Stabilization of thermosolutal convective instabilities in Ni-based single-crystal superalloys: Carbide precipitation and rayleigh numbers

Abstract

The influence of carbide precipitation on grain-defect formation during unidirectional solidification of experimental single-crystal Ni-based superalloys has been assessed over a wide range of compositions with large variations in Re, W, and Ta. In all instances, carbon additions of up to 0.15 wt pct were determined to be statistically significant with respect to stabilizing against the formation of grain defects, such as freckle chains, during solidification. Assessment of the segregation behavior of the constituent alloying additions via a Scheil-type analysis enabled estimation of critical Rayleigh numbers denoting the onset of thermosolutal convection. Precipitation of Ta-rich MC carbides near the liquidus temperature of the alloy was found to interact strongly with the mechanisms associated with freckle formation. Segregation analyses and phase-transformation temperature measurements were used to assess the corresponding Rayleigh numbers for the experimental alloys and to modify the Rayleigh criterion to account for carbide precipitation. Mechanisms pertaining to the interaction of carbides with the onset of thermosolutal convection are discussed.

Fluid Flow and Defect Formation in the Three-Dimensional Dendritic Structure of Nickel-Based Single Crystals

“…Alternatively, the kinematic viscosity and thermal diffusivity are maintained as constants and are, as a product, considered equal to the value 5 9 10 -12 m 4 /s 2 . [11,13,14] Because this consistent approximation is used by various investigators, no discrepancies between the results of this and other investigations are introduced by its use. However, it is unlikely that kinematic viscosity, thermal diffusivity, or their product remain constant over the entire liquidus to solidus temperature range in most alloys, including those presented in this study.…”

“…The Rayleigh numbers calculated with experimentally determined permeabilities are higher than those determined with conventional permeability approximations. [11,14] Additionally, it has been shown that permeability scales inversely with interfacial surface area independent of the degree of flow path tortuosity. As a result, the dimensionless liquid ratio f…”

“…In an effort to employ consistent physical measures and provide a comparison with the permeabilities determined here (Tables I and II), a common approximation for average permeability used by various investigators [11,13,14,41] will also be considered. This approximation assumes the following form:…”

“…Thus, accurate solidification models that can predict the conditions under which these defects form as well as provide input to alloy and component design can clearly provide substantial benefits. [5,6,[8][9][10][11][12][13][14][15][16] During directional solidification, an inversion in the solute density gradient can lead to localized fluid flow, resulting in flow channels compositionally rich in interdendritic solute. [5][6][7][8][9][10][11] These channels or ''chimneys'' are the precursor event to the breakdown of singlecrystal solidification, as the local transport of molten alloy upward through the melt can fragment, erode, and transport solidified material to various locations in the melt.…”

Effect of grain boundary characteristics on hot tearing in directional solidification of superalloys

“…19 However, the enrichment of Ta in the interdendritic melt can be reduced by carbon additions because of the formation of TaC; 19-21 the ␥/␥Ј eutectic content is therefore reduced. 19 CMSX-4 has about 70% higher Ta + Al contents than IN792. In particular, CMSX-4 is carbon-free alloy, whereas IN792 is a carbon-containing alloy.…”