The ductility of cast TiAl-based alloys
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The ductility of cast TiAl-based alloys M H Loretto, Z. Wu, M Q Chu and D Hu IRC in Materials, The University of Birmingham, Edgbaston, B15 2TT, UK ABSTRACT The factors which influence the ductility of cast samples of TiAl-based alloys are briefly reviewed with emphasis on alloys where microstructural refinement has been used in an attempt to improve ductility. The grain size in cast samples of different TiAl-based alloys can be refined either by high additions of about 1at% boron, or by lower additions of about 0.2at%. In addition it is possible to refine the microstructure by massively transforming samples and heat treating the transformed samples in the (alpha + gamma) phase field to precipitate alpha. Significantly different ductilities are found in different alloys with similar grain sizes or with similar microstructures and the origins of the improvements in ductility and of these differences are discussed in this paper. The role of alloying elements in influencing the degree of order in alpha 2 and in turn influencing slip in alpha 2 is discussed. INTRODUCTION In early work it was shown that it was possible through thermomechanical processing to produce alloys based on TiAl which had ductilities of 2% and more [e.g. 1, 2]. It has however become increasingly clear that the main applications of these alloys in the near future will be in the form of cast products, because of the relative costs of thermo-mechanically processed samples and of cast samples. This has been confirmed by the decision announced by General Electric and by Rolls-Royce that the turbine blades to be used in the final stages of their next generation engines will be cast blades made from alloys based on TiAl. The main factor which has led to the decision to use alloys based on TiAl is their density, which is about half of that of the corresponding Ni-based alloys and the strength or the oxidation resistance required for currently envisaged applications are not performance-limiting issues. The fact that these alloys embrittle upon exposure to an oxidising atmosphere at 700ºC [e.g.3, 4] means that even at the low temperatures, which are understood to be relevant in this first application, the alloys will embrittle during service. This in fact is an advantage of using cast samples since the lamellar structure in cast samples embrittles far less than other structures which can be obtained by forging; typically the ductility in lamellar samples is halved whereas it is effectively reduced to zero in duplex samples [3, 4]. Although it is by no means clear what minimum ductility is sufficient for these blades to be used in this application, the main effort which has been put in to improve the properties of cast products has been to obtain ductilities as large as possible. The cast alloys to be used will be hot isostatically pressed (HIPped) in order to close the porosity, which is inevitably present in cast components and it is expected that HIPping will improve the ductility from that of the as-cast product. Progress on controlling microstructures a
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