Observations and analysis of the influence of phase transformations on the instability of the thermally grown oxide in a
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rier systems used in gas turbines have been comprehensively described in recent overview articles.[1–6] These assessments have elaborated the benefits of, and confronted the need for, a system level approach to design and durability. The durability emphasis has been on failure modes governed by the thermally grown oxide (TGO), predominantly ␣ -Al2O3, that forms between the thermal barrier coating (TBC) and the bond coat.[7] This thin layer develops large residual compressive stress because of growth processes and thermal expansion misfit, causing the layer to be unstable against out-of-plane displacements[4,8] (Figure 1). The occurrence of the instability is largely dictated by the mechanical characteristics of the bond coat.[9,10] Nonplanarity and imperfections in the TGO are also important.[11,12] Bond coats most susceptible to this instability are those with compositions in the  -phase field, because of the relatively inferior high-temperature strength of this phase. The most widely documented example occurs in the bond coat referred to as Pt-aluminide,[9,11,13,14] illustrated with the Ni-Al-Pt ternary phase diagram (Figure 2).[15] These  -phase bond coats are susceptible to phase transformations as the Al content depletes during thermal cycling. One such transformation arises because the composition translates from a single-phase  to a two-phase  /␥ ⬘ field[5,16] (Figure 2). It has two potentially significant roles in affecting the performance of the thermal barrier system. One role relates to the different creep/yield strengths of S. DARZENS, Research Associate, and D.R. MUMM, Research Staff Scientist and Lecturer, are with the Princeton Materials Institute, Princeton University, Princeton, NJ 08540-5211. Contact e-mail: sdarzens@yahoo. com D.R. CLARKE and A.G. EVANS, Professors, are with the Materials Department, University of California-Santa Barbara, Santa Barbara, CA 93106. Manuscript submitted May 13, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
the two phases, which affect the ability of the material to accommodate the TGO instability.[8] The other role is associated with the volume strain induced by the phase transformation from  → ␥ ⬘[16] as well as from austenite to martensite in the  phase upon cooling, as evidenced by Chen et al.[17] This strain can interact with the thermal expansion misfit, and the TGO growth, as well as the cyclic plastic straining of the bond coat, to enhance the propagation of the instability. The objective of this article is to characterize the spatial features of the  → ␥ ⬘ transformation, as well as the associated strain fields, and to relate this distribution to the sites where the TGO instability occurs. This spatial information is used in a model that is under development, which addresses the importance of the transformation relative to other effects that contribute to the instability.
II. PROCEDURES The samples analyzed are from among a group described elsewhere.[8] They comprise Rene´ N5 single-crystal superalloy disks (material developed and commercialized b
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