Deformation behavior of differently processed γ-titanium aluminides

doi:10.1016/S0921-5093(01)01552-0

Materials Science and Engineering: A

Volumes 329–331, June 2002, Pages 153-162

https://doi.org/10.1016/S0921-5093(01)01552-0 Get rights and content

Abstract

A short review on forging and rolling of TiAl-ingot material as well as HIPed prealloyed powder is presented. The ingots show a lamellar microstructure with a strong cast texture and alignment of the lamellae, resulting in strong mechanical anisotropies. Hot-forging and heat-treatments are required to obtain isotropic mechanical properties due to a more random texture. The influence of strain rate and temperature on the development of microstructure during forging was studied. The role of twinning and dynamic recrystallization (DRX) will be discussed. After rolling a fine-grained microstructure is observed with a strong modified cube texture, which give rise to improved strength and creep resistance in the transverse direction (TD) of the sheets. Above 1000 °C the sheets can be deformed superplastically. DRX gives microstructures which obey a Zener–Hollomon power law.

Introduction

In the last decade the German material research programs MATFO and MATECH initiated research activities and development on γ-TiAl. Implementation in rotating turbine blade application takes advantage of the high specific stiffness and strength, good creep resistance combined with sufficient oxidation and hot gas corrosion resistance (no titanium fire) [1], [2]. The aim was to exceed the application temperature of advanced titanium alloys and to replace nickel-based superalloys. The production of sheets should open totally new application fields for the TiAl alloys [1], [3], [4], [5], [6], [7]. As a skin-material for hypersonic aircraft, sheet material can save significant weight compared with the superalloys. Furthermore, due to the good hot gas corrosion resistance, TiAl sheets can be used as gas leading parts in aircraft engines.

Processing routes like precision casting, forging and rolling have been developed as well as powder metallurgical production. These research activities have led to a better understanding of the correlation between alloy composition, microstructure, processing behavior and resulting mechanical properties [2], [8], [9], [10]. This paper will mainly describe the influence of processing on the mechanical behavior. Especially the influence of textures is reported, because in TiAl special casting and deformation textures occur, which cause strong mechanical anisotropies [11], [12], [13], [14], [15], [16].

Section snippets

Processing of γ-TiAl based alloys

Most of the described processing procedures were performed at Plansee AG (Austria) or in close connection within the framework of common research programs. Both ingot metallurgy (IM) and powder metallurgy (PM) routes are used. Due to improved melting techniques no HIPing is conducted [17] and isothermal forging is used to produce prematerial for the subsequent hot-rolling process. The rolled sheets are flattened during a so-called ‘primary annealing’ treatment. The sheets can be formed

Thermomechanical treatment to destroy the casting texture

The melting and casting process takes place in vacuum or inert gas atmosphere [1]. The cooling rate after casting is relatively low and a pronounced dendritic microstructure occurs. Fig. 1 shows a typical microstructure observed in an ingot of Ti–48Al–2Cr (at.%) with a diameter of 19 cm. The micrograph was taken in the outer area of the ingot with view in the tangential direction. The dendritic microstructure exhibits a very strong as-cast texture. During solidification the dendrites have grown

Forging of large ingots

In the framework of a German materials research (MATFO) project, forging has been developed to produce fine-grained prematerial which can be rolled to sheet [1], [3]. Two processing routes were developed. ‘Near isothermal’ forging was performed on preheated canned ingots using a cold forging press [1]. The canned material was single-step forged at a temperature within the (α+γ)-phase field with a strain rate of about 1 s⁻¹ and lower [3]. After a reduction in height of about 75–85% the

Sheet rolling

Besides prematerial with a fine-grained equiaxed microstructure, a hot pack-rolling process is needed which fulfils the following conditions [30],

1
nearly isothermal rolling at temperatures within the (α+γ)-phase field;
2
rolling speed and reduction per pass must be carefully chosen in order to prevent cracking on microscopic or/and macroscopic scale and;
3
oxidation of the γ-TiAl alloy must be avoided.

These conditions lead to a narrow range of process parameters within which the rolling of γ-TiAl

Texture and mechanical properties of sheet material

The texture of γ-TiAl sheets in the as-rolled condition and after subsequent heat-treatments have been investigated by X-ray diffraction with Cu–K_α-radiation. The {200}+{002}-, {220}+{202}- and {311}+{113}-pole figures are measured as overlapped pole figures of the pair reflections. Besides these and the strong {111}-reflection the very weak {110}- and {201}-reflections are measured to get information about the orientation differences of 〈100〉 a-axes and 〈001〉 c-axes. From the six measured

Creep behavior

The creep behavior, especially the steady state creep, is influenced by the texture in a similar way as the yield stress because both are mainly determined by the same or by comparable dislocation interactions. Fig. 13 shows creep data of a γ-TAB sheet with two different microstructures. All tests were performed with 225 MPa at 700 °C. In the PA condition the creep rate obtained in RD is nearly ten times larger than in TD. The higher creep rate is obtained in the direction with lower yield

Superplastic forming and dynamic recrystallization

Superplastic forming (SPF) is used worldwide for Ti-base alloys to produce complex shaped parts. The superplastic behavior of γ-TiAl sheet material processed via the IM or PM route was investigated intensively over the last years [18]. For example, tensile tests were performed on Ti–47Al–2Cr–0.2Si sheet material with the PA microstructure. The strain rate sensitivity exponent m was determined with changes of the strain rate [37] between 10⁻³ and 10⁻⁵ s⁻¹ in the temperature range 1000–1100 °C. At

Final remarks

Besides the composition, processing determines the properties of γ-TiAl based alloys. Worldwide extensive works were and are being performed in order to optimize the mechanical properties by optimized casting and thermomechanical processing, such as forging and rolling. The present work shows that the resulting properties are determined not only by grain size, phase distribution and morphology. A very important point is the texture generated during the casting or thermomechanical processing.

Acknowledgements

One of the authors (A. Bartels) gratefully acknowledges support of this work by the Sonderforschungsbereich 371 ‘Mikromechanik mehrphasiger Werkstoffe’ of the Deutsche Forschungsgemeinschaft (DFG). The authors acknowledge the support of the German Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie within the framework of the projects 03M3029 and 03N3034.

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Deformation behavior of differently processed γ-titanium aluminides

Abstract

Introduction

Section snippets

Processing of γ-TiAl based alloys

Thermomechanical treatment to destroy the casting texture

Forging of large ingots

Sheet rolling

Texture and mechanical properties of sheet material

Creep behavior

Superplastic forming and dynamic recrystallization

Final remarks

Acknowledgements

Mater. Sci. Eng. A

Mater. Sci. Eng.

Mater. Sci. Eng. A

Scr. Met. Mater.

Intermetallics

Mater. Sci. Eng. A

Mater. Sci. Eng. A

Prog. Mater. Sci.

Z. Metallkd.

Z. Metallkd.

Metall. Trans. A

Phil. Mag.

Acta Mater.