Single-crystal superalloys are widely used in aircraft engines owing to their excellent high-temperature performance. As key factors in controlling the formation of microstructures, solidification conditions directly influence the comprehensive properties of alloys. Therefore, it is of great significance to understand the mechanisms of alloy microstructure evolution under different solidification conditions and to optimize the solidification process to improve the performance of advanced aircraft engine hot-section components. To investigate the effects of the solidification conditions on the evolution of as-cast and heat-treated microstructures in the second-generation single-crystal superalloy DD6, single-crystal bars oriented along the [001] direction were prepared using high-rate solidification (HRS) and liquid metal cooling (LMC) processes. The results showed that in the as-cast state, the γ′ phases in the dendritic core of the HRS alloy exhibited a relatively regular cubic shape, whereas those in the LMC alloy were irregularly cubic; the γ′ phases in the interdendritic regions of the HRS and LMC alloys were irregularly cubic and larger in size than those in the dendritic core. As the pouring temperature increased, the size of γ′ phases in the HRS alloy first increased and then decreased, reaching a maximum at 1560 ^oC. At the same pouring temperature of 1590 ^oC, the size of the γ′ phases in the LMC alloy was smaller. After the heat treatment, the volume fraction of the γ + γ′ eutectic under all conditions decreased significantly. A trend was observed where the smaller the size of the γ + γ′ eutectic in the as-cast alloy, the greater the reduction in its content, regardless of the initial γ + γ′ eutectic content in the as-cast state. In the heat-treated HRS and LMC alloys, the γ′ phases in the dendritic core and interdendritic regions were uniformly distributed and regularly arranged, and they demonstrated good cubicity. Furthermore, the size uniformity of the γ′ particles in the LMC alloy was superior to that in the HRS alloy. The differences in the microstructures of the as-cast alloys were primarily caused by differences in the temperature gradient during directional solidification. Therefore, increasing the pouring temperature or employing LMC to enhance the temperature gradient helped to refine the dendritic structure of the alloy, reduce microsegregation, and this can simplify the alloy's heat treatment process.