Optimization of the Homogenization Heat Treatment of Nickel-Based Superalloys Based on Phase-Field Simulations: Numerical Methods and Experimental Validation

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Web End = Optimization of the Homogenization Heat Treatment of Nickel-Based Superalloys Based on Phase-Field Simulations: Numerical Methods and Experimental Validation

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Web End = RALF RETTIG, NILS C. RITTER, FRANK MLLER, MARTIN M. FRANKE, and ROBERT F. SINGER

A method for predicting the fastest possible homogenization treatment of the as-cast microstructure of nickel-based superalloys is presented and compared with experimental results for the single-crystal superalloy ERBO/1. The computational prediction method is based on phase-eld simulations. Experimentally determined compositional elds of the as-cast microstructure from microprobe measurements are being used as input data. The software program MICRESS is employed to account for multicomponent diusion, dissolution of the eutectic phases, nucleation, and growth of liquid phase (incipient melting). The optimization itself is performed using an iterative algorithm that increases the temperature in such a way that the microstructural state is always very close to the incipient melting limit. Maps are derived allowing describing the dissolution of primary c/c-islands and the elimination of residual segregation with respect to temperature and time.

DOI: 10.1007/s11661-015-3130-y The Minerals, Metals & Materials Society and ASM International 2015

I. INTRODUCTION

SUPERALLOY castings require proper heat treatment[1] because their microstructure is dendritic[2] and consequently suers from microsegregation of the alloying elements, low melting eutectic phases, and nonide-ally shaped hardening c-precipitates.[35] It is still under discussion if the nal steps of solidication correspond to a eutectic or a peritectic reaction.[610] Also, discontinuous precipitation has been suggested to contribute to the microstructural formation during the solidication 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 solidied remaining melt is concerned. Sometimes, the ne 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.[1315] Often three-step heat treatments are applied. During the initial solution heat-treatment step (typically ranging from 1523 K to 1613 K...