Mechanical characterization of mechanically alloyed ultrafine-grained Ti 5 Si 3 +40 vol% γ-TiAl composites C. Suryanarayana a,n , Rainer Behn a,b , Thomas Klassen a,1 , Rüdiger Bormann a a Helmholtz-Zentrum Geesthacht, Centre for Materials and Coastal Research GmbH, Institute for Materials Research, Max-Planck-Strasse, D-21502 Geesthacht, Germany b Hamburg University of Technology, Department of Materials Science and Technology, D-21073 Hamburg, Germany article info Article history: Received 29 November 2012 Received in revised form 9 March 2013 Accepted 22 April 2013 Available online 2 May 2013 Keywords: Titanium alloys Mechanical alloying Mechanical characterization Electron microscopy Superplasticity abstract Ultrafine-grained ceramic-based composites of Ti–31.6Al–21.6Si (at%) consisting of 60 vol% of ζ-Ti 5 Si 3 and 40 vol% of γ-TiAl were produced by high-energy ball milling followed by hot isostatic pressing (HIP). Because of the cleanliness of the powder and full densification of the HIPed product, the mechanical behavior of the composite could be unambiguously related to the microstructure and chemistry. The starting microstructure after HIPing consisted of intermixed ζ-Ti 5 Si 3 and γ-TiAl phases of approximately equal grain size, the size ranging from about 300 nm to 1 μm depending on the HIP temperature. High- temperature mechanical testing of this ultrafine-grained composite exhibited a strain-rate sensitivity of 40.3. Further, the equiaxed microstructure was retained after mechanical testing, suggesting the possibility of achieving superplastic deformation. Consequently, tensile testing demonstrated elongations of about 150% at 950 1C and a strain rate of 4 Â 10 -5 s -1 . Considering that the present alloy has the ceramic (silicide) phase as the matrix, this temperature at which superplastic deformation is observed is significantly lower than that reported for conventional coarse-grained ceramic materials. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Lightweight intermetallic alloys based on γ-TiAl are promising materials for high-temperature structural applications, e.g., in aircraft engines or stationary turbines [1–5]. Even though they have many desirable properties such as high specific strength and modulus both at room and elevated temperatures, and good corrosion and oxidation resistance, they suffer from inadequate room temperature ductility and insufficient creep resistance at elevated temperatures. Addition- ally, satisfactory creep resistance at temperatures between 800 and 850 1C, an important requirement for elevated temperature applica- tions of these materials, could not be achieved till now. Therefore, current research programs have been addressing the development of high-temperature materials with adequate room temperature ductility for easy formability and ability to increase the high-temperature strength by a suitable heat treatment or alloying additions to obtain sufficient creep resistance. Since the ductility of conventional coarse-grained materials is known to be increased by refinement of microstructure [6], the strategy to achieve improved ductility in intermetallics and ceramics has been to reduce the grain size down to the lowest possible, usually to nanometer dimensions, hoping that these traditionally brittle materials would exhibit increased workability [7]. Ultrafine-grained materials (including nanostructured materi- als, with grain sizes of o100 nm) have interesting combination of properties [8–13]. Even though the available results on the ductility of these materials are inconclusive, adequate ductility has been reported in ceramic-based materials, not at room temperature, but at temperatures significantly lower than those required for coarse-grained materials [13]. It has been shown that the compressive strength of binary γ-TiAl alloys with nanometer-sized grains is about 2600 MPa at room temperature and that, at temperatures higher than about 500 1C, the strength drops very rapidly to low values [14–19]. In fact, the strength was found to decrease at a faster rate for ultrafine-grained materials than for the coarse-grained counterparts. That is, the smaller the grain size of the specimen, the stronger and sharper is the drop in yield strength on increasing the temperature. This observation suggests that monolithic nanostructured materials may not be suitable for achieving the desired creep resistance. Further, recent results indicate that perhaps achieving the finest micro- structure is not the best approach to attain the desired objectives of high strength, improved ductility, and enhanced creep resistance [20–23]. Therefore, alternative strategies need to be adopted. A number of novel production and processing methods like high- energy-milling [24,25], inert gas condensation [26], electrodeposition Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A 0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.04.092 n Corresponding author. Permanent address: Department of Mechanical and Aerospace Engineering, University of Central Florida, Orlando, FL 32816-2450, USA. Tel.: +1 407 823 6662; fax: +1 407 823 0208. E-mail address: Challapalli.Suryanarayana@ucf.edu (C. Suryanarayana). 1 Also at: Institute of Materials Technology, Helmut Schmidt University, University of the Federal Armed Forces Hamburg, Holstenhofweg 85, D-22043 Hamburg, Germany. Materials Science & Engineering A 579 (2013) 18–25