On the detection of MX-precipitates in microalloyed steels using energy-filtering TEM

The properties of microalloyed steels and HSLA steels depend predominantly on the size and composition of nanometre-sized MX precipitates consisting of Nb, Ti, V, C and N, In this work we present a rapid and powerful method for visualizing these small precipitates directly in the steel matrix. This can be achieved by energy-filtering TEM in a 200-kV microscope, We have recorded elemental maps (three-window method) and jump-ratio images (two-window method) using the inner-shell ionization edges; e.g. Fe-M(23), Fe-L(23), V-M(23), V-L(23), Nb-N-23, Nb-M(45), C-K and N-K edges, We have found that precipitates in a steel matrix can be best visualized by recording the jump-ratio image of the matrix element. If the specimens are thinner than 30 nm and high resolution in the images and low detection limits are essential, low energy-loss edges should be used preferentially; in the case of steels the Fe-M(23) edge should be used instead of the Fe-L(23) edge, If the jump-ratio images are recorded with the Fe-M(23) edge, precipitates about 2 nm in diameter may be detected, With thicker samples and if high resolution is not as essential, higher energy-loss edges are preferable, because diffraction effects are less possible. The chemical composition of the larger precipitates (10 nm) could be determined by recording EDX spectra using an STEM probe. We found Nb and V with an atomic ratio of about 3, but we could not detect carbon or nitrogen in the spectra. The Nb and V distribution could be best visualized with a jump-ratio image including both the Nb-N-23 and the V-M(23) edges. However, it was not possible to image the carbon or nitrogen content of the precipitates unequivocally due to overlapping of the C-K and N-K edges with the Nb-M(45) edge.