Aviation has a very high impact on greenhouse gas emissions. In order to tackle CO2 emissions, it has become necessary to increase the operational temperature of aircraft engines, which will result in a higher efficiency and reduced CO2 emissions. However, the higher the operation temperature, the more likely is a catastrophic failure of safety-critical components. Thus, new, advanced high-performance materials for aero-engines are required which can withstand harsher service conditions and maintain structural integrity.
Developing novel materialdesign strategies requires a thorough understanding of the behaviour of these metallic materials at elevated temperatures.Through systematic characterization with near-atomic resolution by atom probe tomo-graphy (APT) combined with electron microscopy, we, a team of re-searchers of the department "Microstructure Physics and Alloy Design" (MA), provide new fundamental in-sights into the role of crystalline imperfections on the lifetime of nickel- and cobalt-based superalloys.
Fig. 1: a) cECCI micrograph showing a high dislocation density in the fully rafted γ/γ’ microstructure of a single-crystal nickel-based superalloy. b) Atom probe reconstruction from a rafted γ’ precipitate, showing dislocations within a γ’ precipitate. c) 1D concentration profile perpendicular to a dislocation revealing segregation of chromium and cobalt at partial dislocations and at the planar defect.
Fig. 1: a) cECCI micrograph showing a high dislocation density in the fully rafted γ/γ’ microstructure of a single-crystal nickel-based superalloy. b) Atom probe reconstruction from a rafted γ’ precipitate, showing dislocations within a γ’ precipitate. c) 1D concentration profile perpendicular to a dislocation revealing segregation of chromium and cobalt at partial dislocations and at the planar defect.
Both groups of alloys derive their outstanding strength from the L12ordered γ′ precipitates. In the case of the nickel-based alloys, their dissolution kinetics under extreme operational conditions are enhanced at elevated temperatures by the presence of high dislocation den-sity. As a consequence, recrystal-lization and directional coarsening of γ′ precipitates occur leading to severely reduced fatigue and creep performance. Direct observation of solutes segregation at dislocations by APT allows us to elucidate the physical mechanism where pipe diffusion initiates the deleterious dissolution of γ′ precipitates and subsequently degrades the properties of superalloys of industrial relevance [1,2].
Fig. 2: a) cECCI overview image showing bright lines corresponding to stacking faults (SFs) in γ’ precipitates and a BF TEM image of an APT tip showing two dark lines depicting SFs. b) HR-STEM of a SF in cubic [110] edge-on condition showing local DO19ordering that is different from the surrounding L12 γ’. c) APT reconstruction showing a partial dislocation (pink colour, 5.6at.% Cr iso-compositional surface) associated to a confined Al depleted stacking fault plane and a schematic that shows the proposed solute diffusional mechanism occurring in the vicinity of the planar defect.
Fig. 2: a) cECCI overview image showing bright lines corresponding to stacking faults (SFs) in γ’ precipitates and a BF TEM image of an APT tip showing two dark lines depicting SFs. b) HR-STEM of a SF in cubic [110] edge-on condition showing local DO19ordering that is different from the surrounding L12 γ’. c) APT reconstruction showing a partial dislocation (pink colour, 5.6at.% Cr iso-compositional surface) associated to a confined Al depleted stacking fault plane and a schematic that shows the proposed solute diffusional mechanism occurring in the vicinity of the planar defect.
Similarly, in cobalt-based alloys, understanding high temperature deformation is of utmost impor-tance. During deformation, dislocations are generated in the γ matrix and their movement across γ’ precipi-tates results in the formation of planar defects such as stacking faults (SF) (bright lines in Fig. 2a) and anti-phase boundaries. Combined high-resolu-tion scanning transmission electron microscopy (HR-STEM) and APT of a stacking fault evidences a distinct structural-compositional contrast with respect to the surrounding lattice [3,4,5]. In Fig. 2b, the stacking fault possess different atomic structure of D019 order compared to the L12 order of the surrounding γ’. Additionally, the 3D compositional field in the vicinity of the planar defect enabled us to know that the diffusivity of the selec-tive solutes to these defects and their diffusion directions are the rate limit-ing steps for the overall creep defor-mation of the superalloy.
These insights will help define strate-gies for alloys’ additions that may help slow down creep deformation and hence enhance the lifetime of safety-critical parts at high temperatures.
References:
1.
P. Kontis, Z. Li, et al.
The effect of chromium and cobalt segregation at dislocations on nickel-based superalloys.
Correlative Microscopy - Novel Methods and Their Applications to Explore 3D Chemistry and Structure of Nanoscale Lattice Defects: A Case Study in Superalloys.
![Fig. 2: a) cECCI overview image showing bright lines corresponding to stacking faults (SFs) in γ’ precipitates and a BF TEM image of an APT tip showing two dark lines depicting SFs. b) HR-STEM of a SF in cubic [110] edge-on condition showing local DO19ordering that is different from the surrounding L12 γ’. c) APT reconstruction showing a partial dislocation (pink colour, 5.6at.% Cr iso-compositional surface) associated to a confined Al depleted stacking fault plane and a schematic that shows the proposed solute diffusional mechanism occurring in the vicinity of the planar defect.](/4130416/original-1575999325.jpg?t=eyJ3aWR0aCI6ODQ4LCJmaWxlX2V4dGVuc2lvbiI6ImpwZyIsIm9ial9pZCI6NDEzMDQxNn0%3D--be027c56bae266e321fc8319f3b1a6cc3d9118aa "Fig. 2: a) cECCI overview image showing bright lines corresponding to stacking faults (SFs) in γ’ precipitates and a BF TEM image of an APT tip showing two dark lines depicting SFs. b) HR-STEM of a SF in cubic [110] edge-on condition showing local DO19ordering that is different from the surrounding L12 γ’. c) APT reconstruction showing a partial dislocation (pink colour, 5.6at.% Cr iso-compositional surface) associated to a confined Al depleted stacking fault plane and a schematic that shows the proposed solute diffusional mechanism occurring in the vicinity of the planar defect.")
![Fig. 2: a) cECCI overview image showing bright lines corresponding to stacking faults (SFs) in γ’ precipitates and a BF TEM image of an APT tip showing two dark lines depicting SFs. b) HR-STEM of a SF in cubic [110] edge-on condition showing local DO19ordering that is different from the surrounding L12 γ’. c) APT reconstruction showing a partial dislocation (pink colour, 5.6at.% Cr iso-compositional surface) associated to a confined Al depleted stacking fault plane and a schematic that shows the proposed solute diffusional mechanism occurring in the vicinity of the planar defect.](/4130416/original-1575999325.jpg?t=eyJ3aWR0aCI6MjQ2LCJvYmpfaWQiOjQxMzA0MTZ9--d8518fae0154d49acc5620953247f4b69a5600da)