Structural and chemical analysis of topologically close packed ( TCP ) phases in high generation Ni ‐based superalloy

Abstract

The investigated material belongs to the new generation single crystal Ni‐base superalloy family. These kind of alloys exhibit excellent high‐temperature creep strength as well as oxidation and corrosion resistance [1,2]. However, due to high content of refractory elements the microstructural stability can be strongly affected by precipitation of topologically close packed (TCP) phases. [3,4] The morphology, structure type as well as chemical composition of TCP phases can significantly vary and depend on alloy's composition and applied heat treatment. It is expected that TCP phases growth from gamma matrix, which is mainly composed of Ni, Re, Co, Ru and Cr in high generation Ni‐based superalloys. For this reason the tested material was subjected to long term aging at high temperature and investigated at different states of TCP phase evolution. The aim of the study is to understand the process of TCP phase precipitation including identification of nucleation sites, the chemical composition and structure type determination at different stages of heat treatment. The microstructural imaging was performed using transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and scanning electron microscopy (SEM). The chemical analysis was accomplished by energy dispersive X‐ray (EDX), and the TEM lamella were cut by focused ion beam (FIB) technique. The results were obtained from two samples which come from the parent material. The material was subjected to standard heat treatment procedure consisting of homogenization and a two step aging treatment followed by long term exposure at high temperature. Sample 1 was derived from the initial state of long term exposure, while sample 2 was obtained from the equilibrium state of TCP phase precipitation. Analysis of the microstructure of the 2 samples shows distinct differences between the states. The microstructure of the initial stage of long term exposure exhibits only few TCP precipitates in the early stage of precipitation, while in sample 2 the extensive formation of TCPs with plate‐like morphology was observed. To fully understand the process of TCP precipitation the determination of nucleation sites is essential. Figure 1 presents a high magnification bright field STEM image taken from sample 1. In this figure it is clearly seen, that TCP starts to nucleate within a gamma channel. After long‐term exposure, gamma, gamma prime as well as TCP phases strongly evolved, what is shown in Figure 2. This figure presents the equilibrium stage of TCP precipitation. It was also shown, that gamma prime phase changed its morphology from regular cubic (beginning of exposure) to rafted irregular shape (after long term exposure). In case of TCP precipitates not only the number, but also their size strongly increased. Consequently, it is expected that a change of creep and fatigue strength will occur with a changing microstructure morphology. The selected area diffraction analysis performed in TEM allowed to identify the structure type of individual phases as well as determination of lattice parameter change during thermal exposure. Moreover the EDX measurements showed tendency to strong segregation of Re and Ru elements into TCP phase at the expense of gamma matrix. All microstructural investigations combined with EDX chemical analysis resulted in evaluation of TCP precipitation process at the structural and chemical aspects.