Diffusive and massive phase transformations in TieAleNb alloys e Modelling and experiments E. Gamsjäger a , Y. Liu a , M. Rester b, c , P. Puschnig d,1 , C. Draxl d, 2 , H. Clemens e , G. Dehm b, c, 3 , F.D. Fischer a, * a Institute of Mechanics, Montanuniversität Leoben, Franz-Josef-Strasse 18, 8700 Leoben, Austria b Department Materials Physics, Montanuniversität Leoben, Franz-Josef-Strasse 18, 8700 Leoben, Austria c Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Leoben, Austria d Atomistic Modelling and Design of Materials, Montanuniversität Leoben, Franz-Josef-Strasse 18, 8700 Leoben, Austria e Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, Franz-Josef-Strasse 18, 8700 Leoben, Austria article info Article history: Received 18 December 2012 Received in revised form 27 February 2013 Accepted 1 March 2013 Available online 29 March 2013 Keywords: A. Titanium aluminides, based on TiAl B. Phase transformation B. Thermodynamic and thermochemical properties E. Phase diagram, prediction E. Simulations, atomistic F. Electron microscopy, transmission abstract The thermodynamic properties of the TieAleNb system are obtained from recently published thermo- dynamic assessments. Based on these data the phase boundaries of the (a-Ti þ g-TiAl) two phase region are calculated by utilizing the CALPHAD approach and are compared to those, obtained by ab-initio calculations. It is found that the ab-initio phase boundaries deviate significantly from those based on the CALPHAD fit to experimental data which can be rationalized by the lack of vibrational entropy contri- butions in the present approach. Consequently a thermodynamic description based on the CALPHAD approach is used to further investigate the kinetics of the massive a / g m phase transformation in the TieAleNb system by means of a recently developed thick-interface model. Simulation of the transformation kinetics results in a massive transformation in the single-phase region only. However, very thin mole fraction spikes are obtained due to comparatively high interface velocities. It is likely that these spikes cannot be fully developed in experiments meaning that diffusion processes are partly suppressed (quasi-diffusionless transformation). A massive transformation in the two-phase region would then be possible. The theo- retical predictions are compared to experimental studies performed on a Tie45Ale5Nb alloy (compo- sition in atomic percent). The alloy is heat treated slightly above the a-transus temperature and subsequently oil quenched to room temperature to generate g m ea 2 interfaces. Energy-dispersive X-ray spectroscopy measurements were performed across g m ea 2 interfaces in a scanning transmission elec- tron microscope to search for chemical spikes. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Phase transformations occur during processing of industrially relevant materials. The kinetics of these phase transformations define the final microstructure and are thus responsible for the resulting properties. In principle a huge variety of phase trans- formation may occur in materials and even different classification schemes for phase transformations exist and have their merits [1]. This work is focussed on massive transformations which constitute a limiting case in the field of diffusive phase trans- formations. According to Hillert [2], “the massive transformation is defined as the reaction by which a one-phase alloy transforms into a new phase by the growth of blocky or massive grains, such that the whole volume of material may transform”. It is generally accepted that diffusion processes are localized at the interface and its nearest surroundings. Hillert [2] argues that for an “ideal massive transformation local diffusion of atoms inside the migrating interface and in a spike in front of the advancing interface” may occur. On the other hand Massalski [3] states that “due to the lack of experimental evidence of such a compositional spike, and because of predicted spikes that are smaller than 1 interatomic spacing it seems a reasonably safe conclusion to consider composition invariance as an established feature of massive transformations”. Therefore, the motivation of this work is to diminish this apparent discrepancy in defining the charac- teristics of massive transformations. * Corresponding author. Tel.: þ43 3842/402 4001; fax: þ43 3842/46048. E-mail address: mechanik@unileoben.ac.at (F.D. Fischer). 1 Present address: Institute of Physics, Karl-Franzens-Universität Graz, Austria. 2 Present address: Physics Department, Humboldt-Universität zu Berlin, 12489 Berlin, Germany. 3 Present address: Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf, Germany. Contents lists available at SciVerse ScienceDirect Intermetallics journal homepage: www.elsevier.com/locate/intermet 0966-9795/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.intermet.2013.03.001 Intermetallics 38 (2013) 126e138