Interdiffusion and Growth of the Phases in CoNi/Mo and CoNi/W Systems
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-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 solution 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. V.D. DIVYA, Postdoctoral Student, U. RAMAMURTY, Professor, and A. PAUL, Associate Professor, are with the Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India. Contact e-mail: [email protected] Manuscript submitted May 11, 2011. Article published online December 7, 2011 1564—VOLUME 43A, MAY 2012
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 crystallographic 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
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