Elsevier

Intermetallics

Volume 15, Issue 4, April 2007, Pages 451-460
Intermetallics

Alloy design concepts for refined gamma titanium aluminide based alloys

https://doi.org/10.1016/j.intermet.2006.05.003Get rights and content

Abstract

The influence of the Al content and the addition of further alloying elements on the cast microstructure of γ(TiAl) + α2(Ti3Al) alloys has been examined. The results show that particularly fine and homogeneous microstructures without strong segregation can be obtained for certain alloy compositions solidifying through the β phase. This behavior can be attributed to the avoidance of peritectic solidification and to the alloying influence on the kinetics of the β  α transformation following solidification. The experimental findings were used to propose a design concept for γ-TiAl + α2-Ti3Al alloys. This concept aims at the production of high-quality castings as well as at ingot material for wrought processing routes because the chemically homogeneous and fine-grained microstructures would be a good precondition for improved workability.

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)

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Permanent address: Institute for Metals Superplasticity Problems, Russian Academy of Sciences, Khalturin Str. 39, 450001 Ufa, Russian Federation.

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