Deformation behavior and high-temperature stability of unidirectionally solidified, twin phase TiAl
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T. Nakano and Y. Umakoshi Department of Materials Science and Engineering, Osaka University, Japan (Received 4 January 1993; accepted 16 March 1993)
Ti—50.8% Al has been cast using unidirectional solidification to produce specimens with a highly directional structure. Each specimen consisted of the intermetallic phases, TiAl and Ti3Al. Both phases existed as single crystal lamellae which were parallel to each other throughout the specimen and aligned along specific crystallographic orientations. Mechanical properties were assessed by deforming specimens such that slip occurred in different directions relative to the lamellae. High-temperature stability was assessed by heating for various times and examining the microstructure. Deformation revealed a highly anisotropic structure since ductility depended upon the principal direction of slip. Poor slip, occurring in a direction through the lamellae, was attributed to the Ti3Al phase where cracking initiated. Heating at 1200 °C resulted in recrystallization of the TiAl phase. The recrystallization started at the ends of the specimens as a result of residual mechanical damage, before spreading along individual lamellae into the bulk.
I. INTRODUCTION The requirement for high-temperature structural materials with improved mechanical properties combined with reduced densities has resulted in a large number of investigations into the Ti-Al alloy system.1 Of particular interest are the Ti-Al compounds with compositions in the region of 50 at. % Al since they display not only excellent high temperature strength and oxidation resistance, but also very low densities.2 However, as with many intermetallics, there are problems of poor roomtemperature ductility associated with the crystallographic phases formed at such compositions.3 The structure of TiAl is very sensitive to compositional variations. Al-rich TiAl exists as the TiAl (y) phase and has the tetragonal L l 0 structure while Tirich TiAl exists as a twin phase structure consisting of lamellae of y-TiAl separated by fine lamellae of the more brittle intermetallic Ti3Al ( a 2 ) having the hexagonal D0i 9 structure.3'4 The twin-phase structure has been found to exhibit improved ductility at room temperature, and this is attributed to the presence of the narrow a2 lamellae.5 A further development has been the production of the twin-phase compound using unidirectional solidification.6 Careful control of such a technique can result in a unidirectional lamellar structure where each lamella is a single crystal consisting of either TiAl or Ti3Al. These phases lie parallel to each other in alternate planes and are connected by specific crystallographic relationships. Due to the lamellar nature of the material, the structures have been found to have 1812
http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 8, Aug 1993
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highly anisotropic mechanical properties, ductility being good when principal slip occurs in a direction parallel to the lamellae and poor when principal slip occurs across the lamel
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