Β-solidifying γ-TiAl Alloy 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.

Effects of hatch spacing on densification, microstructural and mechanical properties of β-solidifying γ-TiAl alloy fabricated by laser powder bed fusion

β-solidifying γ‑titanium aluminide (γ-TiAl) alloys, which typically contain β-phase stabilizers such as Nb and Cr, hold great promise for the aerospace industry and can potentially be processed through additive manufacturing to realize performance unachievable using conventional methods. However, the effects of laser powder bed fusion (L-PBF) parameters on the characteristics of the thus obtained samples remain underexplored. To address this gap, we herein examined the effects of hatch spacing on the densification, microstructural, and mechanical properties of L-PBF-fabricated Ti-44Al-6Nb-1.2Cr (at.%) alloy samples, revealing that the strong influence on thermal history induced the variation in densification and formation of two microstructure types. 0.01 mm hatch spacing resulted in repetitive slow cooling and sufficiently remelting, thus suppressing crack formation, promoting high densification, and inducing a complex phase transformation involving the formation of a basket-weave-structured α2 phase and the 〈001〉 alignment of the β phase along the build direction. 0.06 mm hatch spacing resulted in rapid cooling and insufficient heat accumulation, favoring the massive phase transformation, a jagged morphology α2 phase with a randomly distributed crystallographic texture. Hardness was mainly correlated with phase constitution and volume fraction, whereas compressive properties were jointly determined by additional effects of multiple factors such as grain size and crystallographic texture. This work provides the fundamental insights required to suppress defect formation in β-solidifying γ-TiAl alloys and tailor their microstructure for mechanical property enhancement.

Influence of heat treatment on microstructure of a new β-solidifying γ-TiAl alloy

Influence of heat treatment on microstructure of a new β-solidifying γ-TiAl alloy

Effect of Cooling Rate on Powder Characteristics and Microstructural Evolution of Gas Atomized β-Solidifying γ-TiAl Alloy Powder

Effect of Cooling Rate on Powder Characteristics and Microstructural Evolution of Gas Atomized β-Solidifying γ-TiAl Alloy Powder

Microstructure and Crystallographic Texture Evolution of β-Solidifying γ-TiAl Alloy During Single- and Multi-track Exposure via Laser Powder Bed Fusion

The microstructural evolution and crystallographic texture formation of β-solidifying Ti-44Al-6Nb-1.2Cr alloy were identified under single- and multi-track exposures via laser powder bed fusion (L-PBF) for various process parameters. Under single-track exposure, the microstructure of the melt pool was divided into the band-like α2 phase in the melt pool boundary and β phase in the melt pool center. Numerical and thermodynamic simulations revealed that the underlying mechanism of phase separation was related to the variation in the cooling rate in the melt pool, whereas microsegregation induced a shift in the solidification path. Meanwhile, the crystallographic texture of the α2 phase region was identical to that of the substrate owing to the epitaxial growth of the β phase and subsequent α phase nucleation. In contrast, the β phase exhibited a ± 45° inclined <100> alignment in the melt pool, which was tilted to align along the build direction toward the center of the melt pool corresponding to the simulated thermal gradient direction. Furthermore, the narrow hatch space condition maintained the crystallographic texture to the subsequent scan, forming a continuous band-like α2 phase with a strong selection. However, the crystallographic texture in a wide hatch space condition manifested a random distribution and constituted a fine mixture of the β and α2 phases. For the first time, these results will offer an understanding of an anisotropic microstructure control via the L-PBF process and ensure the tailoring of the mechanical properties in the β-solidifying γ-TiAl-based alloys by approaching hatch spacing control.Graphic

Microstructure Evolution of Gas-Atomized β-Solidifying γ-TiAl Alloy Powder during Subsequent Heat Treatment

To promote the use of γ-TiAl alloys in various domains, such as the aerospace industry, it is pivotal to investigate the unusual phase transformation from rapidly solidified and metastable γ-TiAl toward the equilibrium state. In this study, the microstructure characteristics of gas-atomized β-solidifying Ti-44Al-6Nb-1.2Cr alloy powder, in terms of the effect of rapid solidification on microstructure evolution, were explored in comparison with cast materials. The phase constitution, morphology, and crystallographic orientation between phases were noted to be distinct. Furthermore, subsequent heat treatment was conducted at different temperatures using gas-atomized powder. The transition from the metastable to equilibrium state was observed, wherein firstly, the γ phase precipitated from the retained α2 phase, forming an α2/γ lamellar microstructure. In intensified heat-treatment conditions adequate for cellular reaction, β/γ cells were formed at the grain boundaries of α2/γ lamellar colonies. The findings highlight the overall phase transformation during rapid solidification and continuous microstructural evolution from the nonequilibrium to the equilibrium state. This research can bridge the gap in understanding the effect of the solidification rate on microstructural evolution and contribute to enhanced comprehension of the microstructure in other domains involving rapid solidification, such as the additive manufacturing of γ-TiAl alloys.

Effects of cooling rate on borides morphology and structure in cast β-solidifying γ-TiAl alloy

Borides have been found to play a significant role in determining the mechanical properties of cast TiAl alloys. In order to investigate the influence of cooling rate on the morphology and structure of borides, SEM and HRTEM techniques were utilized to analyze borides in Ti-43Al-4Nb-1Mo-0.5B (at. %) cast plates with varying thicknesses. A higher cooling rate was observed to result in the formation of elongated, curvy borides with high aspect ratios, increased planar faults, and complex structures consisting of a combination of B2 phase and various boride phases. Conversely, a slower cooling rate yielded shorter bar-shaped borides with fewer planar faults. This variation in boride morphology can be attributed to the differing thicknesses of the solute-rich layer, where the eutectic reaction occurs, at the solidification front during fast and slow cooling processes. Bf-TiB with a habit plane of (010) was identified as the dominant phase at all cooling rates. Borides in the [100] and [001] directions demonstrated a faster growth rate compared to those in the [010] direction. Additionally, HRTEM results indicated the presence of TiB2 while Ti3B4 was absent. This can be explained by the limited phase region of Ti3B4 in both the binary Ti-B and ternary Ti-Al-B phase diagrams, which is the smallest among all phases. These findings enhance the understanding of how cooling rate influences the morphology and structure of borides in TiAl alloys, thus providing insights for optimizing alloy design and manufacturing processes.

Influence of upset forging and heat treatment on the microstructure and mechanical properties of a new β-solidifying γ-TiAl alloy

Our previous works revealed that alloying of a β-solidifying intermetallic γ-TiAl alloy with Zr and Hf instead of Nb led to improvement in the strength and creep resistance due to effective solid solution hardening, a strong partitioning preference γ>α2 for Zr and a lower lattice misfit of the γ-TiAl and α2-Ti3Al phases. On this basis, a new β-solidifying γ-TiAl alloy with a composition Ti-44Al-X(Nb,Zr,Hf)-0.15B (at.%) (designated as TNZ γ-alloy) has been designed. The as-cast ingot had a fine lamellar structure with a small amount of the metastable β(βo) phase. Compression tests performed at T = 950–1200 °C showed that the new alloy possessed good forgeability even at 950 °C without signs of strain localization. The alloy ingot was subjected to upset forging at T = 950 °C followed by heat treatments. The applied processing led to formation of refined duplex and lamellar type structures almost free of the β(βo) phase. The obtained microstructural conditions were examined and then the tensile and creep tests were performed. The mechanical tests showed that with increasing the lamellar constituent and decreasing the lamellar spacing the creep resistance expectedly increased and ductility decreased. The obtained results are discussed from the viewpoint of processability, mechanical properties and oxidation resistance of the new alloy.

Thermomechanical characterization and conventional rolling of a novel β-solidifying γ-TiAl alloy with excellent comprehensive properties

The development of the β-solidifying γ-TiAl alloys with good oxidation resistance and thermal deformation ability has always been crucial for the practical application. However, increasing the content of the antioxidant elements, such as Nb, Al, etc., tends to be detrimental to the deformability of the alloys. In this work, a novel Nb, Mn containing β-solidifying TiAl alloy with the nominal composition of Ti-43Al-1.5Mn-3Nb-0.2C-0.2Si-0.1B (at. %) (named as NbMn-TiAl) is invented, which goes through the single β phase as well as the α phase during solidification, resulting in the final as-cast microstructure of nearly full lamellar structure (NFL). Subsequently, the isothermal compression tests were performed within a temperature range of 1100 °C–1300 °C and a strain rate range of 0.001 s−1-10 s−1, the hot processing maps and constitutive equation were established. Moreover, the deformation mechanisms were clarified, including the bending/kinking of the α2/γ lamellar colonies, the dynamic recrystallization (DRX) of γ phase and α phase at different temperatures. Rolled bars with a diameter of 12 mm were then directly fabricated from the ingot by conventional hot rolling process. The as-rolled bars exhibited excellent oxidation resistance and mechanical properties that outperform many available TiAl alloys manufactured using various methods.

Defect elimination and microstructure improvement of laser powder bed fusion β-solidifying γ-TiAl alloys via circular beam oscillation technology

To eliminate metallurgical defects and improve microstructures in laser powder bed fusion (LPBF) β-solidifying γ-TiAl alloys, circular beam oscillation technology was used to fabricate a Ti–43Al–9V-0.5Y alloy in this work. The metallurgical defects and microstructures under conventional linear (non-oscillating) scanning and circular oscillating scanning are first studied in comparison. Then, the microstructural evolution and defect suppression mechanisms under circular oscillating scanning are revealed based on the melt flow behavior. In contrast to the non-oscillating scanning, circular oscillating scanning can better eliminate the pores and cracks of LPBFed Ti–43Al–9V-0.5Y alloy. As the oscillating velocity is decreased, the crack density decreases and the microstructure changes from columnar to equiaxed grains. The crack-free samples with a relative density of 99.98% and fine equiaxed grains are obtained as the oscillating velocity is less than or equal to 100 mm/s. The microhardness (473–581 Hv) of the non-oscillating samples is higher than that (440–487 Hv) of the oscillating samples. The room-temperature compressive strengths (∼1222 MPa and ∼1931 MPa) and tensile strength (∼253 MPa) of the crack-free oscillating LPBFed samples are superior to those of the as-cast counterparts. The suppression of pores is ascribed to the improved keyhole stability, enhanced melt convection as well as escape of bubbles under circular beam oscillation. When the oscillating velocity is less than or equal to 100 mm/s, the sufficient stirring effect of the laser beam promotes the formation of fine equiaxed grains and the reduction of brittle B2 phase content, which effectively inhibits cracking.

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.

The effect of alloying and fluorination on the oxidation behavior of β-solidifying γ-TiAl based alloys

The present work is devoted to study of the oxidation behavior of two β-solidifying γ-TiAl alloys (Ti-43.5Al-4Nb-1Mo-0.1B (TNM alloy) and Ti-44Al-6(Nb, Zr, Hf)-0.15B (TNZ alloy) (at.%)). The as-cast alloys were subjected to upset forging and heat treatment that resulted in similar microstructures in both alloys. Plate-shaped samples were cut from the obtained workpieces, mechanically polished and subjected to oxidation exposure at 800°C (500 h). The samples during annealing were periodically removed from the furnace and weighed. After oxidation exposure the mass gain of the TNZ sample was found appreciably smaller than that of the TNM sample. The preliminary fluorination treatment in a diluted hydrofluoric acid (HF) provided a noticeable increase of the oxidation resistance in the case of the TNM alloy and a significant worsening of the oxidation resistance in the case of the TNZ alloy. At the same time, the non-fluorinated sample of the TNZ alloy showed near the same oxidation resistance as the TNM samples subjected to preliminary fluorination treatment. EDS analysis revealed the competitive formation of aluminum and titanium oxides on the surfaces of the oxidized TNM and TNZ samples. Predomination of the alumina formation contributed to higher oxidation resistance.

Microstructural evolution and embrittlement of a β-solidifying γ-TiAl alloy during exposure at 700 °C

The microstructural evolution and the room-temperature tensile properties of fully lamellar Ti-43.5Al–4Nb–1Mo-0.5B alloy exposed at 700 °C for 500 h in air were investigated. In bulk microstructure, the precipitation and growth of ellipsoidal ω0 particles were found in equiaxed β0 grains, and the parallel decomposition of coarse α2 lamellae into fine α2 + γ lamellar packets was observed. The α2 lamellae near the surface decomposed first into fine α2 + γ lamellar packets and then blocky γ grains after the full consumption of the α2 phase. A mixed oxide scale with obvious stratification mainly composed of TiO2 and Al2O3 formed on the surface of the specimens. The outward diffusion of Ti atoms on the specimen surface led to the formation of an Al-rich and Ti-lean blocky γ zone between the matrix and the oxide scale. The degradation of room-temperature mechanical properties of the alloy after exposure was primarily caused by the evolution of subsurface microstructure, and the evolution of bulk microstructure had little effect on the mechanical properties at this stage. Under axial stress, microcracks nucleated from the tip of a V-shaped “notch microstructure” at the interface between the blocky γ zone and the matrix, causing premature failure. Removal of the surface layer of the thermally exposed alloy led to room temperature mechanical properties essentially similar to those before thermal exposure.

Tailored Fully Lamellar Microstructure of a Newly Developed Mn-Containing β-Solidifying γ-TiAl Alloys Rolled Bar

Tailored Fully Lamellar Microstructure of a Newly Developed Mn-Containing β-Solidifying γ-TiAl Alloys Rolled Bar

The Influence of Nb, Zr, and Zr + Hf on the Lattice Parameters and Creep Behavior of β-Solidifying γ-TiAl Alloys

The Influence of Nb, Zr, and Zr + Hf on the Lattice Parameters and Creep Behavior of β-Solidifying γ-TiAl Alloys

Влияние Nb, Zr и Zr+Hf на параметры решеток интерметаллидных фаз и сопротивление ползучести γ-TiAl сплавов на основе Ti-44Al-0.2B

X-Ray diffraction (XRD) analysis was used to study the influence of alloying with 5 at.% Nb, 5 at.% Zr and 5 at.% (Zr+Hf) on the lattice parameters of the γ(TiAl) and α2(Ti3Al) phase in the intermetallic alloys based on Ti-44Al-0.2B (at.%). Before XRD analysis duplex structures with near the same microstructural parameters were obtained in samples of the alloys. The XRD data were used to calculate the tetragonal distortion (cγ / aγ ratio) of the γ phase, the cα2 / aα2 ratio of the α2 phase and the γ / α2 lattice misfits of the alloys. The highest tetragonal distortion (cγ / aγ) of the γ unit cell (cγ / aγ =1.0124) is observed for the base Ti-44Al-0.2B alloy, followed by the Nb-, (Zr+Hf)- and Zr-containing alloy (cγ / aγ =1.0116, 1.0075 and 1.0069, respectively). The cα2 / γα2 ratio is insignificantly changed depending on alloying. Doping with Zr and Zr+Hf leads to a noticeable decrease in the γ / α2 lattice misfits as compared with the alloy doped with Nb and the base alloy. For instance the γ / α2 lattice misfits determined in both crystallographic directions of the γ phase in the Ti-44Al-5Zr-0.2B and Ti-44Al-5Nb-0.2B alloys were found to be ε110 / ε101= 0.93 / 0.59 and ε110 / ε101=1.38 / 0.79, respectively. It has been recently revealed that γ-TiAl alloys with near the same duplex structures based on Ti-44Al-0.2B and doped with Zr and Zr+Hf demonstrated appreciably higher creep resistance than the alloy doped with Nb and the base alloy. It is assumed that the lower cγ / aγ ratios obtained for the Zr- and (Zr+Hf)-containing alloys contribute to the reduction of creep resistance. The fact that the Zr- and (Zr+Hf)-containing alloys showed higher creep resistance than the Nb-containing alloy should be mostly attributed to the lower γ / α2 lattice misfits in the Zr- and (Zr+Hf)-containing alloys and the higher solution hardening caused by doping with Zr and Hf. Therefore, the impact of alloying on the creep and heat resistance in β-solidifying γ-TiAl alloys should be considered taking into account the changes of the lattice parameters of the γ(TiAl) and α2(Ti3Al) phase, which influence the physical processes determining the creep behavior.

Quantitative characterization of β-solidifying γ-TiAl alloy with duplex structure

For precise plastic deformation, microstructure control is essential especially for β-solidifying γ-TiAl alloy with duplex structure. Based on stereology, the microstructure of isothermally compressed γ-TiAl alloy was divided into β0 grains, remnant α2/γ lamellar colonies, α2 and γ grains. The results show that the volume fraction of β0 grains slightly increases in the isothermally compressed γ-TiAl alloy with the increase of height reduction. Meanwhile, the volume fractions of remnant α2/γ lamellar colonies and α2 grains decrease. However, the volume fraction of γ grains increases from 64.39% to 78.47%. According to the quantitative results, the α→γ phase transformation was investigated in-depth, and it is found that isothermal compression accelerates the α→γ phase transformation. The first α→γ phase transformation is similar to ledge-controlled transformation, through which remnant α2/γ lamellar colonies finally convert into γ grains in isothermal compression. The second is achieved by α/γ phase interface immigration.

Achievement of forging without canning for β-solidifying γ-TiAl alloy containing high content of niobium

ABSTRACT The processes of forging without canning were optimized, such that forging could be successfully performed and forged pancakes without cracks could be obtained. The maximum height reduction could reach 80%. The as-forged microstructure consisted of γ and β/B2 phases. The volume fraction of the β/B2 phase rose slightly compared to the pre-forging state. Near the top or bottom of the pancake, residual lamellae were found, while full dynamic recrystallization was completed at the center and lateral parts. Equiaxed γ grains illustrated that dynamic recrystallization mainly occurred in the γ phase, while the softer β phase coordinated the deformation by adjusting morphology. Two types of Σ3 boundaries, [110] 70° and [111] 60°, were found in the γ phase. At room temperature, the fracture was brittle, and the elongation was 0.07%. With increasing temperature, the yield strength and ultimate strength decreased, while the elongation increased. The brittle–ductile transition temperature was between 800°C and 850°C.

Effect of Nb, Zr and Zr + Hf on the microstructure and mechanical properties of β-solidifying γ-TiAl alloys

The present work has been devoted to study of β-solidifying γ-TiAl alloys based on Ti–44Al-0.2B and doped with Nb, Zr and Zr + Hf. The phase transformation sequences were established for the alloys. On this basis, upset forging followed by heat treatment was performed that resulted in formation of near the same duplex type microstructures in the alloys. Tensile tests in the temperature range of 20–900 °C and creep tests at 700 °C were carried out for the alloys in the duplex conditions. The alloys doped with Zr and Zr + Hf showed appreciably higher strength, higher temperatures of the brittle-ductile transition (BDT) and enhanced creep resistance while retaining near the same ductility below the BDT temperatures as compared to the Nb-containing alloy. Enhanced creep resistance and higher temperatures of the BDT in the alloys doped with Zr and Zr + Hf in contrast to the Nb-containing alloy were attributed to higher solid solution hardening due to larger atomic radii of Zr and Hf versus Nb and to different partitioning behaviors of Zr and Zr + Hf versus Nb leading to higher concentrations in the γ phase of Zr and Zr + Hf in the Zr- and (Zr + Hf)-containing alloys in contrast to that of Nb in the Nb-containing alloy. Both of these factors contributed to lower diffusivity (especially in γ grains) in the alloys doped with Zr and Zr + Hf. In particular, alloying with Zr and Zr + Hf shifted the development of dynamic recrystallization processes towards high temperatures that slowed down the increase of ductility with increasing the test temperature in the BDT range and led to higher BDT temperatures.

Twinning and twin intersections in γ grains of Ti-42.9Al-4.6Nb-2Cr

A nanoscale observation utilizing the transmission electron microscopy (TEM) device was presented to investigate the hot deformation behavior of β-solidifying γ-TiAl alloy (Ti-42.9Al-4.6Nb-2Cr, at.%) with duplex structure, and elucidate twinning and twin intersections in γ grains. Results showed that twinning is tightly linked to twin intersections in γ grains. At the initial stage of isothermal compression, faults in two directions with the acute angle of ∼70° intensively formed in γ grains. With the proceed of isothermal compression, faults paralleling to each other gradually evolved into twins in the lateral direction, which is helpful to accommodate the atomic plane misfit. And dislocations piled-up advantageously promote longitude twinning in γ grains. Once faults in all directions evolved into twins, various twin intersections phenomena would be observed in γ grains.