Single crystal superalloys are used for turbine blades in the hottest parts of gas engines, where they have to withstand high stresses at temperatures exceeding 1000°C. This is only possible because of the well-known γ/γ‘-microstructure of these alloys, forming the basis for excellent creep properties [1]. One of the research objectives is to gain a microscopic understanding of the elementary deformation mechanisms and defect processes that govern high-temperature creep and to use this knowledge for further improvement of single crystal superalloys. Beside Ni-base superalloys which are well established in applications new Co-base superalloys have attracted a lot of research interest since the discovery of stable γ/γ‘-microstructure in these alloys in 2006 [2]. Research on Co-base superalloys is still in its infancy, meaning that there is strong need for fundamental studies on elementary deformation mechanisms and defect processes.

In this contribution we give an overview of our recent research on extended defects that form upon high-temperature creep in Ni- and Co-base superalloys. We demonstrate that the combination of advanced transmission electron microscopy (TEM) techniques, employing dedicated diffraction, high-resolution and analytical methods, is generally required for full characterization of such extended defects and for obtaining a complete picture of their role in elementary creep processes.   

Figure 1 illustrates an example of full characterization of superdislocations in a Ni-base superalloy. First, large-angle convergent beam electron diffraction (LACBED) is employed for reliable determination of the Burgers vector [3], overcoming the well-known problems of conventional Burgers vector analysis of such dislocations. Secondly, using a new FIB cross-sectioning technique [4], the core structure of the same dislocation is studied by high-resolution scanning TEM (HRSTEM). This approach enables scale-bridging characterization of complex superdislocations on the micrometer and atomic scale.

Figures 2 and 3 summarize advanced TEM studies on extended faults that form upon creep of Ni-containing Co-base (CoNi) superalloys [5]. Inside of γ‘ precipitates a characteristic defect configuration is observed in which  superintrinsic stacking fault (SISF) loops are fully embedded in antiphase boundaries (APB). LACBED is used to determine the full Burgers vector (including sign and magnitude!) of the partial dislocation loop [5]. Pronounced segregation of alloying elements at SISF and APB is revealed by HRSTEM and energy-dispersive X-ray spectroscopy (EDXS). Based on these and other results, an elementary creep mechanism is proposed which is characterized by ½<112> slip involving a SISF→APB transformation [5]. Atomic diffusion at creep temperature appears to be a rate limiting step by gradually changing the fault energies which control the kinetics of the transformation.

[1] R.C. Reed, The Superalloys: Fundamentals and Applications, Cambridge University Press, Cambridge, 2008.

[2] J. Sato, T. Omori, K. Oikawa, I. Ohnuma, R. Kainuma and K. Ishida, Cobalt-base high-temperature alloys, Science (2006), 312, pp. 90-91.

[3] J. Müller, G. Eggeler, and E. Spiecker, On the identification of superdislocations in the γ′-phase of single-crystal Ni-base superalloys – An application of the LACBED method to complex microstructures,  Acta Materialia (2016), 87, pp. 34-44.

[4] J. Müller and E. Spiecker, Correlative microscale and atomic scale characterization of individual defects and interfaces in Ni-base superalloys, Proceedings of the Microscopy Conference MC 2015, 6-11 September 2015, Göttingen, Germany, MS4.076, pp. 146-147.

[5] Y.M. Eggeler, J. Müller, M.S. Titus, A. Suzuki, T.M. Pollock, and E. Spiecker, Planar defect formation in the γ′ phase during high temperature creep in single crystal CoNi-base superalloys, Acta Materialia (2016), 113, pp. 335-349.

Financial support by the German Research Foundation (DFG) through the SFB-TR 103 “Single Crystal Superalloys” and the Cluster of Excellence EXC 315 “Engineering of Advanced Materials” is gratefully acknowledged.

Figures:

Figure 1: Full characterization of superdislocation in Ni-base superalloy combining plan-view LACBED for Burgers vector analysis [3] with site-specific FIB cross-sectioning and HRSTEM imaging of dislocation core structure [4].

Figure 2: Planar defect bands with characteristic SISF loops in creep-deformed CoNi superalloy: Confirming elementary creep mechanism by LACBED analysis of sign and magnitude of Burgers vector [5].

Figure 3: Revealing segregation of alloying elements at planar defects (SISF, APB) in creep-deformed CoNi superalloy by combined Z contrast imaging (HRSTEM) and local chemical analysis (ChemiSTEM) [5].

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EMC Abstracts - https://emc-proceedings.com/abstract/combining-advanced-tem-techniques-for-full-characterization-of-extended-defects-in-creep-deformed-single-crystal-superalloys/