Interdiffusion and Growth of the Phases in CoNi/Mo and CoNi/W Systems V.D. DIVYA, U. RAMAMURTY, and A. PAUL Deleterious topological-closed-packed (tcp) phases grow in the interdiffusion zone in turbine blades mainly because of the addition of refractory elements such as Mo and W in the Ni- and Co-based superalloys. CoNi/Mo and CoNi/W diffusion couples are prepared to understand the growth mechanism of the phases in the interdiffusion zone. Instead of determining the main and cross-interdiffusion coefficients following the conventional method, we preferred to determine the average effective interdiffusion coefficients of two elements after fixing the composition of one element more or less the same in the interdiffusion zone. These parameters can be directly related to the growth kinetics of the phases and shed light on the atomic mechanism of diffusion. In both systems, the diffusion rate of elements and the phase layer thickness increased because of the addition of Ni in the solid solution phase, probably because of an increase in driving force. On the other hand, the growth rate of the l phase and the diffusion coefficient of the species decreased because of the addition of Ni. This indicates the change in defect concen- tration, which assists diffusion. Further, we revisited the previously published Co-Ni-Mo and Co-Ni-W ternary phase diagrams and compared them with the composition range of the phases developed in the interdiffusion zone. Different composition ranges of the tcp phases are found, and corrected phase diagrams are shown. The outcome of this study will help to optimize the concentration of elements in superalloys to control the growth of the tcp phases. DOI: 10.1007/s11661-011-0990-7 Ó The Minerals, Metals & Materials Society and ASM International 2011 I. INTRODUCTION THE Ni- and Co-based superalloys are extensively used in turbine blades in aeroengines because of their high tensile, creep, and fatigue strengths, which combine with high ductility and fracture toughness along with excellent high-temperature corrosion. In these alloys, solid solu- tion and creep-rupture strengths are mainly obtained by alloying them with different refractory elements such as Mo, W, and Re. To protect the superalloys from oxidation at high temperatures, bond coats such as b-Ni(Pt)Al and MCrALY (M = Ni, Co, Fe) are used. During deposition of these coatings and subsequent operation of the component, an alumina layer grows on the surface of the bond coat, which in turn hinders the diffusion of oxygen. However, because of compositional differences between the superalloy and the bond coat, an interdiffusion zone forms in the middle. This zone often contains brittle topological-closed-packed (tcp) phases such as l, r, and the Laves phase. A high concentration of refractory elements in current generation superalloys naturally leads to a relatively high fraction of tcp phases. In addition, the loss of Ni from the superalloy leads to a weak interface between the matrix and the precipitates, which in turn leads to easy crack initiation. [1–4] Even the loss of the refractory elements leads to lowered strength and creep resistance. In view of the importance of these alloys, numerous studies have been conducted on the development of binary phase diagrams [5–8] and the evolution of the tcp phases in them. [4,9–11] However, diffusion, which plays a major role in the growth of these phases, has not been examined in detail. A few diffusion studies are available now on binary systems such as Ni-Mo, [12] Co-Mo, [13] Ni-W, [ 14, 15] and Co-W. [16] It is reported that the l phase predominantly occurs in the Co-Mo and Co-W systems, whereas the r phase grows in the Ni-Mo system. Above 1333 K (1060 °C), no intermetallic compounds form in the Ni-W system. Although a few studies are available in the Ni-W system, the activation energies reported differ vastly. [15,17,18] Most importantly, only a few studies are available on ternary Ni-Co-Mo [19,20] and Ni-Co-W [21] phase diagrams, and no studies are available on diffusion related aspects. The change in composition may affect the diffusion rate of the species and the growth of the tcp phases significantly, which has not been examined yet. Hence, the aim of the present study is to identify the synergy between chemistry, diffusion kinetics, growth rate of the phases in the diffusion zone, type of crystallo- graphic lattice, and diffusion mechanism. The first part of the article discusses the role of chemistry in the diffusion kinetics of binary systems such as Co/Mo, Ni/ Mo, Co/W, and Ni/W, followed by ternaries based on Ni-Co-Mo and Ni-Co-W. These experiments have also yielded useful information on phase diagrams. The second part deals with the correlation of the growth rate of different phases with kinetics and mechanisms of diffusion. V.D. DIVYA, Postdoctoral Student, U. RAMAMURTY, Profes- sor, and A. PAUL, Associate Professor, are with the Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India. Contact e-mail: aloke@materials.iisc.ernet.in Manuscript submitted May 11, 2011. Article published online December 7, 2011 1564—VOLUME 43A, MAY 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A