Alloy design concepts for refined gamma titanium aluminide based alloys
Introduction
Light alloys based on the γ-TiAl and minor fractions of the α2-Ti3Al intermetallic phases (hereafter called γ + α2 alloys) represent a unique class of materials. Owing to their low density and attractive high-temperature properties, they have a significant potential for innovative applications in advanced energy conversion systems, in which these materials may replace the heavier nickel-base superalloys at intended service temperatures of 600–900 °C. However, cast γ + α2 alloys generally exhibit low ductility and damage tolerance at room temperature as well as low workability at elevated temperatures that restrict their application. These deficiencies are associated not only with the intrinsic brittleness of the γ and α2 phases, but also with the microstructure that evolves in castings during freezing [1], [2], [3]. A coarse-grained microstructure, a sharp casting texture and significant chemical inhomogeneity are typical characteristics of γ + α2 alloy ingots [1], [2], [3], [4], [5], [6], [7]. The production of sound castings with a homogeneous fine-grained microstructure and without significant texture is, therefore, of great importance for the application of γ + α2 alloys. First, this would enable to manufacture a widespread range of sound cast products with reproducible mechanical properties guaranteeing minimum values for the design of components. Second, owing to the increased workability of such alloys the fabrication of components using different hot-working procedures might be significantly facilitated. However, up to now, there is no elaborated concept of designing γ + α2 alloys in order to produce sound cast materials. Furthermore, the influence of different alloying additions and their content on the microstructural evolution of γ + α2 alloys during solidification and the subsequent solid-state transformations are not completely understood yet.
The present work is devoted to a microstructural study of cast γ + α2 alloys in dependence on the aluminum content, several alloying additions and the cooling rate. The experimental findings are to be used to develop new γ + α2 alloy compositions that are characterized by good chemical homogeneity and a fine-grained microstructure in as-cast condition.
Section snippets
Experimental
Buttons of about 30 g weight were melted in a laboratory arc-melting furnace on a water-cooled copper plate under argon atmosphere. The investigated materials included alloys with varying niobium content (0, 5, 10 and 15 at%), boron additions (0 and 0.2 at%) and other micro-alloying elements (molybdenum, tungsten, carbon). The buttons were melted at least seven times to ensure good homogeneity. High-purity metals and a Ti–49.1Al–0.37C (hereafter in at%) master alloy were used as starting
Influence of the Al concentration on cast microstructure
According to the binary phase diagram (Fig. 1 [10]), γ + α2 titanium aluminide alloys can solidify either completely through the β phase for Al concentrations up to around 44 at% Al or peritectically for higher Al concentrations according to the reaction L + β ⇒ α. Up to now, particularly alloys solidifying peritectically have been investigated and considered for applications. In the as-cast condition, these alloys are characterized by a coarse colony size (d ≫ 100 μm) and a sharp casting texture [6],
Summary
An alloy design concept for the production of sound γ-TiAl + α2-Ti3Al alloy castings with a chemically homogeneous and fine-grained microstructure has been proposed. The concept has been developed according to the following considerations: (i) solidification through the β phase to avoid chemical inhomogeneities resulting from peritectic reactions and to form different orientation variants of the α phase from one β dendrite as proposed in literature [6], [9], [14], [15], [21], [22]; (ii) an
Acknowledgements
One of the authors (V. Imayev) would like to thank the Alexander von Humboldt Foundation for financial support. The authors further gratefully acknowledge valuable discussions with V. Küstner, U. Lorenz and J.D.H. Paul.
References (44)
- et al.
Microstructure development in gamma alloy mill products by thermomechanical processing
Mater Sci Eng
(1998) - et al.
Microstructure and deformation of two-phase γ-titanium aluminides
Mater Sci Eng
(1998) - et al.
Phase equilibria and solidification in Ti–Al alloys
Acta Metall
(1989) - et al.
Solidification structure in a cast γ alloy
Scripta Mater
(2003) - et al.
Directional solidification of TiAl-base alloys
Intermetallics
(2000) - et al.
Microstructure refinement of cast TiAl alloys by β solidification
Scripta Mater
(2004) - et al.
Control of a fine-grained microstructure for cast high-Cr TiAl alloys
Mater Sci Eng A
(2005) - et al.
Phase equilibria and microstructural control of gamma TiAl based alloys
Intermetallics
(1998) - et al.
The decomposition of the beta phase in Ti–44Al–8Nb and Ti–44Al–4Nb–4Zr–0.2Si alloys
Acta Mater
(1998) - et al.
Physical metallurgy for wrought gamma titanium aluminides: microstructure control through phase transformations
Intermetallics
(2005)
Boride morphology in a (Fe, V, B) TiAl alloy containing B2-phase
Acta Metall Mater
Mechanical properties of thermomechanically treated Ti-rich γ+α2 titanium aluminide alloys
Scripta Mater
Phase equilibria in TiAl alloys containing 10 and 20 at.% Nb at 1473 K
Scripta Metall Mater
The compression behaviour of niobium alloyed γ-titanium aluminides
Acta Mater
Phase equilibria among α (hcp), β (bcc) and γ (L10) phases in Ti–Al base ternary alloys
Intermetallics
Tracer solute diffusion of Nb, Zr, Cr, Fe, and Ni in γ-TiAl: effect of preferential site occupation
Intermetallics
Phase equilibria in the TiAl binary system
Acta Mater
Microstructural development during directional solidification of α-seeded TiAl alloys
Acta Mater
Refinement of the lamellar structure in TiAl-based intermetallic compound by addition of carbon
Scripta Mater
Dislocation dynamics in carbon-doped titanium aluminide alloys
Mater Sci Eng
Designing gamma alloys: fundamentals, strategy and production
Scale-up of ingot metallurgy wrought γ-TiAl
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