Abstract
Size-dependent compositional variations under different cooling regimes have been investigated for ordered L12-structured gamma prime (γ′) precipitates in the commercial powder metallurgy Ni-based superalloy RR1000. Using scanning transmission electron microscope imaging combined with absorption-corrected energy-dispersive X-ray spectroscopy, we have discovered large differences in the Al, Ti and Co compositions for γ′ precipitates in the size range 10–300nm. Our experimental results, coupled with complementary thermodynamic calculations, demonstrate the importance of kinetic factors on precipitate composition in Ni-based superalloys. In particular, these results provide new evidence for the role of elemental diffusion kinetics and aluminium antisite atoms on the low-temperature growth kinetics of fine-scale γ′ precipitates. Our findings have important implications for understanding the microstructure and precipitation behaviour of Ni-based superalloys, suggesting a transition in the mechanism of vacancy-mediated diffusion of Al from intrasublattice exchange at high temperatures to intersublattice antisite-assisted exchange at low temperatures.
Highlights
Polycrystalline nickel-based superalloys for turbine disc applications typically employ complex alloy chemistry in order to produce the required properties
Using scanning transmission electron microscope (STEM) imaging combined with absorption-corrected energy-dispersive X-ray (EDX) spectroscopy we have discovered large differences in the Al, Ti and Co compositions for γ′ precipitates in the size range 10 to 300 nm
Absorption-corrected EDX spectroscopy in the S/TEM has been performed on a large number of extracted γ′ precipitates in order to study the size dependant compositional variations present for the powder metallurgy (PM) Ni-based superalloy RR1000 at different solution cooling rates
Summary
Polycrystalline nickel-based superalloys for turbine disc applications typically employ complex alloy chemistry in order to produce the required properties. Changes in the PSD and/or phase chemistry have a direct impact on material performance and controlling the mechanical properties requires accurate understanding of precipitate phase chemistry and its evolution during cooling. Further cooling results in the growth of secondary γ′ precipitates until the low elemental diffusivities of the γ′ stabilising elements make it difficult for these elements to reach the comparatively coarse (hundreds of nanometres) secondary γ′. This results in supersaturation of these elements within the γ matrix and drives the nucleation of additional intragranular γ′
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