Atomic resolution structural and analytical characterization of layered precipitates in special alloys for high-temperature applications

Abstract

Ni-base superalloys (e.g. Alloy 718) are largely used nowadays for manufacturing metallic components like gas turbine engine disc components for land-based power generation and aircraft propulsion to be exploited at high temperatures, due to their excellent mechanical properties and resistance to corrosive or oxidizing environments. The VDM 780 Premium alloy has been recently developed [1] in order to push even further the temperature limits of the widely used Alloy 718. This newly developed Ni-base superalloy is based on the  matrix (Ni-based cubic Fm-3m structure) and ’ hardening precipitates (Ni3Al-based cubic Pm-3m structure). The amount of the various crystal phases, the size and morphology of the different precipitates depend on the particularities of the applied heat treatments. The crystal phases obtained during the thermal treatment at high-temperature determine the grain size evolution and the mechanical properties of the alloy operating at high temperature. This HRTEM study brings a decisive contribution in understanding the structure of the high-temperature precipitates formed in the newly developed alloy VDM 780 Premium. For this purpose, we combined various techniques of analytical electron microscopy, including electron diffraction, atomic-resolution imaging, chemical characterization by EDS and EELS, elemental mapping by STEM-EDS ans STEM-EELS as well as structural modelling. Our results corroborated from SAED patterns, HRTEM imaging and local FFT diagrams have revealed that the high-temperature precipitates in the alloy VDM 780 Premium exhibit a layered structure consisting of alternating  and  phases in the crystallographic orientation relationship [010] || [100], (001) || (010), (100) || (001).