Type of presentation: Oral

MS-9-O-5698 Assessing phase stability and lattice misfit in Co-base superalloys at elevated temperatures by in situ TEM heating experiments

Eggeler Y. M.¹, Müller J.¹, Fries S. G.², Spiecker E.¹

¹Center for Nanoanalysis and Electron Microscopy (CENEM), Department for Material Science, Friedrich Alexander Universität Erlangen-Nürnberg, 91058 Erlangen, Germany, ²Interdisciplinary centre for advanced materials simulation (ICAMS), Department of scale bridging Thermodynamic and Kinetic Simulation (SKTS), Ruhr-Universität Bochum, 44801 Bochum, Germany

yolita.eggeler@fau.de

Co-based alloys, of a composition of Co-12Al-9W, form a stable two phase γ/γ’ microstructure at 900 °C [1]. This microstructure is morphologically identical to the microstructure of Ni base superalloys and promises greater temperature capability due to the higher melting point of Co compared to Ni. γ’ cubes, consisting of the L12 crystal structure are coherently embedded in a solid solution fcc (A1) γ matrix. In contrast to Ni-base superalloys the lattice constant of the γ’ phase is larger than the one of the γ matrix corresponding to a positive lattice misfit. To ensure precipitate hardening at temperatures, which are relevant to practical applications, 700-1100 °C, as experienced in gas turbine applications, the stability of the γ/γ’ phases is of fundamental importance.

In this study the stability of the γ and γ’ phase as well as the lattice misfit in Co based alloys was investigated. Employing in situ heating in the transmission electron microscope (TEM) the dissolution of the γ’ precipitates is directly observed. Small tertiary γ’ precipitates in the channels start to diminish at a temperature of about 800 °C and are fully dissolved at 850 °C. The volume fraction of the γ channels increases at the expense of the γ’ precipitates, Figure 1. During heating, the different thermal expansion of γ and γ’ and the redistribution of alloying elements changes the lattice parameters and therefore the resulting lattice mismatch [2]. In contrast to lattice misfit measurements by x-ray diffraction (XRD) [3], where the contributions from γ and γ’ are often difficult to separate, the diffraction intensities of the two phases can be clearly distinguished in selected area electron diffraction patterns. Combined with in situ heating experiments this enables the determination of the local lattice misfit at various temperatures up to 950 °C .

With the γ and γ’ volume fractions, evaluated from the images, and the lattice parameters at different temperatures, a thermodynamic assessment using the software Thermo-calc , and TCNI6 database (www.thermocalc.com), shows to be in good agreement, as illustrated for the volume fraction in Figure 2.

[1] J. Sato et al, Science (2006), vol.312, p. 90

[2] Yu.N. Gornostyrev et al, Scripta Materialia (2007), vol.56, p. 81-82

[3] F.Pyczak et al, Materials Science & Engineering A (2013), vol.571, p. 13-18

The authors gratefully acknowledge the collaboration with Prof. Pollock from the University of California in Santa Barbara, GE, NSF, and the DFG priority program SFB-TR 103 for financal support.

Fig. 1: Series of TEM dark field micrographs recorded during in situ heating of a Ni containing Co-base alloy. The arrow marks the region, where the tertiary γ’ precipitates show contrast at 708 °C and 801 °C until no contrast is observed at 854 °C, where the tertiary γ’ precipitates are fully dissolved.

Fig. 2: The evolution of phases with temperature for the Ni containing Co-base superalloy shown in Figure 1 calculated with the software Thermo-calc. Dots represent the experimentally obtained volume fraction of the γ’ phase (blue) and the γ phase (red) during in situ heating.