Βo Phase Research Articles

Thermal stability of a newly developed Mn containing β-solidifying γ-TiAl intermetallic compound at 750 °C

A newly low-cost β-solidifying γ-TiAl alloy Ti-44Al-3Mn-0.4Mo-0.4W-0.1B-0.1C (in at. %, named as TMMW) was invented. The rolled bars with a diameter of 12 mm were successfully fabricated by conventional hot rolling process, directly carrying out from the ingot without the pre-forging or hot-packing. The heat-treated TMMW rolled bar, which had a fine-grained nearly lamellar (NL) microstructure, was then air-exposed at 750 °C for up to 3000 h. The changes of the microstructure during this exposure were characterized using electron probe X-ray micro-analyzer (EPMA), transmission electron microscope (TEM), electron backscattered diffraction (EBSD) and high-energy X-ray diffraction (HEXRD). The tensile properties of the samples before and after thermal exposure were also tested. The fine-grained NL microstructure is found to be thermodynamically stable during the exposure. Meanwhile, the combined addition of Mo and W shows a positive effect on the stabilization of βo phase at colony boundaries, with only slight precipitation of the nanometer βo phases with no other brittle phases, such as Laves, in the metastable α2 lamellae. It is found that the thickness of the α2 lamellae decreases after 500 h exposure, while remains basically unchanged from 500 h to 3000 h, which is contributed by the effective pinning effect of the precipitated βo phase in α2 lamellae. As a result, the exposure-induced embrittlement under room temperature and elevated temperature does not take place. Interestingly, the ductility tested at 800 °C improves significantly after exposure. As for the tensile strength, it is only reduced by less than 10 % after 500 h exposure, while it remains essentially unchanged when increasing the exposure time from 500 h to 3000 h. Finally, the mechanisms of the precipitation of βo phases in the α2 lamellae and the tensile properties evolution during the exposure were carefully discussed.

Quantitative analysis on microstructure characteristic of pre-strained β-solidified TiAl alloy during post-heat treatment

The β/βo phase in β-solidified TiAl alloys plays a contradictory role in improving processability at high temperatures but deteriorating performance at service temperature due to its ordering transformation. After exerting its positive role during thermomechanical processing, it must be eliminated or reduced to a minimum through post-heat treatment. In this study, the microstructural evolution of the pre-strained Ti-43.24Al-8.42Nb-0.20W-0.21B-0.24Y alloy on post-heat treatment is investigated, and a quantitative relationship between βo phase content and the pre-strain is established. Due to the difference in driving force of phase transformation, with decreasing pre-strain the microstructure displays varied characteristics from a mixed structure comprising (α2+γ) lamellar colonies, γ blocks, and βo phase, to a nearly-lamellar structure after post-heat treatment at 1270 °C/4h/FC. Only if the pre-strain is less than 0.78, a refined nearly-lamellar structure can be achieved. This work provides important theoretical guidance for practical forging processing of β-solidified TiAl alloys.

Interaction between dynamic recrystallization and phase transformation of Ti-43Al-4Nb-1Mo-0.2B alloy during hot deformation

The β solidified γ-TiAl alloy holds important application value in the aerospace industry, while its complex phase compositions and geometric structures pose challenges to its microstructure control during the thermal-mechanical process. The microstructure evolution of Ti-43Al-4Nb-1Mo-0.2B alloy at 1200 °C/0.01 s−1 was investigated to clarify the coupling role of dynamic recrystallization (DRX) and phase transformation. The results revealed that the rate of DRX in α2+γ lamellar colonies was comparatively slower than that in βo+γ mixed structure, instead being accompanied by intense lamellar kinking and rotation. The initiation and development rates of DRX in α2, βo, and γ phases decreased sequentially. The asynchronous DRX of the various geometric structures and phase compositions resulted in the uneven deformed microstructure, and the dynamic softening induced by lamellar kinking and rotation was replaced by strengthened DRX as strain increased. Additionally, the blocky α2 phase and the terminals of α2 lamellae were the preferential DRX sites owing to the abundant activated slip systems. The α2→βo transformation within lamellar colonies facilitated DRX and fragment of α2 lamellae, while the α2→γ transformation promoted the decomposition of α2 lamellae and DRX of γ lamellae. Moreover, the varied βo+γ mixed structures underwent complicated evolution: (1) The γ→βo transformation occurred at boundaries of lamellar colonies, followed by simultaneous DRX of γ lamellar terminals and neighboring βo phase; (2) DRX occurred earlier within the band-like βo phase, with the delayed DRX in enclosed γ phase; (3) DRX within the βo synapses and neighboring γ phase was accelerated owing to generation of elastic stress field; (4) Dispersed βo particles triggered particle stimulated nucleation (PSN) of γ phase. Eventually, atomic diffusion along crystal defects in βo and γ phases caused fracture of band-like βo phase and formation of massive βo particles, impeding grain boundary migration and hindering DRXed grain growth of γ phase.

Emissivity measurements conducted on intermetallic γ-TiAl-based alloys for aeronautical applications

The directional spectral emissivity of a Ti–48Al–2Nb–2Cr alloy (in at.%), 4822 alloy, and a Ti-43.5Al–4Nb–1Mo-0.1B alloy (in at.%),TNM alloy, used in the aeronautical industry, are measured between 150 and 850 °C. The differences in the emissivity values between both alloys at the lowest temperatures, indicates that the βo phase, only present in TNM, exhibit higher emissivity values. By numerical integration of the measured data, the total directional and hemispherical emissivity have been calculated. At 850 °C the total hemispherical emissivity in vacuum are nearly identical with 0.274 ± 0.006 for the 4822 alloy and 0.273 ± 0.007 for the TNM alloy. The lower emissivity change with temperature measured in TNM alloys is related with the deconvolution of βo phase by diffusion processes. Afterwards, near-normal spectral emissivity measurements are performed in both alloys during isothermal oxidation treatments at 750 °C and 850 °C for 120 h. The emissivity data reveal that the TNM alloy exhibits higher oxidation resistance especially at 750 °C. In parallel, microstructural characterization has been performed before the measurements, after the directional emissivity measurements prior to oxidation and after isothermal oxidations. The formed oxide scale is composed of four layers that coincide with those reported in the literature: an outer layer of TiO2 contiguous with a layer of Al2O3, followed by a TiO2/Al2O3 mixed layer and finally a thin layer of Nb-rich nitride. This mixed layer governs the interferential part of the alloys’ emissivity spectra, which, in combination with the background, determines the overall radiative behavior of the alloys under service conditions.

A refined fully lamellar TiAl alloy extruded at α-phase region: Microstructure and mechanical properties

Multiphase TiAl alloys with refined fully lamellar (RFL) microstructure can balance their mechanical properties at room temperature (RT) and high temperatures, which is vital for their applications. A new cost-effective and compact processing method is needed to reduce the processing cost and widen the extent of applications. In this work, Ti–44.5Al–4Nb–0.5Mo–0.1B (at.%) alloy with RFL microstructure was fabricated using a one-step hot-pack extrusion process at 1280 °C with an extrusion ratio of 4. The microstructure and mechanical properties of the extruded rod were uniform except in a thin layer near the surface. The RFL microstructure exhibited a deformation texture with <11 2‾ 0> α2 parallel to the extrusion direction and (0001) α2 parallel to the radial direction of the rod. The yield strength, ultimate tensile strength and ductility of the rod were simultaneously improved after extrusion due to elimination of βo phase, decrease in the sizes of the lamellar colony and spacing, and the presence of texture. The fundamental reason why RFL microstructure can be obtained by extrusion is that the higher Al and lower Mo contents in the alloy expand α phase region and decrease βo content of the alloy compared with TNM alloy.

Breaking the high-temperature strength-ductility trade-off in TiAl alloys through microstructural optimization

Designing a novel microstructure with higher strength-ductility than lamellar microstructure (LM), which has been considered the most valuable engineering application in the service temperature range, is desired for lightweight TiAl alloys. Our prior publication reported a three-phase tri-modal (T-T) structure for Ti-43.5Al-4Nb-1Mo-0.1B(.at%) with superior mechanical properties than LM at room temperature (Ref: Acta Mater. 225 (2022) 117,585). However, its strength-ductility at working temperature has yet to be fully exploited. This work fills the much-needed gap. The qualitative and quantitative optimizations indicate that the T-T structure's yield strength increases first and then decreases with increasing pearlitic-like microstructure (PM, which consist of three-phase (α2, γ and βo phase) with clear orientation relationships that form in a lamellar/cellular manner) content while the ductility keeps rising. When the T-T structure contains ∼20 % PMs, it exhibits a yield strength about 130MPa higher than LM coupling with doubled ductility at 750 °C. PMs can induce a prominent twinning induced plasticity and dislocation jog dragging (TWIP&DJD) effect, resulting in better strength-ductility below 700°C; above this temperature, the TWIP&DJD effect significantly plasticizes the alloys. The higher strength of the T-T structure at 750 °C is caused by hetero-deformation-induced strengthening between LMs and PMs, which cannot be offset by the plasticizing caused by PM when the volume fraction of PMs is less than 50 %. This study demonstrates that introducing an appropriate amount of plasticizing structures (similar to PM) with excellent work hardening capacity is a promising strategy for enhancing the strength-ductility synergy of TiAl and possibly other brittle materials.

Revealing cellular reaction mechanisms and high-temperature structure stability in β-stabilized TiAl alloy

This study reveals that the cellular reaction (CR) is dominated by grain boundary (GB) migration mechanism controlled by element diffusion, during which the α2 → γ + βo phase transformation occurs. CR transforms supersaturated α2/γ lamellar system from metastable to equilibrium state via microstructure coarsening. Moreover, elements enrichment of Ti and Nb, Mo Ti, Nb and Mo occurs during CR, triggering the α2 → γ + βo phase transformation. Increasing the aging temperature accelerates the CR rate and extending the soaking time transforms more volume of α2/γ lamellae into pearlitic-like microstructure (PM). The PM exhibits almost negligible coarsening with the increase in aging temperature and extension of soaking time in the temperature between 950 and 1100 °C, indicating excellent thermal stability far beyond the service temperatures of TiAl intermetallics. Therefore, PM is a worthy pursuit in microstructural design for high-temperature application of TiAl alloy.

Microstructure evolution and precipitation behavior in a Y2O3-bearing TiAl alloy during creep

The paper studies the microstructure stability of a Y2O3-bearing Ti–48Al–2Cr–2Nb alloy during creep. Tensile creep tests were performed at 800–850 °C under 100 or 275 MPa. Compared to thermal exposure, the low creep stress promotes the microstructure degeneration of lamellar colonies, mainly including α2-lath dissolution, γ-lath coarsening and βo phase precipitation. The high creep stress further accelerates the α2 → γ transformation, but goes against the precipitation of βo grains inside α2 laths. Similarly, more Cr-rich precipitates with the composition of Ti(Al, Cr)2 are observed in creep samples, which are identified as the C14-Laves phase. It has a {0001}C14//{111}γ and <101¯0>C14//<11¯0>γ orientation relationship with the γ phase. The high imposed stress also facilitates the precipitation behavior. It should be noted that there are a few C14-Laves phases precipitating along the Y2O3 interface by heterogeneous nucleation. The high strain energy around Y2O3 particles, especially under the high creep stress, contributes to the precipitation of C14-Laves phase not only at the Y2O3 interface, but also in the γ matrix.

The microstructure evolution and phase transformation behavior of a β-solidifying γ-TiAl alloy during creep

The microstructural evolution and creep behavior of the Ti-43.5Al–4Nb–1Mo-0.1B alloy have been investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM). The excellent creep property was obtained with a fully lamellar (FL) microstructure containing the least grain boundary βo phase (GB-βo). TEM results revealed that after creep testing the α2 →βo phase transformation was observed in the FL microstructure. The formation βo phase is associated with the accumulation of Mo element, which is confirmed by the energy-dispersive X-ray spectroscopy (EDS). Moreover, the formation of βo precipitation in α2 lamellae effectively decreased the generation of dislocations in (α2/γ) lamellae, thereby improving the creep resistance. For the near gamma (NG) microstructure of the as-forged sample, a large number of dislocations and dislocation tangles were observed in the globular γ phase (γ-glob), which are considered to be the dominant creep mechanism. Moreover, the ellipsoidal ωo phase was observed in the GB-βo phase, accompanying with dislocations and sub-boundaries formation. In sum, the excellent creep property of the β-solidifying γ-TiAl alloy is attributed to the fine FL structure with a small amount of GB-βo phase and the formation of βo precipitation in (α2/γ) lamellae.

Phase stabilities and equilibria of the Ti-Al-Nb ternary system at intermediate temperatures III: The 1010 °C isothermal section and Scheil reaction map

This paper is a continuation of our recent publications Xu et al. (2022, 2023). We systematically characterise the diffusion multiples using scanning electron microscopy (SEM), electron probe microanalysis (EPMA), electron backscatter diffraction (EBSD) and transmission Kikuchi diffraction (TKD), and construct the whole composition range of the 1010 °C isothermal section in this work. There are nine stable phases, six three-phase regions and fifteen two-phase regions in the 1010 °C isothermal section. The βo phase is in contact with the β phase in the 1010 °C isothermal section. Two phase reactions occur between 1010 °C and 980 °C. The O phase is formed by the βo+ σ→O reaction at a temperature of 997.5 °C, and an invariant reaction of β + βo→α2 +O occurred at a little above 980 °C. Based on our prior phase diagram works and the present work, we finally constructed the Scheil reaction map from the liquid phase down to 900 °C. These phase diagram and reaction works are mandatory for TiAl based alloys’ design and application, and provides essential raw data to re-optimise the Gibbs free energy descriptions of this ternary system.

A high tensile strength above 900 ℃ in β-solidified TiAl alloy through alloy design and microstructure optimization

As a target for high-temperature applications, β-solidified TiAl alloys have attracted intense attention owing to their low density and superior mechanical properties at elevated temperatures. However, their microstructure instability and the remnant βo phase in β-solidified TiAl alloys result in the deterioration of their mechanical properties. In this work, a novel β-solidified TiAl alloy, Ti-44Al-5.5Nb-0.5W-0.5Cr-0.3Si-0.1C (at%), showing improved high-temperature mechanical properties, was proposed. Appropriate forging and heat treatments were applied to obtain excellent mechanical properties. Hot forging at the (α + β) phase region refined the lamellar colony significantly, and a heat treatment at 1270 °C removed the residual βo phase. Interestingly, undissolved γ laths contributed to the refinement of lamellar spacing. The optimized sample exhibited a nearly lamellar (NL) structure with a colony size of 38.2 µm and lamellar spacing of 29.6 nm, thus presenting a tensile strength of 520 MPa at 950 °C, indicating the present alloy could be used above 900 °C.

Phase stabilities and equilibria of the Ti-Al-Nb ternary system at intermediate temperatures II. The 980 °C isothermal section and reactions

This paper is a continuation of our recent publication [S. Xu, et al. Journal of Alloys and Compounds 900 (2022) 163399–163407]. We systematically characterized the diffusion multiples using scanning electron microscopy (SEM), electron probe microanalysis (EPMA), electron backscatter diffraction (EBSD), transmission Kikuchi diffraction (TKD) and transmission electron microscopy (TEM) and constructed the whole composition range of the 980 °C isothermal sections. The constructed range includes nine stable phases, twenty two-phase regions and ten three-phase regions at 980 °C. Six phase reactions occurring between 900 °C and 980 °C were determined. The ωo phase was formed by the βo+ γ→ωo reaction at a temperature just below 980 °C. The βo phase vanished through the βo→α2 + ωo+O reaction at a temperature just above 900 °C. These experimental results will bridge the existing research gap by providing the missing thermodynamic data. We believe that accurate predictions of phase diagrams and reactions occurring at intermediate temperatures using the obtained data are now feasible and the recomputations will resolve previous inconsistences and disputes.

A TEM study of a dislocations in the ßo phase of an intermetallic TNM-TiAl alloy

The structure and mobility of a<001> dislocations in the βo phase of an intermetallic TNM-TiAl alloy produced by Spark Plasma Sintering were investigated by transmission electron microscopy. We find that these dislocations are dissociated into two identical superpartial dislocations separated by a stacking fault. Furthermore, a<001> dislocations are elongated along their screw orientation and anchored at small pining points assumed to be nano-sized precipitates of the ωo phase. In situ room temperature straining experiments show that dislocation propagation occurs through jumps between these elongated configurations.

Inhibition of ωo phase precipitation in TNM-based TiAl alloys by Cr and Mn

In modern β-solidified γ-TiAl alloys, ωo phase usually precipitates very easily within βo phase during heat treatment at intermediate temperatures. This phase transformation leads to increased brittleness and reduced service performance of the alloy. In this paper, the evolution of the ωo phase is investigated in the TNM-based TiAl alloys by the addition of Mn or Cr elements. The results show that both Mn and Cr have a strong inhibiting effect on the precipitation of ωo phase, while the inhibiting effect with Mn addition is more obvious. Specifically, in the Ti-43.5Al–4Nb-0.6Mn-0.1B alloy exposed to 750 °C for 1000 h, the ωo phase is almost completely inhibited. The composition range in which Mn and Cr effectively suppressed the ωo phase precipitation were determined by reasonable modulation. The phase transformation temperature of ωo phase was lowered by the addition of alloying elements, making it difficult to precipitate at intermediate temperatures. The corresponding phase transformation mechanisms were discussed.

The microstructure evolution and tensile properties of Ti–43Al–4Nb–1Mo-0.2B alloy during hot rolling

Ti–43Al–4Nb–1Mo-0.2B (at.%) (TNM) alloy sheets were fabricated, and the corresponding microstructure developed from lamellar to duplex structure with the rolling reduction increased from 50% to 72.5%. Interestingly, a novel Z-type lamellar structure developed at 67.5% deformation strain, and the formation process was discussed by calculating the Schmid factor. Moreover, α2+γ→βo phase transformation occurred at the kink band boundaries. Subsequently, as the reduction increased to 72.5%, the coarse βo phase decomposed and transformed to α2+γ phases due to the distortion energy stored after multi-pass rolling. Finally, the room-temperature tensile properties of the TNM sheets were tested. With the comparison of the as-forged alloy, the strength and ductility increased significantly by hot rolling, and the ultimate tensile strength reached 880 MPa at 60% total reduction. The relationship between the rolling process, microstructure evolution, and corresponding properties was established.

Decomposition and phase transformation mechanisms of α2 lamellae in β-solidified γ-TiAl alloys

The decomposition of (α2/γ) lamellae during service is an important microstructure degradation phenomenon that is worth studying. In this study, the decomposition mechanism of (α2/γ) lamellae was systematically investigated by transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) techniques. The results showed facilitated α2 →βo phase transformation by [112¯0]α2/[01¯1]γ semi-coherent α2 /γ interface due to its larger misfit than that of [112¯0]α2/[1¯10]γ interface. Ellipsoidal ωo particles were formed at βo /α2 interface and nucleated at the interface dislocations. The energy-dispersive X-ray spectroscopy (EDS) results demonstrated feasible depletion of Mo and enrichment of Ti at α2 /βo interface to form ωo phase. In addition, lath-shaped ωo precipitates were observed between two precipitated large βo grains formed by direct α2 →ωo phase transformation. The formation mechanism of the lath ωo phase was attributed to both element segregation and strain accommodation caused by βo precipitate in α2 lamellae. The α2 /βo interface served as preferential nucleation sites for ωo phase due to the atomic structure of the interface, which crystallographically was closely associated with the structure of ωo phase. Moreover, βo precipitates at low stress introduced stacking faults in α2 lamellae, while delaying the parallel decomposition of α2 lamellae at high stress. In sum, these findings provided profound insights into the decomposition mechanism of (α2/γ) lamellae, relevant as theoretical basis for controlling the microstructures and improving the performance of β-solidified γ-TiAl alloys.

In- and ex-situ study of the deformation behavior of the βo(ωo) phase in a Ti4Al3Nb alloy during high-temperature compression

In-situ synchrotron-based high energy X-ray diffraction (HEXRD) and ex-situ Gleeble tests were conducted to investigate the deformation behavior of the β o (ω o ) phase in a Ti 4 Al 3 Nb alloy. In samples compressed at 600 and 800 °C, the brittle ω o phase deforms mostly elastically, resulting in the failure of the Ti 4 Al 3 Nb alloy by premature fracture. In a sample deformed at 900 °C, the ω o phase mostly transforms into the β o phase under uniaxial loading. Moreover, the ductility of a Ti 4 Al 3 Nb alloy is largely enhanced at this temperature. In a sample deformed at 1000 °C, dynamic recrystallization (DRX) of the β o phase extensively takes place. Direction 1 (D1) and Direction 2 (D2) deviating from the loading direction with an angle of 65° ± 5° and 15° ± 5° are selected to analyze the lattice strain evolution of (110) βo lattice planes. At the late stage of macro strain hardening, deformed<110> βo //D1 oriented grains bear a higher load due to the occurrence of DRX in<110> βo //D2 oriented grains. Subsequently, DRX continues in<110> βo //D1 oriented β o grains. The coordinated deformation of β o grains ensures the good deformability of a Ti 4 Al 3 Nb alloy at 1000 °C. • Cleavage fracture occurred for samples deformed below 800 °C due to the brittle nature of the ω o phase. • Reversal stress-induced ω o →β o transformation was observed at high applied stress. • Loads transfer among differently oriented β o grains was revealed.

Plasticity and brittleness of the ordered βo phase in a TNM-TiAl alloy

In order to identify the reasons for the brittleness of a TNM-TiAl alloy (Ti51.05Al43.9Nb4Mo0.95B0.1 in at.%), the plastic deformation behaviour of the ordered βo phase is investigated. The corresponding powder was densified by Spark Plasma Sintering to produce a near lamellar microstructure made of γ/α2 lamellar colonies surrounded by γ and βo grains. Then, the room temperature tensile behaviour is studied by carrying out tensile tests and additional in-situ straining experiments in a transmission electron microscope (TEM). The TNM alloy exhibits a limited ductility, with the fracture surface of test specimens showing cleavage facets along lamellar interfaces. In addition, the βo grains contain nano-precipitates of the ωo phase and deform plastically by <111> superdislocations dissociated into two superpartial dislocations separated by an antiphase boundary. These dislocations glide in {011} planes and are observed to be localized into pile-ups, which is related to the ωo strengthening precipitation within the βo grains. This localised hardening behaviour leads to stress concentration in grain boundaries which is assumed to be responsible for delamination along lamellar interfaces in neighbouring lamellar colonies.

Thermomechanical instability and deformation behavior of βo(ω) phase region in a Ti-43Al-8Nb-0.2W-0.2B alloy under high-temperature rotary-bending fatigue

Phase transformation and deformation behavior of the βo(ω) phase region under high-temperature rotary-bending (HTRB) fatigue condition are studied in the current work. Rotary-bending fatigue tests of a Ti-43Al-8Nb-0.2W-0.2B (at.%) alloy were performed at 800 °C under 325, 340, 355 and 370 MPa in atmosphere condition. The quasi-fatigue limit (σ-1) was calculated as 355 MPa [4]. Microstructures were examined using scanning and transmission electron microscopy (SEM &TEM) and thermodynamic calculation was adopt to interpret the phase transition mechanisms. Under HTRB fatigue, the phase transition of ωo → βo occurred and the newly formed βo phase is completely coherent with the pre-existing βo phase. Thermodynamic calculation revealed that a tensile stress could promote the phase transition of βo → ωo. The local recrystallization was found at lamellar colony boundary and grain interior, where is consisted of γ and ω grains. The deformation mechanisms of βo phase might be the dislocation glide on the 〈111〉{123} system. The dislocations have a strong tendency for slip localization into slip bands. Disordered ω phase was observed in the residual ωo phase and it may be a transition state in the phase transformation of ωo → βo.

Structural stability and mechanical properties of TiAl+Mo alloys: A comprehensive ab initio study

Titanium aluminides are technologically important intermetallic alloys with also many curious properties interesting from a basic research point of view. When alloyed with Mo, several (meta)stable phases have been reported; their properties are, however, largely unknown due to the alloy processing (e.g. fast cooling) and/or non-existence as a single-phase material. Here we employ first principles calculations to study compositional trends in structural and mechanical properties. We could show that Mo increases the density of all studied phases, leads to their chemical destabilization with the exception of the ordered bcc βo phase, increases their ductility, and enhances the elastic anisotropy. Discrepancies between two employed ab initio methods (special quasi-random structures vs. coherent potential approximation) in the case of the βo and B19 phases are rationalized with significant local distortions which may eventually facilitate a spontaneous phase transformation. Predictions of ordering temperatures solely based on the configurational entropy do not yield values in the experimentally expected ranges.