Reaction behavior and evolution of phases during the sintering of TaeAl powder mixtures Hossein Sina a , Srinivasan Iyengar a, * , Sven Lidin b a Materials Engineering, Lund University, P.O. Box 118, 22100 Lund, Sweden b CAS Chemical Centre, Lund University, P.O. Box 118, 22100 Lund, Sweden article info Article history: Received 14 July 2015 Received in revised form 7 September 2015 Accepted 12 September 2015 Available online 14 September 2015 Keywords: Intermetallics Powder metallurgy Reaction synthesis Microstructure Thermal analysis abstract Intermetallic compounds based on tantalum aluminides are of considerable interest in various industrial applications. In this work, the formation of tantalum aluminides has been studied in elemental powder mixtures containing 25, 50 and 66.7 at% Ta. A differential scanning calorimeter (DSC) was used to heat the samples up to 1500 K at 15 K min À1 . Phase evolution was studied by heating a few samples to temperatures below and above the observed DSC peaks. The heat treated samples were analyzed using scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. The results suggest an exothermic reaction between tantalum particles and molten aluminum, which leads to the formation of Al 3 Ta compound as the initial product. This reaction reached completion for the aluminum-rich samples and the corresponding DSC peak was very broad, containing two distinct steps which indi- cated the effect of a diffusion barrier during the reaction. In these samples, the Al 3 Ta product was stable upon further heating. A different behavior was observed for the equiatomic and tantalum-rich samples, an incomplete reaction with a considerable amount of unreacted tantalum. These samples were asso- ciated with a narrower reaction peak in the DSC plots, followed by a mildly exothermic peak at higher temperatures. The latter was found to correspond to the formation of Al 69 Ta 39 phase in the solid state, together with minor amounts of s (Ta-rich) and 4 (near equiatomic) phases. The Al 69 Ta 39 phase showed a tendency to disappear on prolonged heating. The s and 4 phases were observed to dominate as the major phases in tantalum-rich and equiatomic samples, respectively. Increasing the heating rate shifted the reaction peak for Al 3 Ta formation to higher temperatures and the apparent activation energies were estimated as 383 ± 13 kJ mol À1 and 439 ± 22 kJ mol À1 for the initial and final stages of this reaction. The heat of formation of Al 3 Ta was also estimated as À36 ± 7 kJ mol À1 in the interval 1050e1350 K. Studies on the effect of particle sizes of the reactants showed that, in most cases, the reaction peak shifted to lower temperatures on decreasing the tantalum particle size. A similar behavior was observed for aluminum in the tantalum-rich samples, while an inverse effect was seen in equiatomic and aluminum-rich samples. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Intermetallic compounds based on transition metal aluminides have been regarded as suitable candidates for advanced structural applications. These compounds exhibit an attractive combination of properties such as high melting points and enhanced resistance against corrosion and oxidation at elevated temperatures. This group of materials, including AleTa compounds, offers a variety of applications in different fields such as thin films and coatings [1,2], microelectronics, resistors and capacitors [3,4], diffusion barriers [5,6] and structural materials which can serve at high temperatures [7,8]. Amorphous AleTa alloys have been employed in applications requiring high thermal and chemical stability [1,4]. However, low ductilities of intermetallic compounds, particularly at ambient temperatures, may limit their use in some applications. Despite extensive studies on the AleTa system [3,4,7e12], un- certainties are still associated with some phase relations and the binary phase diagram is not yet fully established. However, there is agreement in literature on the existence, stabilities and * Corresponding author. E-mail addresses: Hossein.Sina@material.lth.se (H. Sina), Srinivasan.Iyengar@ material.lth.se (S. Iyengar), Sven.Lidin@chem.lu.se (S. Lidin). Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom http://dx.doi.org/10.1016/j.jallcom.2015.09.100 0925-8388/© 2015 Elsevier B.V. All rights reserved. Journal of Alloys and Compounds 654 (2016) 103e111