Effect of SPS processing temperature on the microstructure and properties of a Ti 49 Al 47 Cr 2 Nb 2 alloy
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0980-II07-07
Effect of SPS Processing Temperature on the Microstructure and Properties of a Ti49Al47Cr2Nb2 Alloy Alain Couret1, Guy Molénat1, Jean Galy1, and Marc Thomas2 1 CEMES/CNRS, BP 94347, Toulouse Cedex4, 31 055, France 2 DMMP/ONERA, BP 72, Chatillon Cedex, 92332, France
ABSTRACT The sintering of TiAl alloys by the Spar Plasma Sintering (SPS) technique is investigated in the present paper. Compactions are conducted between 1100°C and 1225°C on a Ti49Al47Cr2Nb2 powder. Two-phased γ + α2 and lamellar microstructures are generated at low and high temperatures, respectively. The former exhibits enhanced tensile properties at room temperature but at the expense of a limited creep resistance. On the other hand, the latter suffers from a poor ductility. The deformation systems which are activated are determined by post mortem transmission electron microscopy. The mechanical strength of the two phased γ + α2 microstructure is analyzed in terms of the Hall-Petch law. INTRODUCTION TiAl alloys which are expected to be used for turbine and compressor blades should offer sufficient room temperature ductility and strength, as well as high temperature creep resistance. Since these properties are strongly microstructure dependent, the present work was devoted to optimize the microstructure of one single alloy. A microstructure formed by equiaxed lamellar grains with a controlled grain size is assumed to be a good compromise for mechanical properties. Indeed, the grain size has to be small enough to minimise the stress concentrations leading to failure at low temperature and to ensure reproducible results from one sample to another, and large enough to prevent from intergranular sliding during creep. Moreover, the processing route should be cost-effective and should allow the manufacturing of blades. Several routes have been extensively investigated: ingot metallurgy with its forge or extrusion stage, powder metallurgy and casting. Each of them has exhibited its own limits. In this context, the aim of the present work is to evaluate the potentiality of an infrequently used approach in powder metallurgy: Spark Plasma Sintering. With this processing route, powders are compacted by applying intense direct courant pulses under a uniaxial pressure. Its main advantages are the prospect of manufacturing near net shape blades and the short processing time, allowing a cost reduction and a minimization of physical processes such as grain growth. The SPS capability has been recently reviewed by Munir et al. [1]. During the last decade, many compactions of carbides, nitrides or oxides and only a few of metallic compounds were synthetized. Concerning TiAl alloys, Mei et al. [2] demonstrated that TiAl powders can be easily sintered and Calderon et al. [3] developed nano-grained microstructures which offer very high ductility and strength but which are not suitable for high temperature applications as explained above.
EXPERIMENTAL DETAILS Pre-alloyed powders of Ti49Al47Cr2Nb2 were consolidated via the Spark Plasma Sintering technique wi
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