Papers by Keyword: TiAl

Abstract: The alloys based on γ-TiAl have attractive properties as light weight heat-resisting material. The poor oxidation resistance is one of the drawbacks to limit their application. This results from the high growth rate of TiO₂ in the scale and the spallation of the oxide scale during cooling. In the present study, the influence of Zr addition on the oxidation behavior of TiAl has been investigated with emphasis on scale adherence. The oxidation behavior of Ti-45Al (in mol %), Ti-45Al-6Nb alloys containing 0 - 0.2% Zr, Ti-Nb and Ti-Zr binary alloys were investigated. The oxide scale was analyzed by conventional methods including X-ray diffractometry, SEM and EPMA. The addition of Nb to TiAl results in a remarkable decrease in mass gain, while the Zr addition to TiAl results in a slight decrease. The Ti-45Al-Nb alloy with Zr shows scale spallation after long term exposure. Concerning the binary alloys, the addition of Zr to Ti more than 20 mol% accelerates the oxide growth significantly. The XRD for Ti-45Al-6Nb-0.2Zr after oxidation shows the formation of Zr oxides and Ti-Zr complex oxides in the scale. The scale spallation observed in this alloy is due to the enrichment of Zr at scale / alloy interface and formation of Zr oxide in the scale.

Abstract: Ti-43Al-5V-4Nb (at.%) intermetallic compounds with fully lamellar structure were fabricated by forging method without canning using the blended elemental powders. The process route consisted of powder blending, compacting, sintering and final fabrication by hot forging. During sintering, there existed expansion, which is detrimental to forging process. In order to overcome the difficulty, two-step forging was introduced. The total reduction of two-step forging was up to 65%. Nearly fully lamellar structure of TiAl alloy was obtained. Overall, an optimized and potentially lower cost processing route could be identified.

Abstract: Full dense and highly pure TiAl/Ti₂AlC in situ composites were successfully synthesized by reactive synthesis from the powder mixtures of Ti, Al and carbon black by hot-press-assisted reaction process. The reaction process, microstructure and bending strength of the TiAl/Ti₂AlC in situ composites were investigated in detail. The results show that the as fabricated composites posses three phases, namely, TiAl, Ti₃Al and Ti₂AlC. The matrix phases are mainly equiaxed TiAl with a minor lamellar Ti₃Al phase. Ti₂AlC particles with size less than 1 μm are distributed uniformly in matrix grains as a reinforcing phase. When C content is 0.44 wt %, the flexural strength reaches 426.21 MPa, which is increased by 35 %.

Abstract: Ti-Al nanoparticles and have been synthesized by the flow-levitation (FL) method. The morphology, crystal structure and chemical composition of Ti-Al nanoparticles obtained were investigated by transmission electron microscopy, X-ray diffraction and induction-coupled plasma spectroscopy. Results showed that the average grain size of these nanoparticles is about 30 nm, and it can be controlled availably by altering procedure parameters. The structure of powder is mainly double phase Ti-Al particulate composite. In addition, there are certain corresponding relation between the compositions of evaporate source master material and phase composition of alloy compound nanoparticle. The expected nanoparticle, which is mainly composition of nanocrystalline intermetallic compound, can be obtained through changing the compositions of evaporating source mastermaterial.

Abstract: Actual alloy and process development for high temperature turbine applications in the aerospace sector is strongly aimed at reaching the high demands on reduction of CO2 emissions responsible for the green house effect. Based on weight reduction the main objective resides in improving engine performance and efficiency. Last generation intermetallic titanium aluminides (γ TiAl) have a big potential to reach this goals. γ TiAl is nevertheless a very demanding material requiring very sophisticated processing routes. Access has developed a casting route for production of high quality γ TiAl components based on skull induction melting (SIM) and centrifugal investment casting. Although the feasibility of the technology has been already proven in earlier projects, it is still necessary to improve the process for series production of parts with the high quality standards required by the aerospace industry. With aid of a new developed centrifugal casting facility Access and its partners are conducting a comprehensive qualification process for the production of aerospace components, e.g. low pressure turbine blades. Basic issues comprising casting cluster design based on numerical simulation, process control and quality management are being addressed.

Abstract: In order to make the TiAl Intermetallics-based materials more competitive with conventional superalloys, further work on microstructure modification and alloying process has been carried out specific to the cast and wrought process, respectively. It is proved that directional lamellar microstructures can be formed in the cast TiAl. Those microstructures have demonstrated preponderant mechanical properties under unidirectional load after the cast porosity being removed by HIP at proper temperatures. This microstructure design specific to the rotating parts has benefited the endurance capacity of cast TiAl turbochargers. On the other hand, the additions of Gd have shown significant effect in refining the lamellar microstructures of forged TiAl alloys but do not bring out the detrimental influence to the tensile ductility. The reason is that the Gd-containing precipitates can greatly impede the growth of  grains while themselves dispersing during the thermal process. Upon those technology advances, the further success of TiAl alloys’ applications is prospected.

Abstract: . Intermetallic Ti-Al-Cr layers with small additions of Si, Zr, W, and Y were deposited on γ-TiAl specimens using magnetron sputtering. The oxidation behaviour of the coated γ-TiAl alloy was studied in the temperature range between 950 and 1000°C under cyclic oxidation conditions in air. Compared to the bare substrate material, the coatings exhibited higher oxidation resistance. During prolonged exposure the intermetallic layers degraded losing their capability to form a protective alumina scale. On coated γ-TiAl samples zirconia topcoats were deposited by electron beam physical vapour deposition. These thermal barrier coating systems exhibited lifetimes exceeding the maximum exposure length of 1000 1 h cycles at 950°C, but failed at 1000°C. Failure was caused by degradation of the bond coats resulting in spallation of the thermally grown oxides.

Abstract: In this paper, effect of AlCl3 addition as process control agent (PCA) during high energy ball milling of Al and TiO2 powder mixture was studied. This mechanical activation is aimed at to synthesize an ultra fine grained TiAl/Al2O3 composite. Experimental results show that AlCl3 significantly prevents severe cold welding of Al particles to milling media. Sublimation of this compound by local temperature increase due to balls collisions seems to be the main reason in prevention of severe cold welding. Samples were characterized by XRD, SEM and DTA. Mean crystallite sizes in particles of this sample and those milled with no PCA and milled with stearic acid were calculated and the results showed that smaller crystallite size is obtained in presence of AlCl3. However, DTA results revealed that addition of AlCl3, shifts aluminothermic reduction of TiO2 by Al to higher temperatures and therefore, final composite phases form at higher temperatures. Phase evolutions during further heat treatment of the powder sample milled with AlCl3 were also thoroughly studied.

Abstract: The processing of large or near-net shaped parts from g-TiAl alloys is extremely challenging. The forging of large-scale TiAl-parts is hampered by the unavailability of high-quality, chemical homogenous pre-material. However, using an innovative combination of forging and joining large discs of TiAl with excellent mechanical properties can be produced. Metal Injection Moulding (MIM) facilitates the production of near-net shaped parts of high-strength TiAl-alloys with strength comparable to the cast condition.

Abstract: This paper gives an account of research which has been carried out on electron-beam brazed specimens made of high-Niobium γ-titanium aluminides. The microstructure in the brazing area of two different brazes (TiCuNi and Ni 102) is explained and analysed via scanning electron microscopy and energy dispersive X-ray spectroscopy.