Full length articleNew insights into high-temperature deformation and phase transformation mechanisms of lamellar structures in high Nb-containing TiAl alloys
Graphical abstract
Introduction
Titanium aluminides based on the intermetallic phases γ (TiAl) and α2 (Ti3Al) have been developed over decades for service in high-temperature technologies [1], [2], [3]. Nevertheless, widespread applications are hindered by insufficient ductility and damage tolerance. It is well accepted that deformation of (γ+α2) alloys is mainly provided by the majority γ phase. The α2 phase is less deformable because dislocation glide is largely confined to prism planes with <110> Burgers vectors, which are perpendicular to the c axis of the D019 unit cell. Dislocation glide with shear components parallel to the c axis is strongly impeded by high Peierls stresses and thus virtually impossible [4]. Thus, plastic deformation of monolithic α2 (Ti3Al) is highly anisotropic. It should also be noted that this anisotropy is exacerbated still further at moderately high temperatures of 500–700 °C, depending on composition [5,6]. This behavior of the α2 phase is also a concern for two-phase alloys, when constraint co-deformation of the α2 and γ phases occurs. Due to the disparity of the elasto-plastic properties, high constraint stresses develop during deformation [7], which, together with the susceptibility to cleavage fracture, are the main reason for the poor ductility. The problem can partly be reduced by optimizing the alloy constitution and microstructure (for a review see [1]). In particular, the development of multiphase alloys involving additional constituents like βo or orthorhombic phases, has significantly improved the mechanical performance [8].
The lack of independent slip systems of the α2 phase could, in principle, be reduced by the activation of mechanical twinning as, for example, in the disordered hexagonal counterpart α-Ti. However, in the fully ordered state, mechanical twinning appears extremely difficult because large interchange atomic shuffles are required in order to maintain the D019 structure [9]. Nevertheless, twin structures have been occasionally observed in single-phase α2 alloys [10], [11], [12], [13]. The experimental findings are summarized as follows. Lipsitt et al. [10] observed stacking faults and micro-twins in Ti3Al after 900 °C tensile deformation but did not specify the twinning elements. Lee et al. [11] observed twin structures in hyperstoichiometric Ti–34Al after high-temperature compression between 800 and 1100 °C with twinning elements that correspond to c-axis compression twins in disordered α-Ti. Kishida et al. [12,13] recognized mechanical twinning in Ti–36.5Al single crystals after compression above 1000 °C, when the compression axis was close to the c-axis of the D019 cell. The twin habit plane was , and the twinning elements were , , , . The indices of K1 and η2, were found to be irrational, i.e., they are only approximate. Hence, the twins were categorized as Type-II twins [14]. According to the notation introduced by Kishida et al. [12,13], K1 and η2, were set in apostrophes. No evidence of twinning was reported for stoichiometric α2 alloys [2,15]. In an undeformed Ti–46Al alloy, Wang et al. [16] observed twin structures after high-temperature annealing and quenching, and claimed that the twins are a result of double twinning of (201)[104] and (011)[012] formed in the high temperature α phase. This suggests that excess point defects and thermo-elastic stresses could support twinning.
Little is known about twinning of the α2 phase of (α2+γ) alloys. This is certainly partly due to the usually very fine morphology of the α2 phase in these alloys, which makes the detection of twin structures with electron microscopic diffraction contrast difficult. Zhang et al. [17] observed twin structures in the α2 phase after room temperature compression of a Ti–50Al–2Mn–1Nb (at%) alloy, in which the α2 phase was present in the form of widely separated Widmannstätten platelets. The twinning of the α2 platelets was caused by twins moving in the surrounding γ matrix and striking the α2 platelets. The twinning of the α2 phase, however, was only possible with appropriate orientation of the incoming twin in relation to the α2 platelet. In the other cases, the twins were either reflected back into the γ phase, or the twins bypassed the α2 platelets. The fact that the twin structure was observed after room temperature deformation is difficult to understand, since the required interchange shuffles can hardly be realized at this low temperature. Therefore, it remains unclear whether the twin structure observed in this study was the by-product of high-temperature phase transformation.
Taking together, the factors that apparently support mechanical twinning in Ti3Al alloys are:
- (i)
hyperstoichiometric (Al-rich) composition;
- (ii)
compressive deformation;
- (iii)
high deformation temperature (>800 °C) to allow significant diffusion;
- (iv)
grain orientation favorable for c-type deformation;
- (v)
point defect supersaturation;
- (vi)
presence of local stresses.
In a previous study [18], twin-like structures with the elements were observed in the α2 phase of a Ti–45Al–8.5Nb–0.2W–0.2B–0.02Y (at%) alloy. The major alloy phases are γ (TiAl) and α2 (Ti3Al); minor constituents are βo/B2, ωo and orthorhombic phases, for a review see [3]. It is well documented that this alloy type is prone to constitutional and structural changes upon thermo-mechanical treatments. Thus, the evolution of the twin structures in the α2 phase during high-temperature deformation could be masked by the constraint co-deformation with other phases and various phase transformations. Elucidation of these processes, which are of significant importance with respect to the mechanical properties of such alloys, is the major subject of the paper.
Section snippets
Experiments
The nominal composition of the alloy is Ti–45Al–8.5Nb–0.2W–0.2B–0.02Y (at%). Cylinders in sizes of ∅8 mm × 12 mm were compressed to a strain of 30% in a Gleeble 3500 machine (Dynamic Systems Inc., USA) at 800 °C. Details of the experimental procedures can be found in [18]. After compression, the samples were immediately water-quenched to freeze the high temperature microstructure. Specimens of the original as-cast ingot and the compression test were prepared for electron microscope
Constitution and microstructure of the as-cast alloy
Fig. 1 illustrates the constitution and microstructure of the undeformed alloy. The major constituents are γ, α2 and βo(ωo), as confirmed by the diffraction pattern (Fig. 1(a)). It should be noted that in the as-cast alloy no ω-related phase was recognized within the lamellar colonies, i.e., the ω phase was only observed within the βo phase. The γ and α2 phases are mainly present as lamellar colonies with the orientation relationships [19]
The γ phase occurs in
Discussion
In the literature, mechanical twinning of the ordered intermetallic α2 (Ti3Al) phase has been reported several times. The investigations were mainly carried out on polycrystalline single phase α2 alloys or α2 single crystals and were presented in detail in the Introduction. Although the emphasis in this earlier work was on the kinematics of twinning, it has virtually been agreed that large interchange shuffles were required to maintain the ordered D019 structure. Such atomic exchanges are
Summary
The present study describes the evolution of twin structures in the α2 phase and associated α2→ωo transformations. The combined operation of these processes leads to a novel microstructure, which has been crystallographically characterized. The main conclusions are summarized as follows:
- 1.
The twin-like morphology in the α2 phase can be attributed to {201}<014> twinning of the α2 phase, which formally corresponds to the {101}<012> deformation twinning mode in disordered hexagonal
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (contract nos. 51971175, 51601146 and 51831001) and State Key Lab of Advanced Metals and Materials (contract no. 2017-ZD02). Lin Song is grateful for the fruitful discussion with Dr. Heike Gabrisch.
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