Co-based superalloys with a composition of Co-9Al-9W exhibit a stable γ/γ’ microstructure at 900°C . Investigations have shown that planar fault formation is a dominant deformation mechanism during creep of Co- and CoNi-based superalloys [2, 3]. In the present work a SX Co-based superalloy, referred to as ERBOCo-1, was creep deformed in tension under 400 MPa along the -direction at 850 °C. The alloy contains high amounts of Co, Ni, Al, Cr , W and four minor alloying elements. After heat treatment ERBOCo-1 exhibits cuboidal γ’-precipitates (L12) embedded in a solid solution γ-matrix (fcc).
After creep testing defects were characterized by means of TEM, large-angle convergent beam electron diffraction (LACBED) and STEM in combination with EDX-spectroscopy. In tension <112> slip systems are active in the early stages. Two partial dislocations cut through the γ’-phase leaving behind an ASA-fault (APB-SISF-APB) where superlattice intrinsic stacking faults are enclosed by anti-phase boundaries. For the ASA-fault formation and microtwinning in Co-based alloys, a deformation mechanism with coupled displacive and diffusive processes have been proposed [2, 4].
In this work, we investigated different stages of creep deformation at plastic strains of 0.3 % and 5 %. The local structure of planar faults in both specimens was investigated using HR-STEM in combination with a SuperXDetector for EDXS. A new, site-specific preparation method was applied to prepare atom probe tomography (APT) tips containing single planar defects. This complementary approach allows for 2D- and 3D-chemical characterization. HR-STEM investigations show that the SISFs locally changes from L12 to D019 (as reported by Titus et al. ) and is accompanied by a localized compositional change as shown in figure 1 (a) and (b). It was found that the width of the segregation zone depends on the duration of exposure to temperature and stress. Furthermore, the amount of enrichment of Co, Cr and W and the depletion of Ni and Al also varies with time. These findings were corroborated by means of atom probe tomography as shown in figure 2.
It is suggested that the local phase transformation to the D019-phase is in a non-equilibrium state and is coupled to the planar fault energies. The investigation of the segregation behavior offers the possibility for more “efficient” alloying and a fine tuning of the planar fault energies in future alloy systems.
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