Silicon-induced incipient melting and resolidification as the origin of secondary surface pit grains in nickel-based single crystal superalloy turbine blades

A previously unreported type of surface defect has been identified in nickel-based single crystal turbine blades: localised surface pit grains that emerge after standard post-casting treatments, occurring exclusively within regions previously covered by surface scale. Detailed characterisation using focused ion beam (FIB) sectioning, high-resolution electron microscopy, and micro-XPS analysis reveals that each pit consists of two distinct regions: a central Zone I containing blocky rhenium-rich particles and needle-shaped topologically close-packed (TCP) particles embedded in a γ′ matrix, and a surrounding Zone II characterized by fine γ-phase particles and a gradually recovering γ/γ′ microstructure. XPS analysis confirmed significant silicon enrichment within Zone I , demonstrating that silicon originates from the chemical reaction between the molten alloy and the silica-based mould wall during investment casting. This localised silicon enrichment is proposed to depress the local solidus temperature, leading to incipient melting and subsequent resolidification during heat treatment. The resulting melting-solidification phenomenon produces a misoriented secondary grain ( Zone I ) and a diffusion-controlled transition region ( Zone II ) depleted in γ-forming elements. These findings establish that silicon-assisted localised melting and resolidification, rather than mechanical damage, are responsible for the formation of these secondary surface pit grains. This study provides new insights into mould-metal interactions and microstructural degradation phenomena, emphasising the need to minimise silicon contamination and optimise heat treatment conditions to prevent such surface defects.