Local Two-Dimensional Measurement of Plasticity Induced Strain in Deformed Ni-base SuperalloysWednesday (28.09.2016) 17:45 - 18:00 Part of:
The excellent thermo-mechanical properties of superalloys originate predominately from their unique microstructure comprising cuboidal γ' precipitates coherently embedded in a solid solution γ matrix. While it is well understood how internal stresses build up in an as-heat-treated sample due to unequal lattice parameters of γ and γ', it is much more exacting to measure and understand the stress distribution in a sample filled with dislocations and to derive appropriate input parameters for modelling e.g. the creep process. In the present work we present a technique based on convergent-beam electron-diffraction (CBED) which enables the mapping of the two-dimensional strain distribution in a superalloy’s deformed microstructure with high spatial resolution. A simultaneous chemical analysis acquired during the mapping allows for directly combining chemical and structural information in each data point.
The method is applied to a single crystalline Ni-base superalloy creep-deformed to 0.4 % of plastic strain at 1050 °C and 160 MPa. A weakly rafted microstructure filled with channel dislocations has formed as expected for the early stages of creep in this high temperature and low stress regime. Employing the CBED mapping the strong anisotropy of strain in the two orthogonal γ channels is clearly revealed. At the points where two γ channels meet, the measured strain values are significantly higher than inside the channels consistent with the observation that channel dislocations cannot easily penetrate into the channel crossings. This effect was found for the majority of channel crossings and is therefore assumed to be characteristic for these early stages of creep. Compared to an undeformed sample, only small changes in strain have been identified in the γ' precipitates. Moreover, the chemical analysis showed that a pile-up of e.g. Re, Cr or Co is present in the γ' phase, while Al or Ni are depleted. Despite the large differences in strain in the vertical and horizontal channel, differences in chemical composition have not been identified at this stage of creep deformation.