Characterization of Three-Dimensional Dendritic Structures in Nickel-Base Single Crystals for Investigation of Defect Formation

Abstract

During solidification, solute-induced convective instabilities at the solid -liquid interface can result in the formation of defects such as freckles and misoriented grains in superalloy single crystals. These defects can be particularly detrimental to the properties of single crystal nickel-base superalloys. Unfortunately, detailed understanding of fluid flow at the scale of the dendritic structure has yet to be fully understood, particularly under conditions in which heat extraction is non-axial. The objective of this research is to develop a technique for quantifying the dendritic structure and morphology at the solid -liquid interface for the purpose of providing direct input into computational fluid flow modeling. Using the RoboMET.3D serial sectioning system, threedimensional datasets of dendritic structure at the solidification front have been obtained for René N4 abruptly decanted during solidification. Distribution and arrangement of solid and liquid in the vicinity of dendrite tips is analyzed and the implications for defect formation are discussed.

Modeling fluid flow in three-dimensional single crystal dendritic structures

“…As a result, contiguous regions of cross flow across the data set were unavailable for heights corresponding to f S > 0.6. Nevertheless, the dependence of PDAS and SDAS on cooling rate [34,35] suggested that the decanting process did not significantly influence the structure in the upper half of the mushy zone. Thus the dependence of permeability on volume fraction and dendritic morphology would extend to a fully decanted structure.…”

“…In addition, isolated porosity was identified with a volume fraction in the range typical of single crystal nickel-base castings [31][32][33]. Additional details of the reconstruction and initial characterization of resulting structures have been presented elsewhere [34][35][36]. Here, the focus will be on examination and description of dendritic morphology and its effect on local fluid flow.…”

Porosity in millimeter-scale welds of stainless steel : three-dimensional characterization.

“…Their contribution was the first instance of a self-contained, fully automated, metallographic serial-sectioning system inclusive of preparation, imaging and closed-loop depth removal measurement. In a few years following, Spowart and Mullens would present a solution for fully automated serial-sectioning across a broader variety of systems including very strong and formidable materials such as nickel-base superalloys, human bone, discontinuously reinforced composites and pitch-based carbon foams [31][32][33][34][35][36] . Spowart and Mullens would eventually go on to license their process and the system itself would later become commercially available under the trademark name of Robo-MET.3D…”

Localisation of convection in mushy layers by weak background flow

“…Fowler 1985;Worster 1992), are further susceptible to convective instabilities that can result in the formation of chimneys -dendrite free regions that are observed in the frozen sample as defects called freckles (e.g. Madison et al 2008). In extended systems, chimneys may form a spatially periodic array.…”