Hotter, safer and greener aero-engines

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.

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.

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.
Scr. Mater. 145 (2018)
2.
P.  Kontis,  D.M.  Collins,  et  al.
Microstructural degradation of polycrystalline superalloys from oxidized carbides and implications on crack initiation.
Scr. Mater. 147, 59–63 (2018)
3.
S.K.  Makineni,  M.  Lenz,  et  al.
Elemental  segregation  to  antiphase boundaries in crept CoNi-based single  crystal  superalloys.
Scr.  Mater.  (2018) accepted.
4.
S.K.  Makineni,  A.  Kumar,  et  al.
On  the  diffusive  phase  transforma-tion mechanism assisted by extended dislocations during creep of a single crystal  CoNi-based  superalloy.
Acta  Mater. 155, 362–371 (2018)
5.
S.K. Makineni, M. Lenz, et al.
Correlative Microscopy - Novel Methods and Their Applications to Explore 3D Chemistry and Structure of Nanoscale Lattice Defects: A Case Study in Superalloys.
JOM, 1–8 (2018)

MA authors: P. Kontis, S. K. Makineni, B. Gault

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