Laser Metal Deposition of the Intermetallic TiAl Alloy
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I.
INTRODUCTION
GAMMA titanium aluminides are becoming wellknown candidate materials for replacing nickel-based superalloys in the low-pressure turbine of the aircraft jet engines.[1,2] TiAl attractiveness results from a combination of physical and mechanical properties such as a low density, a high Young’s modulus, high mechanical properties up to 1073 K (800 C), a good hot corrosion resistance, and an excellent fire resistance.[3–6] If the ordered crystallographic c-TiAl structure confers very good mechanical strength at high temperatures, on the other hand, it is also responsible for a lack of ductility which renders manufacturing difficult. A strong aluminum dependence on microstructure and tensile ductility was reported for decades in the literature.[7–9] Indeed, only sub-stoichiometric alloys, at about 48 at. pct Al, with a fine-grained duplex structure that alleviates pile-up dislocations at grain boundaries, show substantial elongation at rupture. This duplex microstructure consists of c grains adjacent to c + a2 lamellar colonies. The fact that even the c grains exhibit a significant ductile behavior is probably due to the entrapment of interstitial oxygen within the a2 phase, which tends to change the stacking faults energy. For alloys with a higher Al content, a coarser single c phase microstructure is then formed, thereby exhibiting a much lower ductility. On the other hand, for alloys with a lower Al content which promote the formation of a fully lamellar microstructure, the higher amount of the
MARC THOMAS is with ONERA – The French Aerospace Lab, 92322 Chaˆtillon, France. Contact e-mail: [email protected] THIERRY MALOT is with ENSAM ParisTech, 75013 Paris, France. PASCAL AUBRY is with Den – Service d’Etudes Analytiques et de Re´activite´ des Surfaces (SEARS), CEA, Universite´ Paris-Saclay, 91191, Gif sur Yvette, France. Manuscript submitted August 12, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
brittle a2 phase has a detrimental effect on tensile ductility.[7,10,11] The effect of the aluminum content and of the related microstructures on mechanical properties was extensively investigated by General Electric for the implementation of the Ti-48Al-2Cr-2Nb alloy in the GEnx gas turbine engines. The conventional centrifugal casting process which limits the formation of gas porosities and of mis-runs enables to obtain sound TiAl parts. At present, this processing route which is used for TiAl low-pressure turbine blades offers the advantage of shaping complex parts. However, direct solidification imparts structural heterogeneities to TiAl parts as well as a strong texture due to the directional cooling which is very difficult to alleviate by subsequent heat treatments.[12–14] Even though the morphological texture could be anticipated for simple parts, it is not trivial for complex parts like turbine blades. Now, this morphological texture and the ordered structure of c-TiAl lead to strong anisotropy of the mechanical properties.[15,16] To be able to ensure minimum mechanical values, regardless of the shape
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