The Effects of Supersaturation on the Nucleation of Precipitates in Nickel-Based Superalloys

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The purpose of this study was to select an alloy with a low driving force for nucleation for a more in-depth study of the nucleation of γ’-precipitates. To do this, three Ni-Cr-Al alloys were chosen close to the solvus line of a Ni-Cr-Al phase diagram, ensuring low supersaturation and therefore a low driving force for precipitate growth. The three alloys chosen — 03-14, 03-15, and 03-16 (Ni-9.0 Cr-7.0 Al atomic percent (at.%), Ni-9.25 Cr-6.75 Al at.%, and Ni-9.5 Cr-6.5 Al at.%, respectively) — were aged at 600° C for 0.25 hours in order to observe forming precipitate nuclei in both the γ-matrix and γ’- precipitates. Three-dimensional atom probe (3DAP) microscopy was performed on these alloys to produce three-dimensional reconstructions of their atomic arrangement to determine the nanostructure and the chemical composition of both the γ and γ’ phases. The results for volume fraction, number density, and mean radius of the precipitates, as well as the concentrations of both the matrix and the precipitate phases, were then determined for the 03-15 and 03-16 alloys by analyzing the 3DAP microscopy data. These attributes were previously established for alloy 03-14 in unpublished results by T. Ziebell and K. Yoon. From these results, all three of these alloys were observed to have a lower driving force for nucleation than the previously studied Ni-14.8 Cr-5.2 Al at.% alloy.1 Of those three, 03-16 had the lowest Al supersaturation, thereby also having the lowest driving force for nucleation. Given this, alloy 03-16 will be used for a more direct study of the nucleation of precipitates. Transmission electron microscopy (TEM) was also used to confirm the structure of the precipitates. The resultant diffraction pattern was conclusive in showing that the precipitates observed in the three-dimensional reconstructions were in fact L12 ordered Ni3Al γ’-precipitates, due to the appearance of superlattice reflections. The presence of γ’-precipitates in these superalloys is important, as L12 ordered precipitates help to strengthen the alloy by impeding the motion of dislocations through the γ-matrix

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  • 07/16/2018
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