Invited Review
Phase transformation and microstructure control of Ti2AlNb-based alloys: A review

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Abstract

In recent years, the Ti2AlNb-based alloys are selected as potential alloys for elevated temperature applications to replace conventional Ni-based superalloys owing to their good creep resistance and oxidation resistance which are related to the O precipitates. In this paper, the precipitation mechanisms of O phase, phase transformation and microstructure control of Ti2AlNb-based alloys are reviewed. Ti2AlNb-based alloys generally consist of B2/β, α2, and O phase with different morphologies which are derived from the various heat treatment processes, including equiaxed α2/O particles, bimodal microstructure, and Widmannstätten B2/β + O structures etc. As a newly developed strengthening phase, O precipitates can be precipitated from the B2/β matrix or α2 phase directly as well as generated by means of peritectoid reaction of α2 phase and bcc matrix. Microstructural control of the Ti2AlNb-based alloys can be implemented by refining the original B2/β grain size and regulating the O precipitates. Multidirectional isothermal forging (MIF) and powder metallurgy technique are two effective methods to refine the original B2/β grains and the morphology and size of O precipitates can be regulated by adding alloying components and pre-deformation process. Moreover, the phase diagram as well as coarsening behavior of Ti2AlNb-based alloys in ageing process is also reviewed. For the further application of these alloys, more emphasis should be paid on the deep interpolation of microstructure-property relationship and the adoption of advanced manufacturing technology.

Introduction

Titanium-aluminum system alloys based on intermetallic (e.g. α-Ti3Al, γ-TiAl, and δ-TiAl3) possess great high-temperature strength, strong oxidation resistance, good creep resistance, and excellent microstructural stability owing to the combined action of metallic bond and covalent bond, which have become research hotspot in high-temperature structural materials [[1], [2], [3], [4]]. However, the development of these alloys was restricted because of their poor intrinsic brittleness in room temperature [[5], [6], [7]]. Consequently, Ti2AlNb alloys based on ordered orthorhombic phase (O) have been developed in the past decades, which were firstly discovered by Banerjee in a Ti3Al-based alloy toughening experiment in 1988 [8]. These new-type alloys own better room-temperature ductility and fracture toughness, as well as crack propagation resistance compared with TiAl-based alloys and better elevated temperature strength and oxidation resistance than Ti3Al-based alloys [9,10]. In addition, compared with Fe-based and Ni-based superalloys, the density of Ti2AlNb-based alloys is reduced by about 40 % without compromising high-temperature performance [11,12]. In summary, these excellent mechanical properties make Ti2AlNb-based alloys the important candidates to replace Ni-based superalloys and indicate broad application prospects in the aerospace fields.

Ti2AlNb-based alloys vary greatly in morphologies and the corresponding mechanical property, depending on the fabrication processes (e.g. casting, forging, powder metallurgy etc.) and heat treatment procedures (e.g. deformation, solution treatment, recrystallization, stress relief annealing, ageing etc). The grain size of the Ti2AlNb-based alloys fabricated by the conventional casting is bulky, accordingly exhibiting a poor performance. Moreover, due to the large difference between the melting point of Al and Nb elements, composition segregation is easy to occur during the smelting process of the alloy, which leads to a microstructural inhomogeneity. In order to solve this problem, thermomechanical processing (TMP) is usually used to treat alloy ingots to achieve the target of grain refinement as well as eliminating defects, such as hot forging and hot rolling [[13], [14], [15]]. However, although superior comprehensive mechanical properties can be obtained by thermomechanical processing, it is relatively complex and high-cost in the fabrication process. Compared to the above manufacturing processes, powder metallurgy technique began to be applied in the preparation of Ti2AlNb-based alloys, which makes full use of the materials and optimizes both the microstructure and properties [[16], [17], [18], [19]]. Hence, it is necessary to understand the precipitation mechanism of the typical microstructures of Ti2AlNb-based alloys (e.g. equiaxed, bimodal, and lath) accurately, and to regulate the microstructure of the alloys by means of hot working, composition design, and heat treatment processes. In this paper, phase composition, microstructural control, and phase transformation of Ti2AlNb-based alloys are reviewed. Besides, Ti-Al-Nb phase diagram, powder metallurgy techniques and coarsening behavior of the alloys are also involved.

Section snippets

Ti-Al-Nb phase diagram

Since the Ti-Al-Nb system intermetallic exhibited superior mechanical properties, numerous investigations have been carried out to clarify the phase transformation process as well as phase equilibrium state by drawing the binary or ternary-system diagrams [[20], [21], [22], [23], [24], [25], [26], [27], [28]]. Such diagram is also called phase equilibrium diagram, which is the phase relationship under the condition of thermodynamic equilibrium. Owing to the significance of phase diagram as

Phase composition of Ti2AlNb-based alloys

The microstructure of the Ti2AlNb-based alloys is generally composed of B2/β phase, O phase and α2 phase, and the lattice parameters of each phase are displayed in Table 1.

As documented in previous studies, the main phase in Ti2AlNb-based alloys and specific conventional titanium alloys is B2/β phase. B2 phase is ordered body-centered cubic (bcc) structure with a space group of Pm3¯m, which belongs to CsCl-type structure. There are so many vacancies inside this kind of crystal lattice that

Microstructure control of Ti2AlNb-based alloys

It is well known that the mechanical properties of Ti2AlNb-based alloys are strongly influenced by the microstructures. Therefore, the optimization of microstructure and the precise control of the morphology are the prerequisites and fundamentals to improve the performances of the alloy, which require the understanding of the microstructure-property relationship as an important basis. With regard to Ti2AlNb-based alloys, the purpose of optimizing microstructure and improving the comprehensive

Conclusions and prospects

The O-phase-strengthened Ti2AlNb-based alloys owns great creep resistance and oxidation resistance at elevated temperature, which are considered as excellent candidates for high temperature applications to replace conventional Ni-based superalloys. Microstructures and morphologies of the Ti2AlNb-based alloys are closely associated with its mechanical properties, accordingly influencing the application of the alloy. By using different approaches to prepare Ti2AlNb-based alloys with different

Acknowledgements

This work was supported financially by the National Natural Science Foundation of China (Nos. 52034004, 51871186 and 51474156).

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