Discontinuous cellular precipitation in a high-refractory nickel-base superalloy
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I.
INTRODUCTION
THE phenomenon of discontinuous precipitation has been documented in numerous alloy systems. Despite differences in the morphological development of discontinuous transformations in different alloy systems, most discontinuous precipitation (DP) reactions have common features in terms of reaction kinetics and driving forces.[1–5] In DP reactions, the metastable parent microstructure is transformed through a combined mechanism of boundary precipitation and interfacial migration, which accelerates the kinetics of the stabilization reaction. The reaction interface, most commonly a grain boundary in polycrystalline material,[6] serves initially as a heterogeneous nucleation site and subsequently as a high-diffusivity mobile reaction front. An abrupt change in constituent phase morphology, chemistry, and crystallographic orientation is apparent at the advancing reaction interface. Discontinuous precipitation is driven by local reductions in chemical free energy.[1,3] Additionally, arguments have been advanced that strain energy reduction also provides a significant driving force for the DP reaction.[7,8] The microstructural disruption associated with the reaction cell typically reflects adversely on mechanical properties of the material. The need for improved elevated temperature mechanical properties in nickel-base single-crystal alloys has led to increasingly higher levels of Group VIB and VIIB refractory J.D. NYSTROM, formerly Graduate Research Assistant with the Department of Materials Science and Engineering, Carnegie Mellon University, is Graduate Student, Massachusetts Institute of Technology, Cambridge, MA 02139. T.M. POLLOCK, Associate Professor, is with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. W.H. MURPHY, Research Engineer, is with the Engineering Materials and Technologies Laboratory, General Electric Aircraft Engines, Cincinnati, OH 45215. A. GARG, formerly Research Associate with the Materials Science and Engineering Department, Carnegie Mellon University, is Research Scientist, NasaLewis Research Center, Cleveland, OH 44135. Manuscript submitted December 11, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
alloying additions (Mo, W, and Re). However, with increasing levels of refractory element additions, phase stability in these single-crystal alloys becomes a concern due to lack of knowledge regarding solubility limits and precipitation kinetics. Precipitation of refractory-rich topological close-packed (TCP) phases may occur only after extended periods of elevated temperature exposure.[9,10,11] Their formation typically results in degradation of mechanical properties through depletion of strengthening additions and/or by providing fracture initiation sites.[9] In single-crystal superalloys containing Re, three different Re-enriched TCP phases have been observed: tetragonal s phase (P42/mnm), rhombohedral m (R3m) phase, and orthorhombic P phase (Pnma).[12,13] These phases may precipitate as isolated plates or in a Widmansta¨
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