In situ microcompression and X-ray nanotomography of an additive manufactured Ni-base Superalloy
To understand the critical parameters, which govern the formation of mechanically detrimental topologically close packed (TCP) phases in single crystalline Ni-base superalloys, is a subject of current research. The TCP-phase precipitation strongly depends on the degree of microsegregations of refractory elements, esp. Re, in the γ-phase.[1, 2] By additive manufacturing processes like selective electron beam melting (SEBM), these microsegregations can be significantly reduced, owing to a dramatic increase in the solidification front velocity and the local thermal gradient resulting in a diminution of the primary dendrite arm spacing by more than one order of magnitude. Therefore after employing a short heat treatment to optimize the γ/γ’-microstructure, the precipitation of TCP-phases is clearly retarded compared to cast materials.[3-5] So far the morphological and chemical nature of TCP-phases and their spatial distribution in SEBM material aged for long times at high temperatures has not been studied in detail. Even more important from the application point of view, the impact of the TCP-phases on the mechanical properties of the respective SEBM microstructures has not been examined, neither at room nor at elevated temperatures.
For that reason, the mechanical deformation of a micropillar extracted from high temperature aged SEBM specimens is investigated by combined pillar compression and 3D imaging using a Zeiss Xradia Ultra 810 Nano-CT equipped with an in situ mechanical loading stage. By this approach the impact of TCP-phases and their 3D distribution on the mechanical properties of the superalloy can be studied in detail providing direct insight into how TCP-phases effect, e.g., crystal slip and crack formation. On top of that deploying a Nano-CT gives the possibility to represent nearly a whole primary dendrite arm (≈ 10-15 µm [3, 4]) within one pillar (≈10 µm) to span the complete characteristic segregation distance. To guarantee a sufficiently strong pillar base for the compression experiments “prepillars” are bared inside bulk material using a picosecond lasercutter followed by final FIB-polishing. Thereby the forces are spread on a huge supporting structure (2x2 mm) composed of the same material, which is glued to a mounting pin for the measurements. In my conference talk ex situ reference microcompression tests will also be included to discuss the validity of the in situ tests.