A first-principles study of the effect of Ta on the superlattice intrinsic stacking fault energy of L1 2 -Co 3 (Al,W) Alessandro Mottura * , Anderson Janotti, Tresa M. Pollock Materials Department, University of California, Santa Barbara, CA 93106-5050, USA article info Article history: Received 6 January 2012 Received in revised form 3 April 2012 Accepted 16 April 2012 Available online 6 June 2012 Keywords: A. Ternary alloy systems B. Alloy design B. Mechanical properties at high temperatures D. Defects: planar faults E. Ab-initio calculations abstract New Co-based alloys containing a L1 2 reinforcement phase display exceptional high-temperature properties. Early research has shown that the quaternary alloy Co-8.8Al-9.8W-2Ta (at.%) has a high- temperature strength comparable to single-crystal Ni-based superalloys above 1200 K. Associated with high strength is an unusual high density of intrinsic stacking faults within the g 0 precipitates. In this work, Density Functional Theory, the Axial Next Nearest Neighbor Ising model and Special Quasi-random Structures have been used to calculate the stacking fault energy of L1 2 -Co 3 (Al,W) and the effect of small Ta additions on the stacking fault energy. The model predicts a superlattice intrinsic stacking fault energy of 90e93 mJ/m 2 , which increases up to 30% when one Ta atom is substituted on the Al/W sub-lattice. This effect can be explained by considering d-band effects resulting from the addition of Ta. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The recent discovery of the g 0 phase and g þ g 0 phase field in the CoeAleW ternary system [1] has generated a strong interest in the development of new g 0 -strengthened Co-based superalloys for use in the hottest parts of gas turbines for power generation and jet propulsion. In the CoeAleW system, the g 0 phase has a L1 2 crystal structure, similar to Ni-based superalloys, but the second sub- lattice is randomly populated by Al and W atoms in similar frac- tions. The g 0 phase is unstable in both the CoeAl and CoeW binary systems at all temperatures. The lattice mismatch between the g and g 0 phase is 0.53%, which allows the g 0 precipitates to grow in the g matrix maintaining a cuboidal morphology [1e3]. The stability of the g 0 phase in the CoeAleW phase diagram has been the subject of several studies. Experimentally, the work of Sato et al. [1,4] argues that L1 2 -Co 3 (Al,W) is a stable intermetallic, however Kobayashi et al. [5,6] have recently suggested that the L1 2 - Co 3 (Al,W) may be metastable, decomposing to fcc-Co, B 2 -CoAl and D0 19 -Co 3 W. Ab initio modeling, using special quasi-random struc- tures (SQSs) [7], has been adopted by Jiang [8] to show that L1 2 - Co 3 (Al,W) has a lower formation energy than D0 19 -Co 3 (Al,W) at 0 K. However, the work of Jiang also shows that the g 0 phase in the CoeAleW ternary phase diagram has a higher formation energy with respect to a mixture of fcc-Co, B 2 -CoAl and D0 19 -Co 3 W. It must be pointed out that, although the g 0 phase may be metastable at 0 K, entropy may render g 0 stable at higher temperatures and diffusion may hinder the decomposition of L1 2 -Co 3 (Al,W) into the three more stable phases. Additionally, higher order alloying additions may stabilize the L1 2 structure. First attempts to produce 4- and 5-element g 0 -strengthened superalloys based on the CoeAleW ternary system resulted in alloys displaying an extraordinary yield strength at high tempera- ture [2,3,9], high melting temperature [3] and low segregation during solidification [10]. Despite these encouraging results, further compositional adjustments are needed to improve oxida- tion behavior, raise the g 0 solvus temperature and improve creep properties [3]. Recently, it has been observed that the addition of 2 at.% Ta to a single-crystal CoeAleW ternary alloy results in a flow stress above 500 MPa at 1243 K [2,9]. Analysis of deformed specimens via transmission electron microscopy (TEM) has revealed that Ta- containing alloys display a very high density of superlattice intrinsic stacking faults (SISFs) within the g 0 precipitates [2]. These result from the splitting of the full ah101i into superlattice partial dislocations of the type a=3h112i (see Fig. 1). These findings lead to the hypothesis that Ta may affect the SISF energy (DE SISF ) of the g 0 * Corresponding author. E-mail address: alemottura@engr.ucsb.edu (A. Mottura). 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. doi:10.1016/j.intermet.2012.04.020 Intermetallics 28 (2012) 138e143