Elsevier

Materials Characterization

Volume 167, September 2020, 110474
Materials Characterization

β0 precipitation in α2 lamellae of the β-solidifying multiple-phase γ-TiAl alloy

https://doi.org/10.1016/j.matchar.2020.110474Get rights and content

Highlights

  • β0 phase precipitated in the α2 lamellae of β-solidifying γ-TiAl alloys at the temperature range of 950 – 1150 °C for 6 h.

  • The nucleation and growth of β0 precipitates and the factors influencing the morphology of β0 precipitates are discussed.

  • The interaction between the α2→β0 and α2→γ phase transition is proposed.

  • Provide a guidance for controlling the amount of β0 precipitates in α2 laths in β-solidifying γ-TiAl alloys.

Abstract

The precipitation behavior of β0 particles in the α2 lamellae of a β-solidifying multiple-phase γ-TiAl alloy, i.e., Ti-42.05Al-3.96Nb-1.01Mo-0.21B (at.%) alloys, was investigated using conventional transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). It has been found that β0 phase can precipitate in α2 lamellae when aging-treated at 950–1150 °C for 6 h. This is a process controlled by diffusion of Mo element. The β0 precipitates nucleate initially at the α2/γ lamellar interfaces and grow firstly along the α2 lamellar width direction then along the length direction. The lamellar spacing of α2 lamella and growth stage of β0 precipitates codetermined the morphologies of β0 precipitates. All β0 phases precipitated from α2 matrix follow the Burgers OR: 101β0//0001α2,1¯11β0//112¯0α2, and the interface dislocations between α20 interfaces are commanded to accommodate the lattice misfit. Besides, the decomposition of α2 → γ also occurs during the aging treatments. The α2/γ interface ledges promote the nucleation of β0 phase, but the precipitated β0 particles at the α2/γ ledges impede the α2 → γ transformation. The coordination of α2 → β0 and α2 → γ transformations is influenced by the aging temperature. When the aging temperature is 1150 °C, the amount of β0 precipitation in α2 lamellae up to the maximum.

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 α20 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.

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