β0 precipitation in α2 lamellae of the β-solidifying multiple-phase γ-TiAl alloy
Introduction
γ-TiAl alloys are considered to possess the enormous potential for aerospace applications due to their low weight, high temperature strength, excellent burn resistance and quite good oxidation resistance [[1], [2], [3]]. In particular, the 3rd generation of γ-TiAl based alloys with the high containing niobium (about 5–10 at.%) have been developed to improve the hot workability and service temperature. This kind of alloys are known as β-solidifying γ-TiAl alloys because they solidify entirely through the disordered BCC β-Ti(Al) phase, whereby only a weak solidification texture is formed, the microstructure consists of the three ordered phases γ-TiAl (L10-structure), α2-Ti3Al (D019-structure) and β0-TiAl (B2) structure. Further alloy and process development was undertaken to lower the content of creep-deteriorating β/β0-phase within the microstructure prevailing during service, which lead to the finding of the well-known TNM alloy with a nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (all composition are given in at.%) [[4], [5], [6]]. Recently, TNM alloy is being used to manufacture turbine blades for PW1100G™ engines by isothermal forging and tried to serve at higher temperature [7,8].
It is well known that the lamellar structure in TiAl alloys shows outstanding characteristics for high-temperature structural applications [[9], [10], [11]]. As a result, the thermal stability of lamellar structure under service conditions has been becoming a main concern. Recent researches show that the α2 lamellae are thermodynamically unstable under the stress or long-term thermal exposure, which can decomposed into γ phase and β0 phase through α2 → γ [[12], [13], [14], [15], [16], [17], [18]], α2 + γ → β (β0 + ω) [19,20] respectively. For the transformation of α2 → β0 in γ-TiAl alloy, it has been reported that which can occur after 1000-h exposure at 700 °C in air [21] and also can happen after compression creep tests at 700 °C under 100 MPa [22]. In the TiAl alloys, the β0 phase is the ordering of the BCC β-phase generating at the β single-phase field. The formation of β0 phase below Teut, mainly due to the addition of β-stabilizing elements. Zhu et al. [23] reported that β0 precipitates nucleate and grow by the dissolution of α2 phase and the diffusion of β stabilizers, such as W, Cr, Nb, Mo [6], to achieve phase equilibrium during aging treatment at 950 °C. It is worth noting that the nucleation and growth processes of β0 precipitates remain unclear, and studies based on the transformation of α2 → β0 phase are still lacking in the β-solidifying multiple-phase alloys.
In this work, several aging treatments are conducted on one β-solidifying multiple-phase γ-TiAl alloy, i.e., Ti-42.05Al-3.96Nb-1.01Mo-0.21B, at the temperature range of 850–1150 °C. The β0 precipitates, composition analysis and the interface features of α2/β0 are characterized by scanning electron microscope (SEM) and high-resolution transmission electron microscopy (HRTEM). This study aims at fully revealing the precipitation behavior of β0 phase within the α2 lamellae during aging treatments and tries to provide guidance for the design of β-solidifying multiple-phase γ-TiAl alloy.
Section snippets
Materials and methods
The alloy used in the present study was fabricated by induction skull melting and casting followed by hot isostatic pressing (HIP) at 1260 °C for 4 h under a pure argon pressure of 140 MPa. As shown in Table 1, the actual chemical composition is Ti-42.05Al-3.96Nb-1.01Mo-0.21B, which satisfies the baseline composition of TNM alloys, i.e., Ti-(41–45)Al-(3–5)Nb-(0.1–2)Mo-(0.1–0.2)B. As a result, the transformation pathway can reference the phase diagram of the typical TNM alloy:
Microstructure evolution during aging treatments
Fig. 1 shows the microstructure of as-HIPed alloy, as the initial microstructure. It can be seen that the alloy exhibits a uniform nearly lamellar structure, mainly containing α2/γ colonies, ordered β0 phase and globular γ grains distributing at triple junctions and colony boundaries, as seen in Fig. 1(a). The average α2/γ colonies size is about 85 μm, listed in Table 2. The α2/γ lamellar interface is very regular, as shown in Fig. 1(b). It should be pointed out that no β0 precipitates can be
Conclusion
The current research characterized the α2/γ lamellar structure evolution in the 42.05Al-3.96Nb-1.01Mo-0.21B alloy after heat treated at 850–1150 °C for 6 h. The following conclusions are drawn:
- 1.
There are no evident differences in the mesoscale microstructures of 42.05Al-3.96Nb-1.01Mo-0.21B alloy after aging treatments under 850–1150 °C for 6 h. The heat-treated specimens still exhibit nearly lamellar microstructure with average colony size below 100 μm, the volume fraction of β0 phase
Declaration of competing interest
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (No. 51571162), and the Research Found of the State Key Laboratory of Solidification Processing (NWPU), China (No. 2020-TZ-03).
Date availability
All data included in this work are available upon request by contact with the corresponding authors.
References (35)
Gamma titanium aluminides as prospective structural materials
Intermetallics
(2000)Review of alloy and process development of TiAl alloys
Intermetallics
(2006)- et al.
In and ex situ investigations of the β-phase in a Nb and Mo containing γ-TiAl based alloy
Intermetallics
(2008) Wrought TiAl blades
Mater. Today Proc.
(2015)Microstructural evolution and mechanical properties of a forged gamma titanium aluminide alloy
Acta Metall. Mater.
(1992)- et al.
The relationships of microstructure and properties of a fully lamellar TiAl alloy
Intermetallics
(2000) - et al.
Phase transformation mechanisms in a quenched Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y alloy after subsequent annealing at 800 °C
J. Alloys Compd.
(2017) - et al.
Thermal exposure induced α2+γ→B2(ω) and α2→B2(ω) phase transformations in a high Nb fully lamellar TiAl alloy
Scr. Mater.
(2003) - et al.
Microstructural design and mechanical properties of a cast and heat-treated intermetallic multi-phase γ-TiAl based alloy
Intermetallics
(2014) - et al.
Investigation of α/γ phase transformation mechanism under the interaction of dislocation with lamellar interface in primary creep of lamellar TiAl alloys
Mater. Sci. Eng. A
(2002)
The role of interface dislocations and ledges as sources/sinks for point defects in scaling reactions
Acta Metall. Mater.
Phase transformation mechanisms involved in two-phase TiAl-based alloys - I. Lamellar structure formation
Acta Mater.
Phase equilibria among α (hcp), β (bcc) and γ (L10) phases in Ti-Al base ternary alloys
Intermetallics
γ α2 B2 lamellar domains in rolled TiAl
Scr. Mater.
Titanium aluminide applications in the high speed civil transport
Gamma Titan. Alum.
Technology and mechanical properties of advanced γ-TiAl based alloys
Int. J. Mater. Res.
Design of novel β-solidifying TiAl alloys with adjustable β/B2-phase fraction and excellent hot-workability
Adv. Eng. Mater.
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2023, Acta MaterialiaCitation Excerpt :However, Klein et al. observed that accumulated Mo elements at α2 /γ lamellar interface facilitated the formation of βo precipitate in α2 lamellae [8], demonstrating the influence of β-stabilizing elements on the stability of lamellae. Similar results have been observed in as-cast Ti-43.5Al-4Nb-1Mo-0.1B (TNM) alloys [13]. Moreover, the structural differences between α2 and βo phases led to a similar atom arrangement at α2 /γ interface as that of B2 structure, beneficial to βo formation [15].