Insights into the microstructure and high temperature creep mechanism of Hafnium doped fourth-generation single crystal superalloy

The influence of minor Hf on the microstructure and creep deformation mechanism at 1100 ºC/150 MPa was systematically investigated in a fourth-generation single crystal superalloy. Although minor Hf optimized the alloy microstructures and reduced the volume fraction of solidification pores (S-pores), the creep life decreased. An increase in Hf content induced the morphological transition of MC-type carbides from script-shaped to block-shaped, driven by the enhanced lattice misfit of carbide/matrix resulting from Hf enrichment within the carbide phase. After standard heat treatment, MC-type carbides in both two alloys transformed into small block-shaped and more Hf atoms entered it which was verified by first-principles calculation. Interestingly, the strongest diffusivity belonging to Hf element aggravated the formation of homogenization pores (H-pores) near carbide and eutectic regions. Subsequently, the interconnect cavities originated from H-pores, carbides and recrystallization boundaries resulted in the prematurely fracture and the decreased creep life of tertiary stage. Particularly, an increased Hf content decreased the lattice misfit of γ/γ′ and led to the formation of sparser dislocation network, thereby weakening the interfacial strengthening effect and exacerbating the cutting of superdislocations which decreased the creep life of secondary stage.