Papers by Keyword: Titanium Aluminides

Abstract: Co additions to titanium aluminides were assessed to decrease the typically high sintering temperatures necessary for the densification of intermetallics. Compositions based on the binary Ti-45Al with ternary Co variations between 1-10 at.% were investigated. The specimens were cylinders with 8 mm diameter prepared using blended elemental powders followed by cold uniaxial pressing. Sintering was carried out under an argon atmosphere at different temperatures ranging from 1100 to 1400 °C for 2 hours in a tube furnace. The results indicated that there was a systematic increase in densification with Co additions. The relative density of the reference material Ti-45Al was approximately 53%, however, Co addition of 10% led to densifications in the order of 80%. A strong effect of decreasing the sintering temperature was achieved with Co additions. The microstructure changed from fully lamellar with 1 at.% Co sintered at 1400 °C to duplex with higher Co additions sintered at 1200 °C. Besides the γ-TiAl and α₂-Ti₃Al equilibrium phases, the formation of a CoAl₂Ti intermetallic was identified. The addition of 7% Co led to the highest hardness of approximately 450 HV.

Abstract: The studies results of the titanium with aluminum diffusion interaction at a temperature of 650 oC are presented. The phase and chemical composition of the diffusion interaction zone, the nature of the change in its thickness from the exposure time are determined. It is shown that accelerated cooling of explosion-welded composites from the heat treatment temperature leads to spontaneous separation of the aluminum layer with the formation of a coating based on the TiAl₃ intermetallic compound on the titanium surface.

Abstract: The influence of mechanoactivated reagents cladding on the structural-phase state of the SHS-products was investigated. Titanium and aluminum powders were used as reagents. Mechanical activation was performed on the AGO-2 planetary ball mill. The coating on Ti+Al mechanocomposite was carried out using magnetron installation “VSE-PVD-Power”. At deposition time of 40 minutes, the reaction start temperature increases from 525 °C to 648 °C (compared to reagents without cladding). It can be assumed that an increase in the thickness of the deposited SiO₂ film serves as a barrier to the reaction start, thereby increasing ignition temperature. Apart from pretreatment, the phase composition of the final product contains intermetallic compounds TiAl, TiAl₃, Ti₃Al₅, as well as the small amount of residual Ti. The main phase is TiAl.

Abstract: The investigations of (Ti + Al)-SiO₂ surfacing by magnetron deposition are presented in this article. The correlation between film`s thickness deposited on the particles by mechanocomposites and magnetron sputtering time was established. It was determined that the rational time for surfacing by Ti-Al mechanocomposites system is about 40 minutes. According that deposition time the thickness of deposited SiO₂ films were obtained as 5.2 microns.

Abstract: Samples of TiAl-based matrix in-situ composite with the chemical composition Ti-46.4Al-5.1Nb-1C-0.2B (at.%) reinforced with a low volume fraction of primary Ti₂AlC particles were prepared by vacuum induction melting in graphite crucibles and centrifugal casting into graphite moulds. The hot isostatic pressing (HIP) of the as-cast samples and subsequent heat treatments leads to the formation of equiaxed grains with fully lamellar α₂(Ti₃Al) + γ (TiAl) microstructure and uniformly distributed Ti₂AlC and TiB particles. The minimum creep rates of the in-situ composite are significantly lower compared to those measured for the counterpart low carbon benchmark alloy with the chemical composition Ti-47Al-5.2Nb-0.2C-0.2B (at.%) at temperatures ranging from 800 to 900 °C and applied stress of 200 MPa. The studied in-situ composite shows also significantly improved creep resistance compared to that of some TiAl-based alloys with fully lamellar, convoluted and pseudo-duplex microstructures at a temperature of 800 °C and applied stress of 200 MPa.

Abstract: Isothermal forging processes are typically used for near-net shaping of high-performance components such as turbine discs and blades. Recent developments have introduced isothermally forged titanium aluminides into commercial jet engines. Titanium aluminides are lightweight intermetallic compounds with excellent creep properties but very limited ductility. Their low workability requires isothermal forging at slow strain rates, which is typically kept constant in the process. This work explores the possibility of controlling the strain rate during the process using model predictive control, so that the process time is reduced while the microstructure transformation and the amount of damage introduced into the workpiece are controlled. The results of isothermal compression with friction show that both an acceleration of the process and a reduction of damage are possible using the suggested control strategy.

Abstract: Metal forming processes may induce internal damage in the form of voids in the workpiece under unfavorable deformation conditions. Controlling the amount of damage induced by metal forming operations may increase service performance of the produced parts. Damage is crucial in high-performance components of limited workability such as jet engine turbine blades. Recent developments have introduced forged titanium aluminides into commercial jet engines. Titanium aluminides are lightweight intermetallic compounds with excellent creep properties but very limited ductility. Their low workability requires isothermal forging at slow strain rates, which is typically kept constant in the process. This work explores the possibility of increasing the ram speed during the process so that the process time is reduced while the amount of damage introduced into the workpiece is controlled. The results show that a 25% reduction in process time seems viable without increase in damage by solving an optimal control problem, in which the ram speed profile is determined off-line by minimization.

Abstract: In powder metallurgical processing the sintering process, as well as heat treatments, can drastically influence microstructure formation. In the case of γ-titanium aluminides, it is critical to achieve certain microstructure parameters, such as colony size, porosity and grain boundary morphology in order to obtain appropriate mechanical properties. In this study, the effect of a heat treatment implemented after sintering with the objective of varying the colony size was investigated. Specimens of Ti-45Al-5Nb-0.2B-0.2C prepared by metal injection moulding and uniaxial pressing of feedstock were used to evaluate the tensile and creep properties. Heat treatments conducted at 1350 and 1400 °C for 3 h led to colony sizes of approximately 100 and 200 μm, respectively. Classically, there is an inverse relationship between grain size and creep resistance, nonetheless, for γ-titanium aluminides, the morphology of the colony boundaries was also found to play a role. The larger colony sizes achieved with the heat treatments did not improve the primary creep resistance, which was explained by the change in the morphology of the colony boundaries as they became larger.

Abstract: The paper is focused on the analysis of the role of lamellar microstructure in fracture performance of model TiAl intermetallic compound. Coarse lamellar colonies and, at the same time, fine lamellar morphology were prepared by compressive deformation at 1553 K (region of stable α phase in TiAl equilibrium diagram) followed by controlled cooling to 1473 K (region of α+g phase) with delay on this temperature and then cooling down. The fracture toughness was evaluated by means of chevron notch technique. In addition, because of enhanced toughness, crack resistance curves were obtained by load - unload technique of pre-racked beams, namely in two directions of crack propagation relative to lamellar structure. Extensive development of shear ligament toughening mechanism was observed in fracture surfaces leading to quite good fracture toughness thanks to the heat treatment applied.

Abstract: Challenging issues concerning energy efficiency and environmental politics require novel approaches to materials design. A recent example with regard to structural materials is the emergence of lightweight intermetallic TiAl alloys. Their excellent high-temperature mechanical properties, low density, and high stiffness constitute a profile perfectly suitable for their application as advanced aero-engine turbine blades or as turbocharger turbine wheels in next-generation automotive engines. Advanced so-called 3^rd generation TiAl alloys, such as the TNM alloy described in this paper, are complex multi-phase alloys which can be processed by ingot or powder metallurgy as well as precision casting methods. Each process leads to specific microstructures which can be altered and optimized by thermo-mechanical processing and/or subsequent heat treatments.