High Resolution EBSD Mapping of Martensitic Steel

Introduction

Martensitic steels are developed in order to improve toughness and hardness, but the formation of highly-deformed martensite laths via quenching austenitic steel can result in significant challenges for microstructural characterisation using EBSD. Martensite contains supersaturated carbon, distorting the crystal lattice away from body centred cubic and resulting in a high density of dislocations, the primary cause of strengthening of the steel. In addition to the lattice distortion, the laths are often very small (< 1 μm), necessitating EBSD analysis using lower beam energies and higher diffraction pattern resolutions in order to achieve high indexing hit rates.

In this application brief we show how the combination of sensitivity and speed of the CMOS-based Symmetry detector, coupled with the fact that high speeds are also achievable with good pattern resolution, enables fast and effective characterisation of a martensitic stainless steel sample.

Methodology and Results

A sample of untempered martensitic stainless steel was polished down to a final stage using colloidal silica, and then analysed in a field emission gun SEM. The beam accelerating voltage was initially kept relatively low, at 12 kV, in order to minimise the EBSD pattern source volume and therefore maximising the pattern quality, enabling a high indexing hit rate of ~90%. Patterns were acquired at a resolution of 311 x 256 pixels, with an exposure time of 2 ms giving a final acquisition speed of 495 pps. The first analysis took 75 minutes, covering an area of 50 x 40 μm with a measurement step size of 30 nm.

A second, significantly larger analysis was carried out on the same sample, using an accelerating voltage of 15 kV and a higher pattern resolution of 622 x 512 pixels. Patterns were exposed for slightly longer, giving a final acquisition speed of 294 pps and a higher indexing hit rate of 96.5%. An area of 150 x 115 μm was scanned with a 40 nm step size, taking 10 hours in total (almost 11 million analysis points). In all analyses the beam current was between 5 and 10 nA.

The results of the smaller first area are shown in Fig. 1. The pattern quality (band slope) map shows the variation in the sharpness of the Kikuchi bands: a few of the martensitic laths have relatively bright shades of grey, indicating sharper patterns associated with lower distortion in the crystal lattice. However, most of the laths have relatively low pattern quality and are significantly under 1 μm in width, making successful EBSD analyses challenging. The orientation map (Fig. 1b) clearly shows the hierarchical nature of the martensitic structure, with the individual laths forming blocks, groups of blocks forming packets and, finally, groups of packets defining the prior-austenitic grains.

Fig. 1a. Pattern quality map (band slope) of the first analysis area.

Fig. 1b. Orientation map (IPF colouring scheme) of the first area.

The area of the second analysis covers many more prior-austenitic grains, as shown in the pattern quality (band contrast) map in Fig. 2a and the orientation map in Fig. 2b. In the first area, it appeared that prior-austenitic grains were typically ~20 μm diameter, but in this larger area analysis it is clear that some prior austenite grains are >100 μm diameter. For example, the blue and red coloured area at the top of Fig. 2b (marked with "A") can be reconstructed into a single prior austenite grain. The pole figure from this grain (Fig. 2c) shows the clear martensite variant selection from a single, original austenite orientation.

Fig. 2a. Pattern quality map (band contrast) of the second area.

Fig. 2b. Inverse pole figure orientation map of area 2. “A” marks a prior austenite grain analysed in Fig. 2c.

Fig. 2c. {100}, {110} and {111} pole figures for the prior austenite grain marked “A” in Fig. 2b. The colours correspond to those in Fig. 2b.

It is well documented (e.g. Kitahara et al., 2006) that many of the misorientations between martensite variants in adjacent laths satisfy coincident site lattice (CSL) relationships. This can be seen in Fig. 3, in which the proportion of all high angle boundaries in area 2 that are classed as CSL boundaries is plotted.

Fig. 3. Histogram showing the proportion of high angle boundaries in area 2 that satisfy coincident site lattice (CSL) relationships. Note that >40 % of the boundaries can be classed as Σ3.

Conclusion

This application brief demonstrates the power of the Symmetry EBSD detector for effective analyses of challenging materials samples at relatively low accelerating voltages. The results from an untempered, martensitic stainless steel, achieved at acquisition rates of 290–500 pps, show that the sensitivity of the detector, coupled with high pixel-resolution diffraction patterns, enables high indexing rates as well as relatively short analyses times.

This allows a detailed investigation into the martensitic variant selection as well as a characterisation of the inter-lath boundaries from a relatively large area without the need for post-analysis reprocessing or pattern enhancement. Never before has the power of the EBSD technique for measuring martensitic structures been so apparent.

Reference

H. Kitahara et al. (2006). Crystallographic features of lath martensite in low-carbon steel. Acta Materialia 54, 1279–1288.