Grain growth and carbide precipitation in superalloy, UDIMET 520
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INTRODUCTION
THE properties of a material are determined by its chemical composition and microstructure. In superalloys, besides the size and distribution of the hardening intragranular precipitates and the dislocation substructure, the controlling microstructural features include the size and distribution of grains, the size, morphology, distribution, and nature of grain boundary precipitates, and whether the grain boundary morphology is planar or serrated. The grain size plays an important role in controlling the mechanical properties of superalloys, including the creep rupture[1] and the creep crack growth properties.[2] The influence of grain boundary precipitates (for example, M23C6 carbides) on the properties of Ni-base superalloys has also been recognized[1,3,4] The kinetics of grain boundary precipitation and the precipitate distribution are critical. In order to optimize the creep rupture properties, a discrete distribution of grain boundary M23C6 carbides is usually preferred in superalloys after standard heat treatments. The importance of these microstructural features has been discussed at length in a recent comprehensive review article.[5] In addition to grain boundary M23C6 carbides, the MC carbide precipitation reactions also influence the properties of superalloys.[6,7] The primary MC carbides form during solidification. The secondary MC carbides may precipitate during an annealing or aging treatment at temperatures below the MC carbide solvus temperature, after cooling from a solution annealing temperature above this solvus temperature. The precipitation kinetics of both M23C6 and MC phases are mainly influenced by the chemical composition and the microstructural state prior to aging. The microstructural state prior to aging is governed by S. XU, formerly Ph.D. Student at Ecole Polytechnique de Montreal, is with the Materials Technology Laboratory, CANMET, Natural Resources Canada, Ottawa, ON, Canada K1A 0G1. J.I. DICKSON, Professor, is with the Department of Metallurgy and Materials Engineering, Ecole Polytechnique de Montreal, Montreal, PQ, Canada H3C 3A7. A.K. KOUL, Senior Research Officer, is with the Structures, Materials and Propulsion Laboratory, Institute for Aerospace Research, National Research Council of Canada, Ottawa, ON, Canada K1A 0R6. Manuscript submitted April 28, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
the solution treatment conditions employed and the relative magnitude of the solutionizing temperature with respect to the grain coarsening temperature (GCT). The GCT of an alloy is defined as the transition temperature above which grain growth occurs very rapidly within practical times.[5] The GCT of an alloy containing a high g' volume fraction is often related to the solvus temperature of either g' phase or MC carbides.[8] Grain growth in low g' volume fraction forged alloys usually begins when the solution temperature is sufficiently high to dissolve the primary carbides in the grain boundary regions. As the temperature surpasses the MC carbide solvus temperature,
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