STEM microanalysis of precipitates and their nuclei in a nickel-base superalloy

Abstract

The problem of analysing small MC carbide precipitates in a complex nickel-base superalloy has been undertaken using a variety of microanalytical techniques available on the scanning transmission electron microscope (STEM). These techniques are X-ray analysis (EDX) on both extraction replicas and thin foils, and transmission electron energy loss spectroscopy (EELS) on thin foils. It is shown that the carbides are chiefly TiC, with molybdenum, chromium and aluminium in solution, and that most of these carbides contain a nucleus 20–60 nm in diameter. These nuclei are of considerable practical importance because of their role in promoting carbide precipitation along prior boundaries. They are shown to be oxides of zirconium, and probably also of aluminium and titanium. This identification is only possible following the use of standards for EELS work, and further suggests that additional useful information can be obtained from the examination of the low energy loss structure. Finally, the need to employ both EELS and EDX techniques and their complementary nature is stressed by this work.