Polycrystalline nickel-based superalloys for turbine disc applications typically employ complex alloy chemistry in order to produce a high volume fraction of gamma-prime (γ′) precipitates for the optimisation of mechanical properties [1]. The precipitate coarsening causes a gradual loss of coherency between γ′ precipitates and γ matrix when materials serving elevated temperatures, therefore resulting in the degradation of its mechanical performance [2]. In this work, we report new experimental observations for diffusion-mediated secondary γʹ precipitate coarsening (See Fig. 1) within a near-zero misfit alloy RR1000 in a cyclic manner that these precipitates coarsen and split periodically [3].

Using absorption-corrected energy-dispersive X-ray (EDX) spectroscopy within the scanning transmission electron microscope (STEM) [4], compositional variations for secondary γ′ precipitates as a function of coarsening behaviour under have been investigated. We have observed clear cyclic variations in the elemental concentrations of Co, Ti and Al within the secondary γ′ as a function of ageing time. STEM/EDX spectrum imaging and electron tomography on individual secondary γ′ have revealed local enrichment of Co within the core of secondary γ′ (See Fig. 2). STEM-EDX analysis of the γ-γ′ interface revealed nanoscale enrichment of Co and Cr and a depletion of Al and Ti within the γ matrix region near the γ-γ′ interface (See Fig. 3). Our experimental results, coupled with complementary modelling and synchrotron X-ray diffraction analysis, demonstrate the importance of elastic strain energy resulting from local compositional variations for influencing precipitate morphology. In particular, we show that elemental inhomogeneities, produced within both matrix and precipitates, are induced by complex interactions between thermodynamics and diffusion kinetics. These elemental inhomogeneities will likely affect the kinetics of coarsening and therefore must be taken into account when predicting the microstructure likely to be produced when the material is exposed to different heat treatment regimes. More generally, our findings suggest the importance of considering diffusion kinetics when attempting to understand the microstructural evolution of advanced superalloys. Our discovery renders the potential to retain the overall γ-γ′ coherence in nickel-based superalloys when exposed to elevated temperatures, and therefore to improve its creep properties.

 References:

[1] Reed, RC. Cambridge University Press, 2008.

[2] Acharya, MV et al. Materials Science and Engineering: A 381 (2004): 143-153.

[3] Chen YQ et al. Acta Materialia, (accepted) 2016.

[4] Chen, YQ, et al. Ultramicroscopy 144 (2014): 1-8.

Figures:

Fig. 1 SEM images illustrating the temporal cyclic evolution of precipitate size distributions produced for the alloy RR1000 after different ageing times from as-cooled to 7.5 h at 1073 K. The green arrows highlight where splitting has occurred at 2.0–2.5 h, at 4.5–5.0 h, and at 7.5–8.0 h annealing times.

Fig. 2 HAADF images (top), and X-ray count ratio map of Co/Ni (bottom) according to the temperature scale (right side of the maps). Comparing the ratio maps of large-scale secondary precipitate at different ageing time a) as-cooled, b) 0.5 h, c) 1.5 h and d) 7.5 h shows the reduced chemical inhomogeneities. It is noted that the limited number of small particles agglomerated onto these particles make negligible contribution to ratio maps.

Fig. 3 HAADF image a) of a FIB-sectioned thin foil and corresponding quantified EDX elemental maps of b) Cr, c) Co, d) replot of Co using a different colourmap to reveal the details within γʹ, e) Al and f) Ti. g) is the extracted elemental line profile across an interface labelled by the black hollow arrow a). Note that there is no well-defined γ-γʹ interface visible from the compositional variation.

« Back to The 16th European Microscopy Congress 2016

EMC Abstracts - https://emc-proceedings.com/abstract/on-the-diffusion-mediated-cyclic-coarsening-and-reversal-coarsening-in-an-advanced-ni-based-superalloy/