New insights into high-temperature deformation and phase transformation mechanisms of lamellar structures in high Nb-containing TiAl alloys

Abstract

The paper describes the microstructure evolution by high-temperature compression of a high Nb-containing TiAl alloy. The paper extends a previous publication [Song, et al. Intermetallics 109 (2019) 91–96], in which a unique twin-like morphology in the α₂ (Ti₃Al) phase was reported. However, the origin of these structures could not be clarified without doubt. The present study is focused on phase transformations that in this multiphase alloy can be associated with deformation. Particular attention is paid to local transformations of the α₂ phase into O phase or ω-related phases, which, because of structural and chemical similarity of these phases with α₂, can easily occur and could mistakenly be considered as a twin structure. The details of the atomic processes involved are elucidated by electron microscopy. Given the large shufflings and the atomic site interchanges required for the operation of this twinning system, it is concluded that twinning of the α₂ phase is a diffusive-displacive process. Within the α₂ phase, ω_o is heterogeneously nucleated. The nucleation sites are defect-rich areas, which are subjected to high local stresses. The study strongly emphasizes the close relationship between high-temperature deformation and phase transformations in multiphase titanium aluminide alloys.

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 $\frac{1}{3}$<1$\overline{2}$10> 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 β_o 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 $\left\{20\overline{2}1\right\}$, and the twinning elements were ${\mathit{K}}_1=̓\left\{\overline{2}12\overline{10}3\right\}̓$, ${\mathrm{K}}_2=\left\{20\overline{2}1\right\}$, ${\mathrm{\eta }}_1=<5\overline{146}>$, ${\mathrm{\eta }}_2=̓<\overline{1}3\overline{2}2>̓$. The indices of K₁ and η_2, 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 ($\overline{2}$201)[1$\overline{1}$04] and (0$\overline{1}$11)[01$\overline{1}$2] 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:

• hyperstoichiometric (Al-rich) composition;

• compressive deformation;

• high deformation temperature (>800 °C) to allow significant diffusion;

• grain orientation favorable for c-type deformation;

• point defect supersaturation;

• presence of local stresses.

In a previous study [18], twin-like structures with the elements $\left\{20\overline{2}1\right\}$$<\overline{1}014>$ 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 β_o/B2, ω_o 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.