Speaker
Description
Ni-base single-crystal (SX) superalloys find application in turbine blades for gas engines due to the high-
temperature and high-stress strength originating from the coherent γ/γ’ microstructure. It is well-known
at sufficiently high stresses, two 1/2<101> dislocation families with different Burgers vector can react
and dissociate into two partial dislocations in γ channels. This allows the leading 1/3[-1-12] Shockley
partial dislocation continuously gliding on {111} planes to cut into γ’ precipitates where they create
planar faults [1]. We study the segregation behaviours of alloying elements across the planar faults by
performing the [11-2] (111) creep shear experiments, to intentionally activate the slip system [11-2]
(111) with the highest Schmid factor of 1 where the resolved shear stress is exactly equal to loading
stress. The creep-deformed specimens are interrupted after 1% and 2% shear strain under 250 MPa at
750 °C. The resulting microstructure is investigated using conventional transmission electron
microscopy (TEM), analytical scanning TEM (STEM) with energy-dispersive X-ray spectroscopy (EDXS)
focussing on structural, physical, and chemical details of the local deformation.
We investigated the specimen perpendicular to the (111) plane with the [1-10] direction parallel to the
electron beam. Numerous stacking faults (SF) are observed after 1% and 2% creep strains. Fringe
contrasts under two-beam conditions indicate inclined stacking faults, where the 2% strain sample has
more planar faults within one γ’ precipitate indicating a higher density of planar faults in the 2% sample.
High-resolution STEM micrographs illustrate the superlattice extrinsic nature of stacking faults (SESF) in
the 1% and 2% strained samples. The chemical distributions across SESF are measured by EDXS and the
corresponding concentration profile of 1% and 2% samples. Both samples show almost similar
segregation tendency, which is that γ forming elements Cr, Co and Re are enriched across the SESF while
γ’ alloying elements Ni and Al are depleted, which is partly in agreement with theoretical predictions [2].
For these measurements all microscope parameters and sample thickness for EDXS analysis are kept
the same to quantitatively find out how creep strain and time affect the evolution of segregation.
| Category | Solid State (Experiment) |
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