Tensile, Creep Properties and Microstructural Correlations in an Extruded Ti-48Al Alloy
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TENSILE, CREEP PROPERTIES AND MICROSTRUCTURAL CORRELATIONS IN AN EXTRUDED Ti-48A1 ALLOY G. Babu Viswanathan and Vijay K.Vasudevan Dept. of Materials Science and Engineering, University of Cincinnati, Cincinnati, OH 45221 ABSTRACT In this study, new combinations of heat treatment and extrusion conditions was employed to produce fine-grained, nearly lamellar microstructures in a binary Ti-48 at.% Al alloy. The effect of subsequent heat treatment on microstructure was studied and room temperature tensile properties were determined for some representative microstructures with relatively high volume fractions of lamellar grains. High tensile strengths and ductilities (upto 2.3%) were obtained and microstructural factors thought to be responsible for these are discussed. Constant load tensile creep properties for the same microstructures were evaluated in the temperature range of 700 to 815°C for stresses from 103 to 241 MPa. This data was used to determine minimum creep rates, activation energies and stress exponents and their dependence on microstructure. A significant effect of microstructure, particularly volume fraction of lamellar grains on the creep rates was observed, and creep rates comparable to those reported in quarternary Ti-48AI alloys with similar microstructures were obtained. The various results suggest that it is possible to obtain an attractive and balanced combination of room temperature ductility and high temperature creep resistance in these materials by suitably varying thermomechanical processing and heat treatment conditions. INTRODUCTION Alloys based on the TiAI compound are candidate materials for high temperature aerospace applications because of their unique combination of properties. The most promising alloys, which are based on the Ti-48 Al composition (compositions in at.%) with ternary or quarternary additions, are characterized by the two-phase Ti3AI+TiAl (a2+7) lamellar microstructure [1]. The properties of these alloys are quite sensitive to microstructure; duplex structures show higher tensile ductilities, for example up to 2.2% in binary Ti-48AI [2,3] and up to 4% in ternary or quarternary alloys[2,4-6], whereas fully lamellar structures have poor ductility [3,6]. On the other hand, the creep resistance of the fully lamellar structure is markedly superior than that of the duplex structure [6]. One factor that controls ductility of these materials is grain size, and generally finer the grain size higher is the ductility [4]. A major challenge with regard to the application of these materials is how to balance room temperature ductility and high temperature creep resistance. In this regard, fine-grained, fully lamellar microstructures have the potential to provide this balance. However, the heat treatment conditions generally used to produce the fully lamellar microstructure lead to a coarse grain size, so that alternative processing routes for producing fine-grained, fully lamellar microstructures must be explored. In this paper, results of such a study are presented and it is shown t
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