Atomic relaxation processes in an intermetallic Ti–43Al–4Nb–1Mo–0.1B alloy studied by mechanical spectroscopy

Abstract

An advanced intermetallic γ-TiAl-based alloy containing Nb and Mo has been studied to understand the microscopic mechanisms taking place during thermal treatments carried out to adjust a fine, nearly lamellar microstructure. The evolution of the microstructure has been characterized by high-energy X-ray diffraction and electron microscopy, while the atomistic mechanisms of defect mobility have been studied through internal friction and dynamic modulus measurements. An internal friction relaxation peak has been observed at about 1050 K (for 1 Hz) in the initial oversaturated α₂-Ti₃Al phase, whose intensity strongly decreases after precipitation of the γ-TiAl laths. The activation parameters of this relaxation have been measured, H_act = 3.1 ± 0.05 eV, τ₀ = 8.3 × 10⁻¹⁷ s and β = 1.3, and the relaxation is attributed to a point defect mechanism taking place inside the supersaturated α₂-Ti₃Al phase. A new Zener-like atomistic model based on stress-induced reorientation of Al–V_Ti–Al dipoles has been developed to explain the observed relaxation, resolving the controversy concerning the relaxation peak at 1050 K present in many γ-TiAl-based alloys. Precipitation of the γ-lamellae has also been considered as being responsible for the dynamic modulus hardening and, additionally, for another contribution to the internal friction at higher temperature than the described relaxation. Finally, the theoretical Debye equations as a function of temperature, for both internal friction and dynamic modulus, have been applied, using the measured activation parameters, to perform a deconvolution of the relaxation and precipitation contributions. The obtained results agree well with the experimental ones at different frequencies, allowing a global interpretation of the involved atomic processes.

Introduction

Engineering structural intermetallics are multiphase materials composed of near-stoichiometric metallic phases that exhibit outstanding stability at high temperatures [1]. As a consequence of the atomic ordered lattices of the main phases, which decrease the mobility of defects, these intermetallics are considered as high-temperature and creep-resistant materials for future applications in the automotive, aerospace and energy production industries [1], [2], [3]. Among the different families of intermetallics, γ-TiAl (L1₀, P4/mmm)-based alloys are envisioned as good candidates for advanced automotive and jet engines, including the new generation of hypersonic vehicles [4], [5], [6]. In the 1980s intensive work on the fundamental aspects of the first generation of binary TiAl alloys was performed, followed by the development of a second generation of more complex ternary alloys Ti–(45–48)Al–(1–3)Cr, Mn–(2–5)Nb, Ta, Mo (at.%) [7], [8], [9]. During the last 15 years a third generation with high Nb content Ti–45Al–(5–10)Nb–(0–0.5)B, C, so-called TNB alloy, was developed to improve both room-temperature ductility and high-temperature creep resistance [9], [10], [11]. Parallel basic thermal treatment parameters were defined to adjust the microstructure from fully lamellar to duplex and near-gamma [8]. In spite of all this effort, these alloys were not able to meet an important requirement for industrial-scale development, namely hot-working processing at reasonable costs. The challenge was to design an alloy fulfilling that particular requirement, leading to the development of a new type of TiAl alloys which contains both Nb and Mo in well-balanced quantities. In the following this class is referred to as TNM alloys. The basic strategy [12] was to develop β-stabilized γ-TiAl-based alloys, which allow a near conventional processing and the adjustment of a creep-resistant microstructure suitable for long-term service up to about 1025 K [12]. To achieve this goal an Al content of 43 at.% was selected and a combination of Nb, which among other things slows down diffusion processes [13], [14], with a stronger β-stabilizer such as Mo [15], [16], and a small but precise amount of boron as grain refiner [17], [18]. The phase and phase fraction diagrams [12], [19], [20], the α(A3)-α₂(D0₁₉, P6₃/mmc) and β(A2)-β₀(B2, $\text{P}\text{m}\overline{3}\text{m}$) ordering reactions [12], [21] for such compositions, as well as microstructural evolution during thermal treatment and the decomposition kinetics were studied [22], [23]. More information on the development, microstructure–property relationships and applications of advanced TiAl alloys in general, and TNM alloys in particular, can be found in a recent review article by Clemens and Mayer [24].

However, optimizing these newly designed TNM alloys requires precise control of the microstructure and a deep understanding of the microstructural phenomena contributing not only to the precipitation processes taking place during processing and thermal treatments, but also an understanding of the atomistic mechanisms susceptible of being activated during further high-temperature service conditions.

Thus, the aim of the present work is to study the processes involving the mobility of lattice defects (point defects, dislocations, etc.) taking place during thermal treatments performed to adjust the microstructure over a very large temperature range up to 1460 K. To study the mobility of defects up to this temperature we have applied an innovative approach, namely measurement of the internal friction (IF) and dynamic modulus (DM) variation by mechanical spectroscopy, due to the high sensitivity of this technique to the atomistic relaxation processes [25], [26]; this method has been successfully applied to study several intermetallics such as Fe–Al [27], [28], Ni–Al [29], [30] and binary and ternary γ-TiAl alloys [31], [32], [33], [34] at high temperature.