The reasons why many academic and industrial laboratories are involved in TiAl investigations lie in its remarkably good oxidation, burn resistance and high-temperature strength retention up to 800°C. [1] An application concerning gas-turbine engines arouses expectation, in particular blades of the high-pressure compressors in the air-admission zone and blades of the low-pressure turbines located in the gas ejection zone. For these high added value applications, the material has to be temperature and time resistant to the centrifugal force induced by the engine rotation, in other words the material has to be creep resistant at high temperatures (650-800°C). Additionally, the material has to exhibit a minimum of room-temperature strength and ductility, in particular for handling and assembly purposes. Now, one concern is related to the fact that ductility and creep resistance are experimentally observed as antagonistic properties. [1] To provide the best TiAl microstructure for the expected properties, investigations were concentrated on the chemical composition but above all, on the processing routes and related heat treatments: ingot metallurgy (IM) followed by forging or extrusion steps, powder metallurgy (PM) and casting followed by HIP and heat treatments. These three routes have at least one common drawback, which is the number of expensive steps to achieve the adequate microstructure because of the required complex thermal and thermomechanical treatments. In this context, a new cost-effective processing route was developed, a powder consolidation by spark plasma sintering (SPS).
Spark Plasma SinteringIn this technique, pulses of high intensity direct current are going through the graphite die and the powder under an uniaxial pressure (Fig. 1). A fast and controlled heating is therefore created which allows both compaction and thermal treatment. Such a fast processing also involves deep structural changes compared with other processes, due to limited diffusion mechanisms, making possible the formation and the stabilization of microstructures which are not attainable by common routes.The origins of SPS go back as early as 1933 with the first patent describing methods in which current is used for sintering powders. [2] This route for compaction has really been significantly used during the last ten years, mainly in carbides, nitrides or oxides. [2] As far as metallic powders are concerned, in spite of the identification of PM-SPS as an advanced route, [3] and even though some isolated publications can be found in the 70's, [4] only a few investigations have been reported up to now: in a recent review of the SPS sintering method, [2] only 3 references are reported for metal powders out of the 60 selected references.The recent literature reveals that different metallic systems are studied for SPS consolidation, such as pure metal powders as aluminium, [5] nickel, [6] or titanium, [7] such as sintering of aluminium alloys, [8] or such as intermetallics like NiAl, [9] FeAl, [10] NiTi, [11] or Al 3 Ti. [12] Concern...
