Formation of the polycrystalline structure in the interdiffusion zone of a PtAl-coated third-generation SX superalloy

Abstract

The polycrystalline structure of the interdiffusion zone (IDZ) significantly affects the mechanical properties of the PtAl-coated single crystal (SX) superalloys, while its formation mechanism has not been clarified yet. This study systematically investigated the formation of the polycrystalline structure in the IDZ of a PtAl-coated third-generation SX superalloy by analyzing the interdiffusion and microstructural evolution during the coating deposition process, which mainly consists of grit blasting, Pt electroplating, interdiffusion at 900 °C, and aluminizing. The results indicated that the polycrystalline structure in the IDZ was formed during interdiffusion between the Pt plating layer and the SX superalloy, and was determined by the diffusion rather than the grit blasting. Notably, the phase transformation from cuboidal γ/γ’ phases to Pt-rich γ₁/γ’₁ phases with irregular shapes occurred in the IDZ during interdiffusion at 900 °C, and the phase boundaries and grain boundaries between the IDZ and the SX superalloy coincided. The formation of the polycrystalline IDZ was mainly attributed to the grain boundary (GB) migration induced by the phase transformation, similar to the discontinuous precipitation. Specifically, Pt diffused from the nano-crystalline Pt-plating layer to the SX superalloy mainly along the GBs, inducing the nucleation of Pt-rich γ₁/γ’₁ phases at the GBs. Subsequently, γ₁/γ’₁ phases grew in the direction of Pt diffusion, and the GBs migrated along the growth direction of γ₁/γ’₁ phases concurrently. This study will be helpful for reducing the formation of the polycrystalline structure in the IDZ of the PtAl-coated SX superalloy.