Mesoscopic quantitative chemical analyses using STEM-EDX in current and next generation polycrystalline Ni-based superalloys

Ultramicroscopy. 2019 Sep:204:55-72. doi: 10.1016/j.ultramic.2019.04.015. Epub 2019 May 16.


Quantitative chemical analyses of Ni3Al based hardening precipitates (γ') in polycrystalline Ni based superalloys have been conducted using energy dispersive X-ray spectroscopy (EDX), coupled with a scanning transmission electron microscope (STEM). The aim of the current investigation is (1) to evaluate the accuracy of calibration (k factor determinations and absorption corrections using a combination of differential X-ray absorption (DXA) and convergent beam electron diffraction (CBED)) by comparing with thermodynamic calculations and (2) to demonstrate the importance of the EDX chemical analysis by taking advantage of its unique capabilities to analyse sub-micron scale chemistries within a mesoscopic field of view under STEM. Our experimental findings show good agreement with the mole fraction ratio of γ' to the disordered γ matrix predicted using the Lever rule on a thermodynamically stabilised unimodal superalloy, RR1000. The significance of analysing a statistically viable number of samples in thermodynamically metastable superalloys and the chemical fluctuations found in coarse γ', sized above 200 nm on a scale of a few hundred nanometres in the context of solving a complex morphological evolution of γ' particles is demonstrated.

Keywords: Convergent beam electron diffraction (CBED); Differential X-ray absorption (DXA); Gamma prime (γ′); Quantitative energy dispersive X-ray (EDX) analysis; Superalloys; Thermo-Calc Software.