On APFIM of Grain Boundaries in a Nickel Base Superalloy

Abstract
Experimental details of atom probe field ion microscopy (APFIM) of a Ni-16Cr-9Fe alloy are presented. In particular, studies concerning analysis of intergranular carbides and borides and segregation to grain boundaries are described. It was found that Ni₂₃B₆ precipitates are very sensitive to preferential field evaporation, which necessitates optimization of the analysis conditions. In the FIM images, nickel- and boron-rich precipitates exhibited either dark or bright contrast with respect to the austenitic matrix. Analysis of a grain boundary that contained a high level of segregated carbon, revealed that carbon is partially field evaporated as complex ions. In this analysis, chromium and carbon often appeared next to each other in the ion chain. Together, these two observations indicate nucleation of chromium carbides, which are known to form after extended heat treatment at the applied temperature (600°C).