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Ni-base single-crystal (SC) superalloys have excellent high temperature properties. In order to maximize their properties, it is necessary to homogenize the dendritic segregation by solution heat treatment. On the other hand, to reduce the processing cost, lower temperatures and shorter heat treatment times are required. Therefore, methods to predict the homogenization during heat treatment are needed. The objective of this research is to establish an equation for time dependence of segregation index.
The nominal composition of TMS-238 is Ni–4.6Cr–6.5Co–4.0W–1.1Mo–5.9Al–7.6Ta–0.1Hf–6.4Re–5.0Ru(wt%). SC ingot was cast in a directional solidification furnace. The specimens were solution heat treated at temperatures ranging from 1300 to 1340°C for times ranging from 2 to 100 h. The segregation index δ(=(Cd-Cid)/(Cd0-Cid0)), which is used to evaluate homogenization during solution heat treatment, was calculated. Cd and Cid represent the maximum and minimum element concentrations in the dendrite core and inter-dendritic region after heat-treated specimens, Cd0 and Cid0 are Cd and Cid of the as-cast ingot, respectively. These concentrations were obtained by EPMA analysis.

The results show that the segregation of Re was most severe in alloying elements, and then the homogenization rate was the smallest. For this reason, the apparent diffusion coefficient of Re(Dapp,Re) was used in this study as a representative value for homogenization of TMS-238. The linear equation describing the time dependence of lnδ at 1300°C can be expressed by applying of Dapp,Re=2.41×10-16 m2/s. This was about 1/12 of the diffusion coefficient of Re in pure Ni at the same temperature, most likely due to the interaction of alloying elements. Dapp,Re can be used for prediction of homogenization during solution heat treatment.

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  Copyright year; 2024 ASM International. This article was published in Advances in Materials, Manufacturing, and Repair for Power Plants: Proceedings from the Tenth International Conference and is made available as an electronic reprint with the permission of ASM International. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplications of any material in this article for a fee or for commercial purposes, or modification of the content of this article is prohibited.

キーワード: apparent diffusion constant, dendritic segregations, homogenization, nickel single crystal superalloys, solution heat treatment

会議: 10th International Conference on Advances in Materials, Manufacturing & Repair for Power Plants - EPRI 2024 (2025-02-25 - 2025-02-28)

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更新時刻: 2025-03-14 16:30:18 +0900

MDRでの公開時刻: 2025-03-14 16:30:19 +0900