Search Results for gamma-titanium-aluminide alloys

Effects of microstructure constituents of α 2 -Ti 3 Al/γ-TiAl lamellae, β-Ti grains and γ grains, with various volume fractions on room-temperature ductility of γ-TiAl based alloys have been studied. The ductility of the alloys containing β phase of about 20% in volume increases to more than 1% as the volume fraction of γ phase increases to 80%. However, γ single phase alloys show very limited ductility of less than 0.2%. Microstructure analysis have revealed that intragranular fracture along γ/γ grain boundary occurred in γ single phase alloy whereas it does not along β/γ interphase in alloys containing β phase. In addition, local strain accumulations along β/γ interphase have been confirmed. The present results, thus, confirmed the significant contribution of β phase, especially the existence of β/γ interphase to enhancement of the room-temperature ductility in multicomponent TiAl alloys.

Electron beam melting (EBM) is one of the candidate manufacturing processes for TiAl alloys which have been considered as next generation high-temperature structural materials. The microstructure and mechanical properties of Ti-48Al-2Cr-2Nb (48-2-2) alloy bars fabricated using EBM were investigated, with a particular focus on the effect of processing parameters such as input energy density and building direction. We observed that the microstructure of the alloy bars fabricated using EBM depends strongly on the processing parameters used during the fabrication process of alloy. In particular, the alloy bars fabricated under appropriate processing parameters have a unique layered microstructure composed of duplex regions and equiaxed γ-grain regions (γ bands). Because of their fine microstructure and deformable soft γ bands, the alloy bars with the unique layered microstructure exhibit higher strength and higher ductility at room temperature (RT) than that of cast alloys. In addition, the alloy bars fabricated at an angle between the building direction and the loading axis of 45° show good fatigue properties at RT even without hot isostatic pressing treatment.

The creep behavior of a γ-TiAl based alloy at 1073 K was investigated, examining three different microstructures: equiaxed γ (Eγ), γ/γ fully lamellar (FLγ), and equiaxed γ with α 2 phase on grain boundaries (Eγα 2 ). The aim was to understand the influence of lamellar interfaces and grain boundary α 2 phase on creep behavior. Initially, creep rates were consistent across all specimens upon loading. However, Eγ exhibited a gradual decrease in creep rate compared to Eγα 2 and FLγ. Notably, the minimum creep rate of Eγ was one order of magnitude lower than that of Eγα 2 and FLγ. Conversely, Eγα 2 and FLγ displayed a slight acceleration and the longest rupture strain, albeit with the shortest rupture time compared to Eγ. Upon microstructural analysis of of the creep-test specimens, it was observed that numerous dynamic recrystallized grains (DXGs) and sub-grains formed along grain boundaries and interiors in Eγ, whereas they were limited to the region along grain boundaries in FLγ. In contrast, very few DXGs were formed in Eγα 2 . These findings indicate that γ/γ interfaces inhibit the extension of DXGs into grain interiors, suggesting that the grain boundary α 2 phase effectively suppresses the formation of DXGs.

Tensile deformation behavior of γ-TiAl based alloys consisting of α 2 -Ti 3 Al/γ lamellar colonies, β-Ti grains, and γ grains were investigated by in-situ scanning electron microscopy and digital image correlation technique, in order to identify the role of each microstructure constituents in deformation. The alloy with nearly lamellar microstructure, in which the volume fraction of β/γ duplex ( V DP ) is 10%, shows elongation of only 0.14%, whereas the alloy with nearly globular β/γ duplex microstructure with V DP of 94% shows elongation of 0.49%. In α 2 /γ lamellar microstructure, obvious strain localization occurs along lamellae and develops at specific regions with loading. In the case of β/γ duplex microstructure, strain localization is observed in γ grains and in β phase regions near the β/γ phase boundary, although no obvious deformation is observed in the β grains. β/γ phase boundaries enhances room temperature ductility of TiAl alloys by inducing multiple slip in γ phase and deformation of β phase.

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, 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.

The competitive effect of Nb and V additions on the high-temperature oxidation behavior of Ti- 30Al alloys were studied at 800°C in air. Oxidation performance increased with increasing Nb content, however, V additions eliminated the beneficial effect of Nb on oxidation performance, causing higher oxidation mass gains. In-situ high-temperature XRD by means of synchrotron source suggested dissolution of Nb 5+ but lower valence of vanadium ions in the TiO 2 oxide scale during oxidation. Dissolution of Nb and V ions with different valence in TiO 2 during oxidation could cause the beneficial and detrimental effects observed on the performance of high-temperature oxidation of Ti-30Al.

In this study the effect of Widmanstätten-type morphology α 2 plates on creep has been investigated by preparing nearly equiaxed γ (N γ ) and nearly equiaxed γ having Widmanstätten-type α 2 plates within grain (Wα 2 ). Creep tests were conducted at 1073 K under constant stresses, high stress and low stress, in air. At the high stress, Wα 2 shows creep rate smaller than N γ in transient stage, both specimens show similar minimum creep rate and the creep strain at minimum creep rate is 3 % for Wα 2 and 10 % for N γ, since N γ shows prolonged primary region. In acceleration stage, both show similar behavior with rupture time of about 50 h and rupture elongation of 60 %. At the low stress, on the other hand, reverse behavior occurs, that is, W α 2 shows creep rate higher than Nγ in transient stage. The regions near grain boundaries progressively deformed for both specimens at high stress level, whereas deformed region is extended within grain interiors. From these results it is suggested that α 2 plate act as the obstacle for dislocation motion in the γ matrix at high stress and that interfacial dislocation promote the creep deformation at low stress.

Interstitial carbon (C) in β-Ti, α-Ti, α 2 -Ti 3 Al and γ-TiAl phases present in the γ-TiAl alloys with and without substitutional elements (M: transition element) is quantitatively analyzed using soft X-ray emission spectroscopy (SXES), in order to reveal the effect of solute carbon on the phase equilibria. SXES for carbon analysis was used and the peak intensity of the second reflection of carbon Kα is analyzed using the fully homogenized sample having different C content under the optimum condition to make the accurate calibration curves. The obtained calibration curve is in an accuracy of ± 0.07 at. % C. In all heat treated alloys, no carbide is observed. In Ti-Al binary system, the α+γ phase region shifts toward higher Ti side, and the volume fraction of γ phase increases slightly with the carbon addition. In all system, carbon preferentially partitions into the α phase, followed by less partitioning in the γ and β phases in order. The carbon content in the β phase remains unchanged of almost 0.05 at. % regardless of carbon addition in Ti-Al-V system and the partition coefficient of carbon between the α and γ phases becomes larger in Ti-Al-V system than that in TiAl binary system.

The aspiration to deploy Nb-based alloys as viable upgrade for Ni-based superalloys is rooted in their potential for superior performance in high-temperature applications, such as rocket nozzles and next-generation turbines. However, realizing this goal requires overcoming formidable design hurdles, including achieving high specific strength, creep resistance, fatigue, and oxidation resistance at elevated temperatures, while preserving ductility at lower temperatures. Additionally, the requisite for alloy bond-coatings, to ensure compatibility with coating materials, further complicates the design process. QuesTek Innovations has its Integrated Computational Materials Engineering (ICME) technologies to design a superior performance high-temperature Nb-based superalloy based on solid solution and precipitation strengthening. Additionally, utilizing a statistical learning method from very limited available data, QuesTek engineers were able to establish physics-based material property models, enabling accurate predictions of equilibrium phase fraction, DBTT, and creep properties for multicomponent Nb alloys. With the proven Materials by Design methodology under the ICME framework, QuesTek successfully designed a novel Nb superalloy that met the stringent design requirements using its advanced ICMD materials modeling and design platform.

The EU NextGenPower-project aims at demonstrating Ni-alloys and coatings for application in high-efficiency power plants. Fireside corrosion lab and plants trials show that A263 and A617 perform similar while A740H outperforms them. Lab tests showed promising results for NiCr, Diamalloy3006 and SHS9172 coatings. Probe trials in six plants are ongoing. A617, A740H and A263 performed equally in steamside oxidation lab test ≤750°C while A617 and A740H outperformed A263 at 800°C; high pressure tests are planned. Slow strain rate testing confirmed relaxation cracking of A263. A creep-fatigue interaction test program for A263 includes LCF tests. Negative creep of A263 is researched with gleeble tests. A263 Ø80 - 500mm trial rotors are forged with optimized composition. Studies for designing and optimizing the forging process were done. Segregation free Ø300 and 1,000mm rotors have been forged. A263 – A263 and A293 – COST F rotor welding show promising results (A263 in precipitation hardened condition). Cast step blocks of A282, A263 and A740H showed volumetric cracking after heat treatment. New ‘as cast’ blocks of optimized composition are without cracks. A 750°C steam cycle has been designed with integrated CO 2 capture at 45% efficiency (LHV). Superheater life at ≤750°C and co-firing is modeled.