Quantitative study of thickness debit effect on the microstructure and elemental segregation characteristics in as-cast and heat-treated nickel-based single crystal superalloys

Reducing blade wall thickness has become essential with advancements in blade cooling technologies such as air film cooling, impact cooling, and transient cooling. Heat dissipation is improved by this reduction, which significantly extends the lifespan of the blades. The impact of thickness variations on the microstructure and elemental segregation of as-cast and heat-treated nickel-based single crystal superalloys is investigated in this study. Thin-walled specimens of four different thicknesses (1.513 mm, 2.370 mm, 3.060 mm, and 5.790 mm) were directly cast using a shell mold equipped with varying cavity thicknesses. The microstructure and elemental segregation of these samples were quantitatively analyzed using optical microscope (OM), scanning electron microscope (SEM), and energy-dispersive spectroscopy (EDS). This research not only involves thin-walled specimens of various thicknesses but also includes specimens of the same thickness positioned at different distances from the casting core. Experimental data demonstrate that the size of the γ′ phase follows the log-normal distribution, and its circularity conforms to the four-parameter Dagum distribution. In the as-cast specimens, increases of the thickness from 1.513 mm to 5.790 mm led to 33.7 % increase in the average primary dendrite arm spacing, 86 % increase in the average percentage of the eutectic structure, 44.2 % increase in the average γ′ phase size, and 8 % decrease in the average circularity of the γ′ phase. The degree of elemental segregation also increases, particularly for elements such as W and Re that exhibit significant increases in segregation as the sample thickness increases. The differences in casting thickness lead to initial state differences. Approximately 95 % of the eutectic structures are eliminated during heat treatment, but the efficacy of the process is constrained by differences in the initial state of the alloy. The greater the initial state heterogeneity, the more stringent the required heat treatment time and conditions, and the more limited the effectiveness of the heat treatment. The final microstructure and elemental distribution are the results of the coupled effects of the initial state and heat treatment. In the heat-treated samples, as the thickness increases from 1.513 mm to 5.790 mm, the average primary dendrite arm spacing increases by 33.5 %, the average percentage of eutectic structures increases by 275 %, the average size of the strengthening phase (γ′ phase) increases by 66.7 %, and the average circularity of the γ′ phase decreases by 33 %. The variations in microstructural parameters for specimens at different distances from the casting core also show regular patterns. Key findings indicate that decrease in casting thickness significantly affects γ` phase characteristics, elemental segregation, and microstructural uniformity. These findings can provide valuable guidance for optimizing the design and manufacturing processes of the nickel- based single crystal alloys.