Intelligent alloy design: Engineering single crystals superalloys amenable for manufacture

Abstract

The solidification characteristics of high refractory content nickel based superalloy single crystals have been studied over a wide range of composition. To assess the influence of composition on distribution coefficients, quantitative segregation mapping experiments have been conducted on as cast single crystals. Additionally, differential thermal analysis was utilised to measure the composition dependence of liquidus, solidus and γ′ solvus temperatures. Over the range of composition characteristic of the most advanced commercial single crystal alloys, synergistic interactions existing between select constituent elements were found to mitigate the severity of Re and W segregation during dendritic solidification. Minor additions of carbon, in the range of 0·05–0·15 wt-%, were also found to reduce the degree of Re and W segregation during solidification. In both instances, controlling the extent of Re and W segregation during solidification was shown to strongly influence the tendency of the alloy to form macroscopic grain defects, such as freckles, during unidirectional solidification. Moreover, precipitation of primary carbides within the mushy zone during solidification was also found to impact the solidification characteristics of the alloy. The implications of these alloying approaches for developing novel, high refractory content, single crystal superalloys compositions that are amenable for manufacturing are discussed.

Characterisation of surface recrystallisation in a grit-blasted single-crystal superalloy

“…The single crystal pieces were grit blasted at a pressure of 0.25 MPa on the (001) plane for 30 s. SiO 2 spheres with a radius of 75 µm were used [19]. Considering that the superalloy substrate is required to sustain large mechanical loads at temperatures of up to 1100°C despite the use of ceramic thermal barrier coatings [20], the grit-blasted pieces were tubed in vacuum silica glass, and then subjected to heat treatment at 1100°C with annealing time of 2 and 4 h followed by air cooling to evaluate their thermal stability.…”

Stacking faulted P phase in a grit-blasted Ni-based single-crystal superalloy after thermal exposure

“…The requirements for components in gas turbine engines have been met by nickel-based single-crystal superalloys for decades, owing to their excellent hightemperature properties above 1000°C [1][2][3]. The DD6 alloy is a second-generation nickel-based single-crystal superalloy containing 2 wt-% Re [4], with tensile and creep rupture properties comparable to CMSX-4 [5].…”