ON THE APPLICATION OF ENERGY-FILTERING TEM IN MATERIALS SCIENCE .1. PRECIPITATES IN A NI CR-ALLOY/

Electron spectroscopic imaging(ESI) in the transmission electron micro scope (TEM) can be efficiently used to detect nanometer sized precipit ates 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 ha ve investigated a Ni/20%Cr alloy containing a variety of precipitates: TiN (100-150 nm diameter), Y2O3 (similar to 5 nm) and twinned Y2O3-Al 2O3 precipitates (10-50 nm) and Cr-metal at grain boundaries. We have evaluated the use of jump ratio images (two window method) and element al distribution maps (three window method) for precipitate visualizati on and have found that in most cases the jump ratio images provide sig nificant advantages: the jump ratio images show elemental contrast sim ilar 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 whi ch occur with very low concentration, if background subtraction is beh indered by EXELFS modulations and if ionization edges are overlapping. However, the jump ratio images have to be used with care, because the y are sometimes susceptible to artifacts. One main finding of this wor k is that the jump ratio image of the matrix element, in this case Ni, can be used to visualize all precipitates occurring regardless of cry stallographic orientation and chemical composition. Furthermore, we ha ve applied image correlation techniques to the ESI elemental and jump ratio images for direct visualization of the chemical phases.