Effect of hot rolling and primary annealing on the microstructure and texture of a β-stabilised γ-TiAl based alloy

Abstract

Titanium aluminide alloys based on the ordered γ-TiAl phase are intermetallic materials well suited for lightweight high-temperature applications. The TNM alloys, a specific, β-solidifying group among them with a nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (at.-%), offer a homogeneous and fine-grained microstructure upon casting. This advantage has been exploited to develop a lab-scale hot rolling process in which a cost-effective ingot breakdown of the starting material is omitted. The present work establishes a fundamental understanding of the processes prevailing in the material during hot rolling and primary annealing. Microstructural analysis and texture measurements conducted at a synchrotron radiation source allow to study deformation, recovery, recrystallisation, as well as phase transformation mechanisms in detail. Different hot rolling procedures conducted within the (α+β) and (α+β/β_o+γ) regions of the phase diagram are considered and investigated with regard to the prevalent mechanisms. Hot rolling in the (α+β) region entails an α₂ phase texture novel in γ-TiAl based alloys. Hot rolling in the (α+β/β_o+γ) region near the γ-solvus temperature particularly promotes the breakdown of the initial microstructure in TNM alloys. A specially designed hot rolling process prevents the accumulation of a modified cube texture component in the γ-TiAl phase that is typically linked to anisotropic mechanical properties.

Introduction

Intermetallic γ-TiAl based alloys provide a combination of engineering properties that are essential for lightweight high-temperature applications. Especially their low density, high strength at elevated temperatures, and good oxidation and hot gas corrosion resistance suggest their utilisation, for example, as structural materials in aerospace applications. For some of these applications, γ-TiAl based alloys have been processed and used in the form of hot-rolled sheets [1], [2], [3], [4], [5]. Due to their attractive properties, the potential of γ-TiAl based sheets is large. Nevertheless, they are not yet fully commercialised as they are difficult to manufacture. Details on the sheet rolling of TiAl alloys, which is a multi-pass process, are reviewed for so-called 2^nd generation γ-TiAl based alloys in Refs. [1], [2], including a discussion on the advantages and disadvantages of different prematerial routes, i.e. ingot and powder metallurgy.

For the hot rolling of γ-TiAl based alloys, the demands on the starting material are uncompromising. To obtain a crack-free sheet within the narrow range of processing parameters, the microstructure has to be fine-grained and free from inhomogeneities [1]. When ingot material is used, it is usually subjected to a thermo-mechanical processing, i.e. an ingot breakdown, prior to hot rolling [3]. However, for γ-TiAl based alloys that solidify via the body-centred cubic (bcc) β phase instead of following a peritectic solidification path, a preceding ingot breakdown can be omitted as has recently been demonstrated [5], [6], [7]. In the β-solidifying TNM alloys, which represent a particular group of γ-TiAl based alloys with a nominal chemical composition of Ti-43.5Al-4Nb-1Mo-0.1B (in at.-%), the ensuing hot rolling is additionally facilitated due to an elevated amount of β phase present at high temperatures [8], [9], [10]. The β phase in the TNM alloy is deliberately stabilised by the alloying elements Nb and Mo, which in combination give its name to this class of alloys [11]. The as-cast and hot-isostatically pressed microstructure is equiaxed, fine-grained, homogeneous, and basically free from segregation or texture [12].

At room temperature, TNM alloys consist primarily of the three intermetallic phases α₂-Ti₃Al (D0₁₉ structure), β_o-TiAl (B2 structure), and γ-TiAl (L1₀ structure). Two of these phases, α₂ and β_o, undergo an order/disorder transition at elevated temperatures (T_eut = 1160–1175 °C and T_βο→β = 1175–1205 °C [13]) and thereby form the related α (A3 structure) and β phase (A2 structure). Upon heating, the γ-TiAl phase remains ordered until reaching its dissolution temperature T_γ,solv at roughly 1255 °C [13], the exact transformation temperatures being dependent on the actual chemical composition of the material. TNM alloys can be hot-rolled in different regions in the phase diagram under a variety of conditions regarding the predominating phase fractions [5]. As a result, different microstructures and textures can be created.

First texture measurements on γ-TiAl based alloys have been conducted in the nineteen-nineties by Fukutomi et al. [14] and Hartig et al. [15], [16]. They investigated the formation of texture in the γ phase upon compression, and its stability in the course of recrystallisation and heat treatments. Following rolling experiments, Hartig et al. [15] described the appearance of a cube-like recrystallisation component, which is today known as modified cube texture. Bartels et al. [3], [17] investigated the development of textures during casting, thermo-mechanical treatments, hot rolling in the (α+γ) region, and primary annealing of γ-TiAl sheets. The appearance of the modified cube texture within the γ phase was linked to the anisotropic mechanical properties of the sheets [17]. As a result of the higher amount of α/α₂ phase in hot-rolled TNB and γ-TAB alloys, Schillinger et al. [18] described for the first time the deformation texture of the α₂ phase in γ-TiAl based alloys. Due to similarities to textures reported for two-phase Ti-base alloys, the established terminology of basal and transverse texture components was chosen to describe the main textural features. The deformation texture of the β/β_o phase in γ-TiAl based alloys has been described by Stark [19], who investigated the hot rolling of Nb-rich TNB alloys. The texture of the β_o phase was found to be composed of texture components characteristic of bcc metals.

In all of these previous studies, hot rolling was performed either in the (α+γ) region, or in the (α+β/β_o+γ) region with minor amounts of β phase. However, besides hot rolling in the (α+β/β_o+γ) region with adjustable phase fractions (e.g. majority of α phase, majority of γ phase, or both with respect to β), TNM alloys offer the opportunity to be hot-rolled in the (α+β) region. This opportunity introduces deformation mechanisms and transformation pathways not yet investigated in γ-TiAl based alloys. In this regard, a type of α₂ texture novel in γ-TiAl based alloys is reported and explained in the present work.

By adopting the approach of texture measurements using synchrotron radiation [20], [21], the present work establishes a correlation between deformation, recovery, recrystallisation, and phase transformations and the hot rolling temperature as an important processing parameter. In this context, the impact of these fundamental mechanisms on the microstructure and texture of β-stabilised TNM alloys is detailed. An original approach to minimise the accumulation of texture components typically linked to anisotropic mechanical properties is proposed, underlining the importance of fundamental studies for process design and optimisation.