Abstract
Different cooling paths from a supersolvus temperature have been applied to FGH96, a polycrystalline nickel base superalloy for turbine disc applications, in order to simulate the different microstructures that exist through the thickness of a disc following an industrial heat treatment. Secondary and tertiary γ′ precipitate size distributions and morphology have been analysed and compared for the different heat treatments using SEM and atom probe tomography (APT). Detailed compositional data for both γ′ precipitate and γ matrix are presented, and compared to equilibrium compositions calculated by Thermo-Calc. For the heat-treatments studied, the secondary γ′ composition indicates a shell of differing composition to that towards the precipitate core. From sequential equilibria compositional calculations, it is suggested that the ‘shell’ forms at a lower temperature than the precipitate core. The fine tertiary precipitates do not show the core-shell compositional differences on continuous cooling. W peaks are noted at the γ/γ′ interfacial region, which is of significance for retarding coarsening. A γ′ depletion zone surrounds the secondary precipitates, within which the γ matrix composition differs significantly to the γ far-field values, Finally, a precipitate nucleation and growth mechanistic model is suggested based on the experimental data and Thermo-Calc calculations.
Original language | English (US) |
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Pages (from-to) | 765-776 |
Number of pages | 12 |
Journal | Journal of Alloys and Compounds |
Volume | 732 |
DOIs | |
State | Published - Jan 25 2018 |
Funding
The authors are grateful to Beijing Institute of Aeronautical Materials Co. , AVIC for the material used in this study and the financial support. Atom-probe tomography measurements were performed at the Northwestern University Centre for Atom-Probe Tomography (NUCAPT) and the LEAP tomograph was purchased and upgraded with funding from the NSF-MRI ( DMR 0420532 ) and ONR DURIP ( N00014-0400798 , N00014-0610539 , N00014-0910781 ) programs. NUCAPT is a Shared Facility of the Materials Research Centre of the Northwestern University, supported by the National Science Foundation's MRSEC program ( DMR-1121262 ). We are also grateful to the Initiative for Sustainability and Energy at Northwestern (ISEN) for grants to upgrade NUCAPT's capabilities.
Keywords
- Atom probe tomography
- Nickel base superalloy
- Nucleation and growth
- Particle size distribution
- γ/γ′ composition
ASJC Scopus subject areas
- Mechanics of Materials
- Mechanical Engineering
- Metals and Alloys
- Materials Chemistry