Influence of oxygen on solidification behaviour of cast TiAl-based alloys J. Zollinger a , J. Lapin b , D. Daloz a, * , H. Combeau a a Ecole Nationale Supe ´rieure des Mines de Nancy, LSG2M, Parc de Saurupt, CS14234, F-54042 Nancy, Cedex, France b Institute of Materials and Machine Mechanics, Slovak Academy of Sciences, Racianska 75, 830 02 Bratislava, Slovak Republic Received 27 October 2006; received in revised form 2 April 2007; accepted 10 April 2007 Available online 27 June 2007 Abstract The influence of oxygen on solidification behaviour of cast TiAl-based alloys containing 40e48 at.% of Al was studied. Twelve alloys with fixed Ti:Al ratios ranging from 1.08 to 1.5 and oxygen content of 0.1, 0.8 and 1.5 at.% were prepared by induction melting and casting in a cold crucible under protective atmosphere. Increasing oxygen content affects significantly the macrostructure of the as-cast ingots, increases volume fraction of the a phase formed during peritectic solidification and leads to a change of the b primary solidification phase to the a phase in ternary Tie44.2Ale1.4O, Tie47.3Ale0.9O and Tie47.2Ale1.5O (at.%) alloys. In the case of the alloy with the a primary solidification phase, the partition coefficients achieve values of k s=l Al=a ¼ 0:9 and k s=l O=a ¼ 1:29. In alloys containing 1.5 at.% of oxygen, it has been observed that the b/a solidification phase transition is shifted to a lower aluminium contents when compared to that of binary systems. Oxygen extends the stability of single a phase region. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: A. Titanium aluminides, based on TiAl; B. Phase transformations; C. Casting; D. Microstructure 1. Introduction Since the last two decades, TiAl-based alloys have attracted attention as potential candidates for high-temperature structural applications in the aerospace and automotive industries. Due to low density, high specific strength, high Young’s modulus and oxidation resistance at high temperatures, these materials repre- sent a good alternative to nickel-based superalloys [1,2]. How- ever, their metallurgical preparation including melting and casting is still constrained by several limitations. One of them results from their high reactivity with a large number of elements and especially with oxygen. Despite of the use of vac- uum or protective atmosphere processes (vacuum arc remelting, plasma arc remelting, electron beam melting, etc.), contamina- tion by oxygen cannot be fully avoided. Hence, industrial alloys must be considered at least as ternary TieAleO systems with an oxygen content ranging usually from 300 to 500 wt. ppm [3e5]. It should be mentioned that interactions between titanium aluminides and interstitial elements such as C, O and N were widely studied to understand the effect of these ele- ments on solid state phase transformations, microstructure evo- lution and mechanical properties [6e13]. Some works were devoted to the interactions between mould materials and melts with the aim to improve hardness by oxides and to elucidate the effect of ceramic particles on mechanical properties of TiAl- based alloys [14e17]. The ternary TieAleO system has also been studied from the point of view of oxidation behaviour of Ti-based alloys or with the aim to develop new refractory oxide materials [18e21]. However, to our knowledge, no data exists on the effect of oxygen on solidification behaviour of TiAl- based alloys in the available literature. Solidification path and the heterogeneities ensuing from it have a great influence on grain size and subsequent heat treatments of these alloys [22,23]. * Corresponding author. Tel.: þ333 83 58 42 05; fax: þ333 83 58 40 56. E-mail address: daloz@mines.inpl-nancy.fr (D. Daloz). 0966-9795/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2007.04.002 Intermetallics 15 (2007) 1343e1350 www.elsevier.com/locate/intermet