grains.
Authors: Z.X. Li, Xia Huang, L.C. Qi, Chun Xiao Cao
Abstract: The beneficial effects of boron addition on microstructure transformations and mechanical properties of γ-TiAl alloys were investigated. Two growth mechanisms of boride (TiB2) in γ-TiAl alloy were confirmed, the curved flaky borides are products of irregular eutectic reaction growing coupled with matrix, while some faceted blocky borides in boron-rich alloy are primary TiB2 phase growing directly in melt. The core of flaky TiB2 is ultra-fine B2 phase and there has an orientation relationship [1210] TiB2//[001]B2, (1010) //(010)B2. In addition to the well-known grain refinement effect, boron addition can suppress the formation of metastable feathery and Widmastätten structure and broadens cooling-rate-range for the formation of fully lamellar structure, consequently, it improves thermal stability of the lamellar structure and accordingly prolongs the creep rupture life significantly. Another beneficial effect of boron addition is that boride can restrain discontinuous coarsening on lamellar grain boundary by pinning action and accelerates recrystallization of γ grain by introducing TiB2/matrix interfaces as nuclear sites during homogeneous treatment at 1150°C. Therefore, compared with boron-free alloy more homogeneous and refined near γ microstructure can be obtained in boron modified alloy.
Authors: C. Coelho, Ana Sofia Ramos, Bruno Trindade, M. Teresa Vieira, José Valdemar Fernandes, M. Vieira
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