Α2 Lamellae Research Articles

In-situ synchrotron high energy X-ray diffraction study on the compressive creep behavior of an extruded Ti-45Al-8Nb-0.2C alloy

Wrought high Nb containing (high Nb-TiAl) alloys are potential materials for low pressure turbine blades in aero-engines. The performance and microstructure evolution under high temperature creep condition is of importance to the service stability of these materials. In this study, the internal strain and FWHM evolution of an extruded TNB-based high Nb-TiAl alloy during compressive creep are characterized by in-situ high energy X-ray diffraction (HEXRD) technique for the first time. The compressive creep test was conducted under a constant load of 300 MPa at 900 °C for 10 h in vacuum. The final creep strain is approximately 1.72 %. The microstructure and phase constitution after creep shows little difference from that before creep. Lattice strain analysis shows that γ phase is plastically deformed while the α2 phase deforms elastically due to the low creep strain. The lattice strain of α2 grains is dependent upon the deformation of surrounding γ grains. Creep induces dynamic recovery, exerting a softening effect. A high number of dislocations are visible in the γ lamellae while almost no dislocations exist within the α2 lamellae. α2 lamellae are partially decomposed and refined via the α2→γ transformation.

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.

Twins and β0 precipitation effects on the lamellar degradation in TNM alloy during creep

In this paper, the microstructure instability of the TNM alloy during creep at 800 °C under 150 MPa and 250 MPa was investigated. The results indicate that, both β0 precipitation (inside lamellae) and primary γ twins formation are initiated during the second stage of creep. Compared to low creep stress conditions (150 MPa), high creep stress conditions (250 MPa) accelerate the initiation of the third stage of creep, inducing the precipitation of ω0 phase and secondary twins. The nanoscale ω0 particles formed inside β0 or at the interface of the β0/β0. The intersection of secondary γ twins and α2 lamellae leads to stress concentration and dislocation accumulation, providing nucleation points for β0 phase and promoting the precipitation of β0 within α2 lamellae. The precipitation of the secondary γ twin introduces a unit dislocation of 1/3 [111]γ in the (111) atomic planes, promoting the movement of adjacent atomic planes within the (111) atomic planes. The 1/3 [111]γ dislocations will dissociate into two partial dislocations via 1/3 [111]γ→1/4 [11 1‾]γT +1/9 [0 1‾1]γM. The adjacent atomic planes of the γ matrix and secondary γ twin can transformation to β0 through shear along 1/9 [0 1‾1]γM and 1/4 [11 1‾]γT, respectively. The size and content of β0 phase significantly increase in the third stage of high creep stress, accelerating the decomposition and fragmentation of the α2 lamellae, leading to rapid creep failure.

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.

Origin of the enhanced creep properties of high Nb containing TiAl alloys with the addition of C and Y2O3

In this paper, Ti–43Al–6Nb–1Mo–1Cr, Ti–43Al–6Nb–1Mo–1Cr-0.5C and Ti–43Al–6Nb–1Mo–1Cr-0.5C-0.05Y2O3 alloys were prepared by induction skull melting under argon atmosphere. Creep tests were carried out on three alloys to reveal the effects of C and Y2O3 on the creep properties of alloys and to clarify the precipitation behavior of Ti2AlC and B2 phases. The results show that adding C and Y2O3 can significantly reduce the steady-state creep rate and improve the creep properties of the alloy. After adding C and Y2O3, the parallel decomposition of α2 lamellae occurs, and the fine γ lamellae precipitates from coarse α2 lamellae. The lamellar degradation occurs obviously during creep, and the reaction is α2→γ+B2. The B2 phase preferentially grows into α2 lamellae after nucleation at the lamellar interface. Furthermore, the precipitated B2 phase can induce the formation of stacking faults in α2 lamellae. The strengthening mechanism of C is mainly solid solution strengthening and the precipitation hardening. The dynamically precipitated Ti2AlC during creep can hinder the movement of dislocations and improve the bonding strength of the lamellar interface. Y2O3 mainly plays a role of second phase strengthening, which can share the stress of the matrix and prevent the crack propagation. Finally, the voids nucleation and crack propagation process of the alloy are analyzed and summarized.

Investigation on the effect of electroshock treatment on the microstructure and mechanical properties of Ti–45Al–5Nb alloy

This study investigated the effects of electroshock treatment (EST) on the microstructure and mechanical properties of Ti–45Al–5Nb alloy. Microstructure results showed that no new phase was generated after EST, the peak positions of α2 phase and γ phase were shifted, the relative intensity of the diffraction peaks were enhanced, and the grain orientations were varied. The original relatively coarse α2 lamellae decomposed into fine γ+α2 lamellae in a direction parallel to the original γ/α2 lamellae. The segregation of Nb element was weakened and the distribution was more uniform. Texture research showed that after EST with 60 ms and 80 ms, the maximum relative texture intensity of γ phase was enhanced. After EST with 100 ms, the maximum relative texture intensity of γ phase weakened to 17.86, and the grain orientation became dispersed. The texture distribution was more uniform. The maximum relative texture intensity of α2 phase was weakened and the orientation was dispersed after EST. Mechanical property results showed that after EST, the yield strength and fracture strain of the material was improved. The main reason is that the precipitation of γ phase forms fine γ/α2 lamellae. All results indicate that EST provides a new way to manipulate the microstructure and properties of TiAl alloys.

Regulating phase ratios and mechanical properties of polysynthetic twinned TiAl single crystals via annealing

Polysynthetic twinned (PST) TiAl single crystal specifically refers to a fully lamellar TiAl single crystal with parallel phase interfaces and twin interfaces grown by directional solidification. In this paper, PST single crystals with different phase ratios are obtained by annealing at specific temperatures and holding times. The results show that the diffusion rates of Ti and Al elements at various temperatures directly trigger and propel the surface recrystallization and variation in the internal phase ratio. When the temperature is lower than 1448 K, the diffusion rate of Ti is obviously higher than that of Al, which causes one dense α2 recrystallized layer to form on the surface of TiAl single crystals. Meanwhile, as more Ti elements migrate to the surface, the α2 phase ratio inside the TiAl single crystal thereby decreases. When the temperature exceeds 1448 K, the diffusion rate of Al gradually reverses to exceed that of Ti, which forms the surface sandwiched recrystallization dominated by γ phase and simultaneously increases α2 phase ratio inside the TiAl single crystal. The variation in the two-phase ratio directly induces a significant change in the lamellae thickness, which exhibits different tensile behaviors of PST-TiAl single crystal. When the α2 phase content is less than 20 %, wider γ lamellae make it easier for dislocations to be activated within its lamellae and continuously move across the γ/α2 interfaces, thereby obtaining better tensile plasticity. As the α2 phase content exceeds 30 %, finer γ lamellae inhibit the dislocation initiation, resulting in the fracture occurrence of TiAl single crystal before yielding. No matter how the phase ratio changes, the crack preferentially initiates within α2 lamellae. However, the crack propagation follows different paths based on various γ lamella thicknesses. The fracture mode of PST-TiAl single crystal also changes from shear fracture along slip bands within the γ lamella to brittle fracture along the {11¯00} planes within α2 lamella.

New insights into lamellar and twinning transformations in TiAl/Nb composites with core-shell structure during high temperature annealing

Lamellar and twinning transformations play a crucial role in stabilizing the ordered phases in TiAl based composites for property improvement. In this work, we have systematically investigated the lamellar and twinning transformations in TiAl/Nb composites with core-shell structure and proposed novel insights on γ → α2 and γ → γT processes from an atomistic perspective. The results show that high temperature annealing facilitates the formation of lamellar microstructure including γ + α2 and γT twin lamellae in the vicinity of core/shell interface, due to rapid diffusion of principal atoms followed by atomic shuffling. Specifically, plenty of lenticular γT can nucleate in B2 regime by accidental misalignment of atoms on a 110B2 plane adhering to Kurdjumov-Sachs relationship, and then grow even into α2 lath driven by the release of internal stress via Shockley partial slips on the 111γ plane. The formation of γT mediated by 9R involves γ → 9R and γ ← 9R → γT transformations. Among them, a full period of atomic stacking …ABC|BCA|CAB… (LPSO) can be achieved through three Shockley partials being sequentially activated on the C, A and B atomic layers in the stacking of …ABC|ABC|ABC… (L10) with the total shift of 3δA. Then 9R will change synchronously to γ and γT once the migrating Shockley partials react with Frank partials to yield a full dislocation, i.e., δA + Dδ→DA and δA + δC + δD→BD, which can also be considered as detwinning and twinning behaviors, respectively. Additionally, the transition of bulk γ to α2 or γT lamellae is closely associated with the stacking faults, i.e. whether the activated atomic layer is located at the boundary or periphery of the updated stacking fault region.

Tailoring lamellar orientation and tensile properties of TNM alloy via extrusion

The rectangular billet of TNM alloy with oriented lamellar microstructure was obtained by extrusion above its α transus temperature. The as-extruded TNM alloy exhibits a nearly lamellar microstructure with an average colony size of approximately 60 μm, and the proportion of lamellar colonies with an angle between the interface and the extrusion direction less than 30° exceeds 70 %. EBSD results show that strong basal texture of α phase with basal planes parallel to the extrusion direction was formed during the extrusion process in the (α+β) two-phase field. During the subsequent cooling process, the γ lamellae precipitates along the (0001) plane of α phase according to the Blackburn orientation relationship, allowing the lamellar structure to inherit the orientation of the parent phase. Tensile test results indicate that the strength and fracture elongation of the as-extruded TNM alloy along the extrusion direction (ED) increased simultaneously, which were 1100 MPa and 1.26 %, respectively. The hindrance effect of interfaces on dislocations and stacking faults in lamellar colonies parallel to the extrusion direction helps to improve strength, while the activation of the prismatic slip and pyramidal slip systems in the α2 lamellae is beneficial to the plasticity. This study shows that it is feasible to control the lamellar orientation and improve the mechanical properties of TNM alloy by extrusion above its α transus temperature.

Improvement in compressive creep resistance of Ti-43.5Al–4Nb–1Mo-0.1B alloy via micro-alloying with C and Si

In the present work, the effects of the addition of C and Si on the compressive creep behavior of Ti-43.5Al–4Nb–1Mo-0.1B (TNM) alloy at 850 °C and 900 °C under 260 MPa were investigated. The results show that micro-alloying with C and Si can significantly strengthen the compressive creep resistance of TNM alloy. During compressive creep, obvious dynamic recrystallization occurs in TNM alloy, including major discontinuous dynamic recrystallization and minor continuous dynamic recrystallization. The development of dynamic recrystallization greatly softens the alloy, which leads to large creep deformation. Meanwhile, with the increase of creep strain, the bearing area increases and the actual stress decreases, which eventually results in the secondary creep behavior. In addition, the content of α2 phase increases during compressive creep, which can be attributed to two phase transformations. After adding C and Si, the B2 phase content is decreased and the lamellar colony size is increased, which contributes to the improvement of creep resistance. In addition, fine and dispersed Ti3AlC and Ti5Si3 particles precipitate in α2 lamellae during compressive creep, and maintain definite orientation relationships with α2 and γ phases. This will delay the decomposition of α2 lamellae and greatly improve the lamellar stability. Then, the excellent lamellar stability will inhibit the development of dynamic recrystallization, thus effectively reducing the creep rate.

Effect of simultaneous heating on oxidation behavior of niobium added titanium aluminide (Ti-45Al-5Nb-0.5Si) developed by laser direct energy deposition (LDED)

The present study aims at understanding the effect of simultaneous heating (780 °C) during laser direct energy deposition (LDED) on kinetics and mechanism of cyclic oxidation (from 700 to 900 °C in air) of niobium added tailored titanium aluminide (Ti45Al5Nb0.5Si) developed by LDED using a Nd:YAG laser using 500 W applied power, 300 mm/min scan speed and 2.2 g/min powder feed rate. The microstructure of LDED Ti45Al5Nb0.5Si consists of γ (TiAl) + α2 (Ti3Al) lamellae, however, the mass fraction of γ (TiAl) is found to be higher in case of simultaneous heating as compared to the same without heating. Simultaneous heating increased the activation energy of cyclic oxidation from 231 kJ/mol (for the sample without heating) to 249 kJ/mol. The mechanism of cyclic oxidation is explained.

High-temperature microstructure evolution of an advanced intermetallic nano-lamellar γ-TiAl-based alloy and associated diffusion processes

Nano-lamellar advanced γ-TiAl based alloys doped with small amounts of C and Si are being developed to improve the creep resistance in order to increase the performances of this kind of alloys applied in the low-pressure turbine of aircraft engines. In order to extend the service temperature up to 1073 K or even above, the control of the microstructure stability is key. In this work, a complete study of the microstructure evolution during high-temperature exposure up to 1153 K has been approached through different electron microscopy techniques including HRTEM and HRSTEM with microanalysis. The nucleation and growth of the ordered βo precipitates and the ζ silicides inside the α2 lamellae has been carefully characterized and new orientation relationships and the misfit between all crystalline lattices has been determined, as well as the chemical concentration of the different atomic species on each phase. The electron microscopy study shows that βo and ζ precipitates inside α2 prevents or retards the dissolution of the α2 lamellae and its final disintegration in favor of the γ lamellae. This phenomenon has been discussed in terms of the phase coherence and diffusion processes. These important results allow conclude that the coarsening of the γ lamellae is delayed because of the βo and ζ precipitation, allowing to explain the observed enhancement of the creep resistance in this γ-TiAl based alloy exhibiting a nano-lamellar microstructure.

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.

Effects of Sn on the densification and microstructure of a Ti-48Al-2Nb-0.7Mn-0.3Si-1Sn alloy fabricated from cold-pressed powders through vacuum melting

The density and microstructure of a modified second-generation γ-TiAl intermetallic alloy with a nominal composition of Ti-48Al-2Nb-0.7Mn-0.3Si-1Sn (at. %) were investigated and compared with similar low-Nb containing alloys reported in the literature. The alloy was successfully fabricated from blended and uniaxial cold-pressed precursor powders and subsequently arc re-melted under vacuum and heat-treated. From the obtained results, the alloy showed improved castability as evidenced by the absence of shrinkage cavities and the highest relative density of 99.87%. In as-cast condition, the microstructure comprised fine-grained dendritic structures of γ-phase and γ + α2 lamellae with a mean grain size of about 49 μm which transformed into a well-developed duplex structure consisting of γ grains and γ + α2 lamellae, and traces of Ti5Si3 precipitates after heat treatment. The alloy exhibited a more significant grain refinement in as-cast condition than its counterparts reported previously.

Tensile creep behavior of a heat treated β-solidified γ-TiAl alloy: The development of dynamic recrystallization and the role of B2 phase

The tensile creep behavior of a heat treated β-solidified γ-TiAl alloy has been investigated at 800 °C under 250–300 MPa. Results indicate that the heat treatment procedure proposed in this study can improve creep resistance of the alloy. Combined with kinetic analysis (stress exponent n = 3.36) and TEM observation, it can be inferred that creep deformation is dominated by viscous dislocation glide, and assisted by mechanical twinning. It is found that both twin boundary and TiB can hinder dislocation movement and strengthen creep resistance. During creep, there are obvious dynamic recrystallization (DRX) and B2 phase precipitation. On the one hand, DRX volume fraction increases with the increase of creep stress or the extension of creep time. Meanwhile, TiB can act as nucleation sites of recrystallized γ grains. On the other hand, there are two kinds of B2 phase precipitation behaviors: (ⅰ) thin B2 laths are formed in α2 lamellae; (ⅱ) blocky B2 phases are formed at colony boundary. Creep deformation is mainly completed by γ phase, B2 phase can involve in the deformation only under specific creep conditions. Due to poor deformation ability of B2 phase, creep voids prefer to nucleate at the γ-B2 interface and B2-colony interface. When voids are connected, cracks are formed and begin to propagate, eventually leading to creep failure in brittle-ductile mixed fracture.

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.

Impact of microstructure on elastic properties in the alloy Ti-42Al-8.5 Nb

The microstructure of a γ-TiAl alloy containing niobium undergoes continuous transformations during annealing at 550 °C. These take place within the α2-phase of lamellar (α2 +γ) colonies. We have characterized these changes by transmission electron microscopy and made a correlation to the elastic constants determined by resonant ultrasonic spectroscopy during in-situ annealing at the same temperature. The results show a continuous increase of the E-modulus. This can be attributed to elastic strains acting within α2 lamellae as a result of lattice transformations. After 5000 h, a thermodynamic equilibrium was established with equal fractions of α2-phase and O-phase that differ only in lattice symmetry and Nb content. Generally, the presence of O-phase should contribute to lower elastic moduli, however in the present case the evolving microstructural characteristics were responsible for the observed increase in the E-modulus.

Microstructural stability and creep properties of middle Nb γ-TiAl alloy with a modulated microstructure

In this work, a modulated structure composed of nano-scale Ti2AlNb (O phase) and α2 phase was obtained in middle Nb γ-TiAl alloy. The O phase nucleates at the α2/γ interface and is orthogonal to the α2/γ interface. The creep testing results show that the modulated structure exhibited the better creep performance compared with the fully lamellar microstructure, manifesting in a smaller minimum creep rate and creep strain after creep for 200 h. The O phase in α2 lamellae suppressed the α2 →γ phase transformation and constructed the coherent interface with adjacent γ lamellae, which replaced part of the semi coherent α2/γ interface, to reduce the generation of interfacial dislocation loops, thereby improving the creep resistance. Besides, discontinuous precipitations (DPs) caused by (α2/γ) → β0(ω0) + γ phase transformation were observed in the crept samples, contributing to the microstructure degradation. The present work shows significant engineering significance and scientific value for improving the creep performance and microstructure stability for middle Nb–TiAl alloys.

Enhanced high temperature tensile and creep properties of a β-solidified γ-TiAl alloy with the hybrid addition of C and Y2O3

β-solidified γ-TiAl alloys are popular lightweight structural materials well suited for high temperature applications, however, whose service temperature limit is greatly restricted due to the insufficient high temperature strength and creep resistance. In present work, a hybrid reinforced β-solidified γ-TiAl alloy was successfully developed with the hybrid addition of C and Y2O3 by induction skull melting (ISM) technique, accordingly, the microstructure, high temperature tensile and creep properties of the alloy are detailly discussed. The results indicate that the hybrid addition of C and Y2O3 can improve the high temperature tensile strength of this nearly lamellar TiAl alloy with the increase of about 13.7%. More importantly, creep resistance of this alloy is significantly enhanced, which is attributed to multiple strengthening mechanisms: (ⅰ) the second phase strengthening from multi-scale Y2O3 particles and dynamically precipitated H–Ti2AlC carbides; (ⅱ) mechanical twins and twin intersections can act as special obstacles to dislocation movement, which is of great significance in decreasing the steady-state creep rate. Particularly, this alloy exhibits excellent microstructure stability during creep. On the one hand, nano-scale Y2O3 and dynamically precipitated H–Ti2AlC particles could pin the B2/γ interface to strengthen lamellar colony boundary. On the other hand, dispersed B2 phase particles could precipitate along α2 lamellae and grow up with the consumption of the α2 phase, which will inhibit the degradation of α2/γ lamellae. This study offers a new composition design approach to optimize high temperature performance of β-solidified γ-TiAl alloys.

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.