Nickel-Based / Superalloy - Homogenization during solution heat treatment of Ni-base single-crystal superalloy TMS-238

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(=(C_d-C_id)/(C_d⁰-C_id⁰)), which is used to evaluate homogenization during solution heat treatment, was calculated. C_d and C_id represent the maximum and minimum element concentrations in the dendrite core and inter-dendritic region after heat-treated specimens, C_d⁰ and C_id⁰ are C_d and C_id 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(D_app,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 D_app,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. D_app,Re can be used for prediction of homogenization during solution heat treatment.

This paper is based on results obtained from a project, JPNP21007, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).