ALLOYING EFFECTS IN THE γ ′ PHASE OF CO-BASED SUPERALLOYS Alessandro Mottura 1 , Anderson Janotti 1 , Tresa M. Pollock 1 1 Materials Department, University of California - Santa Barbara, Santa Barbara CA 93106-5050 Keywords: Co-based Superalloys, Ab initio Modeling, Stacking Fault Energy Abstract New γ /γ ′ Co-based alloys appear to be a promising set of materials for use in the hottest parts of gas turbines. Recent work has shown that these alloys behave similar to conventional Ni-based superalloys and preliminary designs have produced alloys with remarkable strength at high temperatures. Under- standing the behavior of solute elements in these al- loys is essential for improving their properties and designing viable alloys. In this work, ab initio sim- ulations are used to study the effects of additional elements on fault energies in the γ ′ phase. Initial in- vestigation shows that both Ta and Ti retain a strong preference for the second sub-lattice in the γ ′ phase. Calculations also show that these elements increase the superlattice intrinsic stacking fault energy of the γ ′ precipitates while promoting the stability of the γ ′ phase relative to other crystal structures. This work indicates that the stability and resistance to disloca- tion penetration of the γ ′ phase in these alloys are closely linked and are strongly affected by chemical bonding rather than the size of solute atoms. Introduction The recent development of new Co-based superalloys is based on the discovery of the γ ′ phase in the Co- Al-W ternary phase diagram [1] and the presence of a γ +γ ′ phase field in the Co-rich corner of the ternary diagram (see Figure 1). Similar to Ni-based superal- loys, the γ ′ phase in Co-based superalloys possesses a L1 2 crystal structure, with roughly equal fractions of Al and W randomly distributed on the second sub- lattice (see Figure 1). The γ ′ phase does not appear in either the Co-Al or the Co-W binary phase di- agrams at typical service temperatures: γ -Co (fcc ) and β-CoAl (B 2 ) coexist in the Co-Al binary phase diagram, while a Co 3 W intermetallic with the D0 19 crystal structure is the most stable phase in the Co-W binary phase diagram. The lattice mismatch between the γ and γ ′ phase in the ternary system is 0.53 %, which is small enough to allow the γ ′ precipitates to grow coherently within the γ matrix, maintaining a cuboidal morphology [1, 2]. Initial attempts to produce 4- and 5-element γ ′ - strengthened superalloys based on the Co-Al-W ternary system resulted in alloys with extraordinary yield strengths at high temperature [2], high melting temperature [3] and low segregation during solidifi- cation [4]. Additionally, it has been observed that the addition of 2 at.% Ta to a single-crystal Co-Al-W ternary alloy results in a flow stress above 500 MPa at 1243 K [2]. Higher densities of super-lattice intrinsic stacking faults (SISFs) were observed within the γ ′ precipitates in deformed samples of the Ta contain- ing alloy when compared to a simple ternary alloy. These stacking faults arise from the combination of two full a/2〈101〉 dislocations in the γ phase forming an a/3〈112〉 that shears the γ ′ precipitates, creating a SISF, and an a/6〈112〉 which remains at the γ /γ ′ interface (see Figure 2). This observation led to the hypothesis that Ta may have an effect on SISF energy (ΔE SISF ) [2]. Further improvements in the properties of these ma- terials will require higher order alloying additions that raise the γ ′ solvus temperature and improve creep properties of these materials [3]. It follows that understanding effects of solute elements in the γ and γ ′ phase on the properties of Co-based superalloys is essential for the development of these alloys. Ab initio simulations can be instrumental in providing insight that will accelerate the design of these alloys. In this research, ab initio methods have been em- ployed to investigate the effects of Ta and Ti, but other elements have been studied to establish trends across the periodic table. Theory Special Quasi-random Structures (SQSs) Simulating a random arrangement of solute atoms ab initio is not trivial since periodic boundary con- ditions enforce periodicity of the lattice. The un- derlying concept to Special Quasi-random Structures (SQSs) is to pick configurations of a small number of 685 Superalloys 2012: 12 th International Symposium on Superalloys Edited by: Eric S. Huron, Roger C. Reed, Mark C. Hardy, Michael J. Mills, Rick E. Montero, Pedro D. Portella, Jack Telesman TMS (The Minerals, Metals & Materials Society), 2012