Papers by Keyword: Gamma Titanium Aluminide

Abstract: With the introduction of TiAl in aircraft jet engines, there is an increasing demand for the evaluation of novel processing routes for gamma titanium aluminides such as additive manufacturing (AM). A Ti-47Al-2Cr-2Nb powder material has been used as feedstock for laser fabrication of 3D samples by means of “Selective Laser Melting” (SLM) and “Direct Metal Deposition” (DMD). A number of processing parameters including laser power, laser scan rate, powder feed rate, have been varied to evaluate their effects on the material soundness. Optimised conditions can significantly reduce the crack sensitivity for this relatively low ductility material. In particular, crack-free experimental conditions have been identified by using additional heating strategies, thus limiting built-up residual stresses during fast cooling. The different samples have been examined using optical and scanning electron microscopy in the as-built condition. The non-equilibrium cooling conditions generate ultra-fine and metastable structures exhibiting high microhardness values. A range of post-heat treatments have been performed to relieve the residual stresses and to tailor more uniform microstructures. Conventional heat treatments in the α+γ two-phase domain or in the α single phase domain have been successfully used to fully restore homogeneous microstructures either duplex or fully lamellar. A comparison is made with microstructures of both laser treated materials and of conventionally processed materials.

Abstract: Gamma titanium aluminides are promising alloys for low-pressure turbine blades. A significant disadvantage of such intermetallic alloys is failure induced during forming processes due to ductile damage and flow instabilities. Previous investigations on a gamma titanium aluminide alloy (TNM), have shown ductile damage due to tensile stress components and instabilities such as shear bands, pores and micro-cracks at low temperatures and high strain rates. The main part of the current work is to delineate damage and unstable regions in the low temperature region. Hot deformation experiments are conducted on a Gleeble®3800 thermomechanical treatment simulator to obtain flow curves to be implemented in a finite element method (FEM) code. Instabilities in the material are described by existing instability criteria as proposed by Semiatin and Jonas and implemented into FEM code DEFORM^TM 2D. Predictions of ductile damage models and the instability parameter are validated through detailed microstructural studies of deformed specimens analysed by light optical- and scanning electron microscopy.

Abstract: Gamma titanium aluminides are innovative materials for high temperature and light weight applications [1]. On the other hand, their hot workability can be limited by failure during hot deformation processes. The prediction of ductile damage in metallic materials can be performed by macromechanical ductile damage criteria [2-4]. If the calculated damage D parameter exceeds a critical value D_c, the material fails. Some macromechanical ductile damage criteria are shown in Table 1, with σ as effective stress, ε as effective strain, σ_max as maximum principal stress, σ_m as hydrostatic stress (mean stress) and εf as equivalent fracture strain. The damage responds to strain localization and thus, to multiaxial stress concentration that increases fracture probability.

Abstract: Gamma titanium aluminide material, cast Ti–45Al–5Nb (at.%), was electrochemically precharged with hydrogen in the cathodic charging mode at a current density of 50 mA/cm2 for times ranging from 6 to 48 h. XRD and microstructure investigations by means of electronic microscopy were used for analyzed the influence of hydrogen on the microstructure.

Abstract: Several diamond coatings were performed on -TiAl substrates by a microwave-plasma assisted CVD, which were made directly to the substrate and indirectly to the TiC, Ti5Si3, Al2O3+TiO2 and Si layers on the substrate. The direct coatings suffered from severe delamination and cracks. The deposited layers on TiC and Ti5Si3 layers partially delaminated, while those on Al2O3+TiO2 and Si layers adhered well without delamination. All the diamond films deposited were characterized using scanning electron microscopy, Raman spectroscopy, and X-ray diffraction. Raman spectra showed that poly- and nano-crystalline diamond films were obtained for the coatings of -TiAl.

Abstract: A material family to replace the current superalloys in aeronautical gas turbine engines is considered to be that of gamma Titanium Aluminide (-TiAl) alloys. Structural components in aeronautical gas turbine engines typically experience large variations in temperatures and multiaxial states of stress under non-isothermal conditions. The uniaxial, torsional and bi-axial thermo-mechanical fatigue (TMF) behaviour of this -TiAl alloy have been examined at 400 – 800oC with strain amplitudes from 0.15% to 0.7%. The tests were conducted at both in-phase (IP) and out-of-phase (OP). The effects of TMF on the microstructure were also investigated. For the same equivalent mechanical strain amplitude uniaxial IP tests showed significantly longer lifetimes than pure torsional TMF tests. The non-proportional multiaxial OP test showed the lowest lifetimes at the same equivalent mechanical strain amplitude compared to the other types of tests.

Abstract: The effect of shot peening and roller burnishing on the fatigue performance of the γ(TiAl) alloy Ti-45Al-9Nb-0.2C was investigated over a wide range of processing intensities. At optimized conditions shot peening and roller burnishing can markedly improve the fatigue strength at ambient temperatures. For temperatures above 650 °C, the residual compressive stresses induced by shot peening and roller burnishing quickly relax. This indicates that, at elevated temperatures, surface roughness and dislocation strengthening become more important for the fatigue performance of mechanically surface-treated components. Roller burnishing leads to much lower surface roughness than shot peening, resulting in more effective improvement of high temperature fatigue performance. However, surface strengthening by shot peening can also be beneficial for the fatigue performance at elevated temperatures, when the surface roughness is reduced by subsequent polishing.

Abstract: The objective of this study was to optimize the casting design of gamma titanium aluminde automotive turbocharger rotor by means of the practical experiment and numerical simulation. Gamma titanium aluminide rotors were produced by centrifugal casting methods on a laboratory scale. Based on the metal-mold reaction of gamma titanium aluminide, the investment molds were manufactured by an electro-fused Al2O3 mold. The experimental results showed that the castings failed to reach the end of the cavities due to insufficient centrifugal force and a lower fluidity compared to the other metals. Although the satisfactory results were not obtained in the numerical simulation, it was concluded that numerical simulation aided to achieve understanding of the casting process and defect formation in gamma titanium aluminide turbocharger rotor castings.

Abstract: In this study, Nb doped γ-TiAl is designed and examined their tensile properties at elevated temperatures. Small compositional changes cause drastic changes of the mechanical properties at 1273K. In order to clarify the deformation mechanism at elevated temperatures, dislocation structures have been observed using transmission electron microscope (TEM). All of the specimens observed by TEM show that at least two slip systems are operating in each grain. The specimen having relatively lower strength and higher tensile elongations shows more than four operating slip systems in the grains. The specimen having a medium strength shows many super-dislocations and their dissociations that block the other dislocation motions. The specimen having the highest strength shows many deformation twins. These observations suggest that deformation twins become the strong obstacles against moving dislocations at elevated temperatures. The relationships between observed dislocation structures and mechanical properties of γ-TiAl are discussed.