Grain Boundary Segregation in NI-Base Alloys: an Integrated Approach using FIB, TEM and SIMS

Abstract

Segregation of elements to the grain boundaries in Ni-base alloys can have a large effect on the mechanical and corrosion properties of these materials. The extent of segregation, whether it is equilibrium or non-equilibrium segregation, is dependent upon the thermo-mechanical treatments applied to the alloy. in order to determine if a particular thermo-mechanical process delivers the desired microstructure, a complete microstructural analysis including an examination of grain boundary segregation must be performed.

In the past, TEM has been used to identify the various phases and precipitates in these materials through the use of bright- and dark-field imaging, electron diffraction, and EDS. However, one of the elements that can play a large role in determining the properties of these alloys is boron. in this group of alloys, boron is generally present in bulk analyses in only a few tens of ppm, and as a result it is difficult to detect using the aforementioned techniques. More commonly, boron segregation to the grain boundaries is usually studied by means of Auger analysis, whereby the sample is charged with hydrogen in order to promote brittle intergranular fracture in-situ in the Auger microprobe. Boron can then be detected at the surface of the clean grain boundary fracture surfaces.