Papers by Keyword: TiAl

Abstract: TiAl alloy is a high-temperature structural material that is highly sensitive to microstructure. Different hot working or heat treatment processes can refine the grain, release stress, and adjust the phase type and distribution. This study investigates the effect of heat treatment and rolling on the microstructure of powder metallurgy TiAl alloys in detail, resulting in TiAl alloys with high plasticity achieved through hot pack rolling. The heat treatment temperature was set between 1290°C and 1330°C, with a temperature interval of 10°C, a holding time of 30 minutes, and cooling with the furnace. Spheroidization of the grains occurred at 1310°C. Phase analysis of the TiAl alloy was conducted after heat treatment at 1290°C, 1310°C, and 1330°C. The microstructure of the TiAl alloy after heat treatment at 1290°C still consisted of γ phase and B2 phase, with no significant change in the content of γ phase and B2 phase. After heat treatment at 1310°C, numerous α₂ phases abruptly emerged in the microstructure. The TiAl sheet exhibits an elongation of 3.6% at room temperature, which increases to 65% at 700°C. The study investigated the phase transformation process, grain morphology, and microstructure evolution of powder metallurgy TiAl alloy under different heat treatment temperatures and rolling. The relationship between microstructure and temperature of powder metallurgy TiAl alloy was established using electron backscattering diffraction.

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: Either at higher temperatures or when a certain alloying element content is exceeded, γ-TiAl alloys contain the β phase (bcc) or its ordered derivate β_o (B2). The relatively soft β phase can facilitate hot deformation, but β_o is detrimental for creep strength and ductility. Thus, knowledge about β_o→β phase transformation is desirable. Surprisingly, even for the binary Ti-Al system it is under discussion whether the ordered β_o phase exists. Also, the effect of alloying elements on the β phase ordering is still unclear. In the present work the ordering of the β phase in binary Ti-(39,42,45)Al and ternary Ti-42Al-2X alloys (X=Fe, Cr, Nb, Ta, Mo) which was experimentally investigated by neutron and high energy X-ray diffraction is compared with the results of first principles calculations using density functional theory. Except for Cr the experimentally determined and the predicted behavior correspond.

Abstract: Improvement of fuel efficiency and reduction of carbon dioxide emission are important issues in the automotive and aviation industries. To achieve these issues, materials that are lightweight and have excellent heat resistance are required. For this reason, various alloys have been proposed. Among them, TiAl intermetallic compounds have excellent low specific gravity and high strength at high temperature. However, TiAl is difficult for machining and easily oxidized, so casting is difficult. For this reason, a method using reaction sintering has been studied, though it is difficult to obtain low oxygen concentration TiAl alloy powder. Therefore, the process to produce TiAl parts from Ti powder and Al powder is studied. However, in this method, when a mixed powder of Ti and Al is sintered, a phenomenon called ignition with a rapid temperature increasing may occurs, and ignited parts are swelling and becomes high porosity.

Abstract: Already, there are several processes to produce intermetalic alloy parts from powder , ex. metal injection molding (MIM) or additive manufacturing (AM). For these processes, pre-alloyed powder made by gas atomized powder is used because of their quality. As other way, intermetallic alloy can be produced combustion reaction process. On this process, ingredient metal powders are mixed and reacted by combustion. However, powders are fused by reaction heat, and they are difficult to keep the powder condition. There for, we are developed the process to produce intermetallic alloy precursor by slow combustion reaction. On this process, temperature of mixed powders increases slower than 0.2K/sec. while the combustion reaction, and powders are reacted without fusing. Using this process, TiAl presursor is synthesized. Relation of reacting condition and quality of the precursor is evaluated, and researched the practical usage of this precurser.

Abstract: The effect of a unique layered microstructure consisting of duplex-like region and equiaxed γ grains (γ bands) on the fatigue properties of Ti-48Al-2Cr-2Nb alloy bars fabricated by electron beam melting (EBM) at an angle (θ) of 90° between the building direction and cylinder (loading) axis was investigated focusing on the layered microstructure and test temperature. We found the room temperature (RT) fatigue strength of the alloy bars fabricated at θ = 90° is higher than that of the bars fabricated at θ = 0°. Moreover, it is comparable to that of the cast alloys with hot isostatic pressing (HIP) treatment in low-cycle fatigue life region, even without HIP treatment. The high fatigue strength of the bars at RT is attributed to the γ band, which acts as a resistance for crack propagation directed perpendicular to the γ band. On the other hand, the fatigue strength of the bars at θ = 90° is lower than that of the bars at θ = 0° in low-cycle fatigue life region at 1023 K. This is because the γ bands dose not act as a resistance for crack propagation at 1023 K. Although the bars at θ = 90° exhibits low fatigue strength in the region at 1023 K, that value is comparable to that of HIP-treated cast alloys due to the fine grain size, which is one of the features for the alloys fabricated by the EBM.

Abstract: The In Situ composites with microstructurally different types of intermetallic matrix such as nearly γ (TiAl) (composite A), multiphase with high amount of lamellar α₂(Ti₃Al) + γ (TiAl) regions (composite B) and fully lamellar α₂ + γ (composite C) were prepared by centrifugal casting and consecutive heat treatments of Ti-44.5Al-8Nb-0.8Mo-3.6C-0.1B, Ti-37Al-7Nb-0.8Mo-5.9C-0.1B and Ti-46.4Al-5Nb-1C-0.2B (at.%) alloys, respectively. The centrifugal casting results in a uniform distribution of coarse primary carbide particles in the as-cast samples. Hot isostatic pressing (HIP) and heat treatments have no effect on the Vickers hardness of the in-situ composite B but lead to a significant softening of the in-situ composites A and C. The in-situ composite C with a coarse-grained fully lamellar matrix shows a higher flow stress at 1000 °C and improved creep resistance at 800 °C compared to those of the in-situ composites A and B.

Abstract: This work is devoted to investigation of the structure of Ti-TiAl₃ composites reinforced by TiB₂ or TiC hard particles and obtained by spark plasma sintering of elemental foils and ceramic powders. Sintering was carried out at the temperature of 830 oC under the pressure of 40 MPa during 10 minutes. Microstructure of the composites obtained was represented by alternated layers of titanium and intermetallic compound TiAl₃. Also, it was found that at the Ti-TiAl₃ interfaces thin intermediate layers were formed. Quantitative elemental analysis of these layers showed that Ti₃Al, TiAl, and TiAl₂ compounds, as well as Ti (Al) solid solution could be formed in these zones. Diffraction analysis did not reveal any transformations of initial reinforcing phases after sintering. Interlayers with titanium diboride had the average microhardness level of 3988 HV, and the average microhardness level of interlayers with TiC was 1610 HV.

Abstract: The details of the lamellar microstructure in TiAl intermetallic alloys, such as the lath thickness and interfaces type governs the strength, ductility, creep properties and the long term microstructure stability of the alloy. The lamellar microstructure coarsening may induce property degradation of materials when the working temperature is high especially for the aero-engine turbine blades. At the same time, the reliability of the structure will decreases dramatically during long term working. In order to customize highly stable microstructure in high temperature, the phenomenon of lamellar formation during the solid-solid α→α₂+γ phase transformation in fully lamellar TiAl alloys was investigated by phase field simulations. The lamellar structure morphology obtained with simulation is coincides with the experimental results. It is found that the independent nuclei and twin-related nuclei co-exist in the nucleation stage for the random noise nucleation. During growth stage, the independent nuclei grow slowly or disappear for the interfacial energy and elastic energy minimization. While most twin-related nuclei survived. During the following coarsening stage, big nuclei swallow small nuclei for interfacial energy minimization. The statistical character of twin area fraction changes complicated during these processes and it will be analyzed in detail. These findings could shed light on the understanding of the lamellar formation and coarsening mechanisms during phase transformation in TiAl alloys.

Abstract: Ti-47Al-2Cr-2Nb-0.15B alloy is a typical γ-TiAl alloy, and powder metallurgy (PM) as a near-net shape method was used to prepare it in this article. Clean pre-alloyed powders were prepared by argon gas atomization, and TiAl alloy was prepared by hot isostatic pressing (HIP) at 1150 °C and 1230 °C. However, surface contamination is inevitable due to chemical reactions with the residual O₂ in the vacuum chamber during gas atomization, or due to physical adsorption of O₂ and H₂O during storage of the powder at room temperature. Infrared spectrometry was used to study this process. We found that the adsorption of gases is mainly H₂O. The adsorbed gas in powders would deteriorate the performance of PM alloy, so a gas protection environment is suggested. Tensile properties of PM TiAl alloy were compared with as-cast alloy. Results showed that PM TiAl alloy had better strength which also had more fine and uniform microstructure.