Phase discrimination between 8 and η phases in the new nickel-based superalloy VDM Alloy 780 using EBSD

VDM Alloy 780 is a new nickel-based superalloy developed for higher service temperatures up to 750 degrees C. In addition to fine gamma '-Ni3(Al,Ti) strengthening precipitates, grain boundary precipitates resembling 8-Ni3Nb and/or eta-Ni3Ti phases are also observed in this alloy. Previous studies on Alloy 718Plus, with chemical composition close to VDM Alloy 780, revealed that phase discrimination between 8 and eta particles is not straightforward and requires the use of high-resolution characterization techniques such as HRTEM. This work aims at evaluating the use of EBSD technique with regards to phase identification of these plate-shaped particles. Classical indexing of diffraction patterns based on Kikuchi band detection in Hough space fails to differentiate between the two phases. However approaches based on the comparison of experimental and simulated patterns, at least partly, could overcome this issue. The advanced dictionary-based approach that involves the simulation of dynamic electron scattering events identified eta phase as the main component of these grain boundary precipitates, consistent with the recently published HRTEM investigation on VDM Alloy 780. Refinement of the classical indexing method under kinematic assumptions implemented in the Refined Accuracy mode on the Oxford AZtec software package also led to appreciable improvement.

Automated summary

The study found that using EBSD technique for phase identification of the grain boundary precipitates in VDM Alloy 780, comparing experimental and simulated patterns can partially overcome the issue of phase discrimination. Additionally, the advanced dictionary-based approach involving dynamic electron scattering event simulation identified the eta phase as the main component of these grain boundary precipitates.