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

Highlights: • δ-Ni₃Nb and η-Ni₃Ti phases are difficult to be differentiated in nickel-based superalloys • Conventional EBSD indexing fails to distinguish between these phases despite their different crystal structures • Dictionary Indexing was able to identify the major component of the grain boundary precipitates as η phase in VDM Alloy 780 • Phase discrimination using EBSD provides better statistics and is less time-intensive compared to TEM based techniques VDM Alloy 780 is a new nickel-based superalloy developed for higher service temperatures up to 750 °C. In addition to fine γ′-Ni₃(Al,Ti) strengthening precipitates, grain boundary precipitates resembling δ-Ni₃Nb and/or η-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 δ and η 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 η 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.