Solidification of high Nb containing TiAl based alloys J. Zollinger* 1 , V. Witusiewicz 1 , A. Drevermann 1 , D. Daloz 2 and U. Hecht 1 Casting of titanium aluminides is an attractive processing route for production of near net shape components: turbocharger wheels, valves and aero-engine components are presently at the heart of casting developments. Among the casting alloys under consideration are a number of niobium rich TiAl based alloys that contain low boron additions for grain refinement and minor additions of other elements to enhance creep resistance. An essential condition that must be met to achieve grain refinement is a solidification pathway competed via b-(Ti), e.g. a pathway that avoids peritectic growth of a-Ti. In this contribution we describe the microsegregation analysis of a unidirectionally solidified sample from the ternary alloy Ti–45Al–8Nb. The corresponding solidification path is discussed on the basis of thermodynamic calculations and is shown to closely follow Scheil predictions with some amount of back-diffusion for aluminium. The analysis indicates that the nucleation undercooling for peritectic a (Ti) in the deep mushy zone is significant. Keywords: TiAl based alloys, solidification, grain refinement, boron additions Introduction Niobium rich and aluminium lean TiAl based alloys 1 with low boron additions are attractive casting alloys, since grain refinement can be achieved in situ, directly in the as cast part. Fully lamellar grains with an average grain size in the range from 20 to 80 mm can thus be obtained in one single process step, the grains being composed of c-TiAl and a 2 -Ti 3 Al. Grain refinement by low boron additions was shown to operate during the solid state transformation of titanium from the bcc b phase to the hcp a-phase. 2 While the detailed mechan- isms of nucleation and growth of a from b-Ti in the presence of boron and borides are still under investiga- tion, one basic condition is clear: peritectic growth of a- Ti should not occur during solidification. Understanding and if possible predicting the solidification path of an alloy as function of composition and process conditions, is therefore desirable. A first step into this direction is calculating the solidification path of an alloy in equilibrium and in Scheil–Gulliver conditions respec- tively, using appropriate thermodynamic software and thermodynamic databases. For TiAl based alloys a comprehensive and reliable thermodynamic database for multicomponent Ti–Al–Nb–B–C alloys is not yet avail- able; however, a new CALPHAD description for the entire ternary system Ti–Al–Nb has recently been elaborated. 3 The investigation presented here aimed to use this thermodynamic description to calculate the solidification path of the ternary alloy Ti–45Al–8?2Nb (at-%) and to compare it to experimental results obtained by unidirectional solidification in a Bridgman furnace. The comparison is based on characterising the chemical microsegregation obtained during solidifica- tion, following the statistical method of element distribution analysis 4,5 that was applied to multi- component Ni base superalloys 6 and aluminium alloys. 7 Solidification path and microsegregation in Ti–45Al–8Nb The solidification behaviour of the TiAl based alloy Ti–45Al–8Nb (wt-%) was investigated using a unidirec- tional solidification experiment (UDS), followed by a final quenching operation in order to freeze the mushy zone and high temperature phases as well as their local chemical composition. The Bridgman–Stockbarger fur- nace used has been described elsewhere. 2 The solidifica- tion experiment was performed on a sample with 6?8 mm diameter and 100 mm length, being contained in densely sintered yttria tubes. The applied thermal gradient G and the solidification velocity v were G520 000 K m 21 and v533610 26 ms 21 respectively. The cooling rate during the final quenching ranged at 100 K s 21 . The integral composition of the sample material was measured after processing by inductively coupled plasma optical emission spectrometry (ICP- OES) for the main elements and by hot extraction for oxygen, and is Ti–44?9Al–8?2Nb with 1575 wt-ppm O in the quenched part. The microstructure and local chemical composition were characterised on longitudi- nal and transverse sections by means of scanning electron microscopy (SEM) in the backscattered electron 1 Access eV, Intzest.r 5, 52072 Aachen, Germany 2 LSG2M, Ecole des Mines de Nancy, Parc de Saurupt, 54042 Nancy Cedex, France *Corresponding author, email j.zollinger@access.rwth-aachen.de ß 2009 W. S. Maney & Son Ltd. Received 17 June 2008; accepted 12 September 2008 DOI 10.1179/136404609X368163 International Journal of Cast Metals Research 2009 VOL 22 NO 1–4 339