Oxidation of Tie46Ale8Ta in air at 700  C and 800  C under thermal cycling conditions M. Mitoraj, E.M. Godlewska * AGH University of Science and Technology, Faculty of Materials Science and Ceramics, Al. A. Mickiewicza 30, 30-059 Krakow, Poland article info Article history: Received 28 July 2012 Received in revised form 29 October 2012 Accepted 30 October 2012 Available online 11 December 2012 Keywords: A. Titanium aluminides, based on TiAl B. Oxidation abstract The Tie46Ale8Ta alloy was tested for oxidation resistance in laboratory air under thermal cycling conditions (1-h cycles) at 700  C and 800  C. Reaction progress was followed gravimetrically. After exposure, the specimens were subjected to systematic analyses comprising X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS). Marker method was used to assess the mechanism of scale growth. Nano- indentation and scratch test were performed to evaluate scale adhesion and micromechanical properties. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Titanium aluminides have been intensively studied for more than 20 years, as prospective structural materials [1,2]. The advanced third and fourth generation two-phase (g þ a 2 ) alloys with a fully lamellar microstructure [3e5], are recognized as the most promising for applications in the automotive [6,7] and aero- space industries [8,9]. They have low densities 3.8e4.0 g/cm 3 , good high-temperature specific strength, excellent creep properties and reasonable oxidation resistance up to 700  C [10,11]. The following properties are regarded as major disadvantages: low room- temperature ductility and fracture toughness [12], insufficient oxidation resistance at temperatures exceeding 800  C [13], difficult and high-cost production [14]. Moreover, at elevated temperatures titanium aluminides are prone to embrittlement caused primarily by dissolved oxygen and hydrogen [15,16]. Further improvement of properties is seen in alloying and/or surface engineering. A great variety of alloying additions and coating systems have been investigated, so far [17e22]. However, only a few papers deal with the oxidation behaviour of the TiAlTa alloys [23,24]. As reported recently [25], titanium aluminide alloys with Ta as a ternary addition have a number of advantages over those with Nb: easier processing, reproducible microstructure and mechanical properties being among the most important. It is not clear, whether and how tantalum affects oxidation resistance. It has been reported lately [26] that compared with niobium, tantalum has a more pronounced effect on oxidation rate, particularly at 1000  C. The aim of this work was to assess the oxidation resistance of a Tie46Ale8Ta alloy in laboratory air at temperatures not exceeding those mostly used in mechanical tests, i.e. 700  C and 800  C. 2. Experimental The experimental material was Tie46Ale8Ta with a fully lamellar microstructure consisting of g-TiAl and a 2 -Ti 3 Al (Fig. 1). An ingot, 13 mm in diameter, produced by horizontal centrifugal casting was cut into pellets (Fig. 2), 0.8e1.0 mm thick, using a dia- mond saw. For the oxidation studies, surface of the specimens was ground with emery paper to 1200 grit number, washed with distilled water and acetone. The weighed specimens were placed in alumina crucibles to collect any spalled material during the heating and cooling cycles. Each thermal cycle consisted of heating to the desired temperature, i.e. 700  C or 800  C, exposure at constant temperature for 1 h (hot dwell time) and cooling in static air. Heating rate was 50  C/min. Overall cooling time was about 25 min. The fastest temperature drop of 100  C/min was registered during the first 3 min. The cold dwell time of 15 min was measured from the moment when the specimens attained the temperature of 100  C. An automatic laboratory setup was used for thermal cycling experiments. Specimens were usually weighed once a day on * Corresponding author. Tel.: þ48 126172536; fax: þ48 126172493. E-mail address: godlewsk@agh.edu.pl (E.M. Godlewska). Contents lists available at SciVerse ScienceDirect Intermetallics journal homepage: www.elsevier.com/locate/intermet 0966-9795/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.intermet.2012.10.014 Intermetallics 34 (2013) 112e121