Optimization of the Homogenization Heat Treatment of Nickel-Based Superalloys Based on Phase-Field Simulations: Numerica
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TRODUCTION
SUPERALLOY castings require proper heat treatment[1] because their microstructure is dendritic[2] and consequently suffers from microsegregation of the alloying elements, low melting eutectic phases, and nonideally shaped hardening c¢-precipitates.[3–5] It is still under discussion if the final steps of solidification correspond to a eutectic or a peritectic reaction.[6–10] Also, discontinuous precipitation has been suggested to contribute to the microstructural formation during the solidification of the remaining melt.[11] Despite this discussion, within the context of the current study, we will always refer to ‘‘c/c¢-eutectics’’ if the solidified remaining melt is concerned. Sometimes, the fine eutectic structure is not visible. This is why also the expression ‘‘primary c/c¢-islands’’ is being used. This microstructure has been well described in the past, for example, by Pang et al.[12] In general, incipient melting during heat treatment is considered as detrimental to the mechanical properties.[13–15] Often three-step heat treatments are applied. During the initial solution heat-treatment step (typically ranging from 1523 K to 1613 K (1250 C to 1340 C) RALF RETTIG, Head of High Temperature Materials Group, NILS C. RITTER, Research Associate, FRANK MU¨LLER, Graduate Student, and ROBERT F. SINGER, Professor and Head of Institute, are with the Institute for Science and Technology of Metals, Department of Materials Science and Engineering, University of Erlangen, Martensstr. 5, 91058 Erlangen, Germany. Contact e-mail: [email protected] MARTIN M. FRANKE, Project Manager Additive Manufacturing, is with the Neue Materialien Fu¨rth GmbH, Dr.Mack-Str. 81, 90762 Fu¨rth, Germany. Manuscript submitted April 24, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
for up to 16 hours), microsegregation and low melting eutectic pools are removed and the c¢-precipitates are fully dissolved. The following two steps are conducted at intermediate temperatures (typically ranging from 1323 K to 1423 K (1050 C to 1150 C) for 2 hours, followed by temperatures ranging from 1073 K to 1173 K (800 C to 900 C) for 24 hours) in order to develop the well-defined cubic c¢-precipitate microstructure.[16,17] The current study will focus on the initial solution heat-treatment (i.e., the homogenization) step. In the last two decades, development of ever-stronger single-crystal nickel-based superalloys for the most demanding applications in aero engine and stationary gas turbine blades was accomplished with the high refractory element additions (rhenium and ruthenium) in the 2nd through 4th single crystal alloy generations.[18,19] At the same time, grain boundary hardening minor elements like carbon or boron have been removed.[16] The consequence of refractory element additions is a change of requirements during the development of the solution heat treatment of these alloys. A full homogenization of rhenium microsegregation is not possible due to its extraordinarily low diffusion coefficient, which is two orders of magnitude lower than that of
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