Hotter, safer and greener aero-engines
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.
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].
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.
MA authors: P. Kontis, S. K. Makineni, B. Gault