On the application of energy filtering TEM in materials science: I. Precipitates in a Ni/Cr-alloy

Electron spectroscopic imaging (ESI) in the transmission electron microscope (TEM) can be efficiently used to detect nanometer sized precipitates in solids. This can be achieved by processing sequences of energy filtered images recorded at different energy losses before and after an ionization edge. To demonstrate the application of the method we have investigated a Ni/20%Cr alloy containing a variety of precipitates: TiN (100-150 nm diameter), Y₂O₃ (∼ 5 nm) and twinned Y₂O₃{single bond}Al₂O₃ precipitates (10-50 nm) and Cr-metal at grain boundaries. We have evaluated the use of jump ratio images (two window method) and elemental distribution maps (three window method) for precipitate visualization and have found that in most cases the jump ratio images provide significant advantages: the jump ratio images show elemental contrast similar to the elemental maps but with lower noise and are nearly free of diffraction artifacts (bend contours, thickness fringes). Jump ratio images can also be useful for obtaining elemental maps of elements which occur with very low concentration, if background subtraction is behindered by EXELFS modulations and if ionization edges are overlapping. However, the jump ratio images have to be used with care, because they are sometimes susceptible to artifacts. One main finding of this work is that the jump ratio image of the matrix element, in this case Ni, can be used to visualize all precipitates occurring regardless of crystallographic orientation and chemical composition. Furthermore, we have applied image correlation techniques to the ESI elemental and jump ratio images for direct visualization of the chemical phases.