Nano- and Microstructural Evolution in Ni- and Co-based Single Crystal Superalloys

The present work uses advanced analytical transmission electron microscopy (TEM) to analyze elementary processes, which govern the microstructural evolution in the γ/γ’-microstructures of Ni- and Co-base superalloy single crystals (SX). Many critical micro-structural processes which govern SX behavior occur on a sub-micrometer scale. Therefore, TEM is required to explore them. TEM can provide microstructural, crystallographic and chemical information and in this respect outperforms other methods like APT (provides only chemical and microstructural information of tiny volumes) and all diffraction methods (provide only crystallographic information). For a good understanding of SX, it is important to explore how microstructures and thermodynamic equilibria are kinetically established. It is equally important, to correctly interpret the role which crystal defects like dislocations and planar faults play during high temperature plasticity. In the present work TEM has been applied to contribute to both fields.
In the thermodynamic/kinetic part of the present work on a Ni-base superalloy, a close to equilibrium reference state was produced by long term annealing experiments (850°C, 100 h). ... mehrIt was shown that high temperature microstructures can be frozen in by fast cooling. This type of associated prior ex situ research is an inseparable part of the work, when the objective is to perform meaningful in situ experiments on the sub μm scale. Then, in situ TEM heating experiments with a novel type of sub-μm diffusion couple allowed to directly observe how γ/γ’-microstructures establish new (chemical and microstructural) equilibria when new conditions are imposed (850 → 900°C). Analytical and numerical analysis of the experimental concentration profiles yielded two important results. First, the rate controlling step in the cross γ-channel interdiffusion process is the interface re-action. Second, the fluxes of individual atoms are coupled. These chemical results (flux analysis) are in good qualitative agreement with the directly observed decrease in γ’-volume fractions (from 950°C upwards).
In the high temperature plasticity part of this work it could be directly observed, how a dislocation climbs in the γ-channel of a Ni-base SX and how, in a Co-based SX, an anti-phase boundary (APB), which had formed during creep, moves from its original {111} plane to an energetically favorable {100} plane. The fact that this APB attracts alloy elements which segregate to this planar fault and that it moreover acts as a nucleation site for the γ-phase, once the γ’-dissolution temperature is exceeded, was observed and documented.
For a Ni-based SX it was shown that advanced diffraction contrast methods like large angle convergent beam electron diffraction (LACBED) can help to identify γ-channel dislocation reactions which contribute to the nucleation of planar faults in the γ’-phase. In a Co-based SX, planar defects in numerous isolated as well as contiguous γ’ precipitates on {111} planes were analyzed. They showed a characteristic APB/SISF/APB configuration, whereby superlattice intrinsic stacking fault (SISF) loops are fully embedded in an antiphase boundary (APB) that typically extends over multiple γ’-precipitates.
Single crystal Ni- and Co-base superalloys are complex engineering materials which consist of up to ten alloy elements and where, depending on their thermomechanical heat treatments, different complex microstructures can form. The present work has shown that even in these complex systems, analytical TEM can help to obtain new and fascinating results and that there are still many open questions which await an answer.