AZtec Reclassify Phase - Discriminating phases in steels

Introduction

Steels are engineered with different microstructures for different applications. Austenite, ferrite, martensite and bainite are common phases in different steel alloys and the relative proportions of these different phases impact how the steel will behave in different regimes. For example, the amount of martensite will influence both strength and toughness. As a result, being able to measure the relative presence of these phases is important.

Electron back-scattering diffraction (EBSD) can readily separate austenite and ferrite due to significant crystallographic difference between these two phases. In contrast, distinguishing martensite, bainite and ferrite is a challenge as the crystal structure of these three phases is essentially the same. All three have a body centred cubic (BCC) structure, with martensite and bainite being only slightly distorted from the ferrite BCC structure. This difference in crystal structure has presented challenges to researchers for many years.

AZtec Reclassify Phase

Martensite and bainite generally have a distorted crystal lattice with a higher density of crystalline defects or residual stresses (generated during processing). As a result the EBSD pattern generated from these phases is of a poorer quality, in terms of intensity and sharpness, than that collected from ferrite.

Therefore discrimination of these phases has focused on the analysis of EBSD pattern quality parameters, including band slope (BS), band contrast (BC) and pattern quality (PQ). Each of these measures can be plotted as a grey scale map, where martensite and bainite appear darker with a lower pattern quality when compared to ferrite.

The example in Fig. 1 shows a band slope map from dual phase steel, containing martensite and ferrite. The martensite has a poorer pattern quality, which is seen here as a lower band slope value, and therefore is the darker regions in the map.

A 'Phase Training' technique is presented. This automatic phase discrimination method provides a robust and easy to use method to separate these phases. This method is incorporated into the AZtec software in the Reclassify Phase mode.

This technique uses one or more of the pattern quality parameters (BC, BS and PQ) for phase classification. Each point on the map is associated with a parameter vector, consisting of map component values for those quality parameters selected. Points with a similar parameter vector are classified as a specified phase. In this way, the software is effectively trained to do the phase discrimination.

Application Examples

1: Dual Phase Ferritic-Martensitic Steel

The dual phase steel discussed above is shown as the first example; the EBSD data collected from this sample was interrogated, a phase map is shown in Fig. 2. This data shown is as collected, with both phases: ferrite and martensite, indexed against Fe BCC match unit (a match unit being the crystallographic parameters used for EBSD indexing). The poor pattern quality for the martensite means that in parts of the map this phase is not indexed at all, shown by the black regions. However, the band slope map shown in Fig. 1 indicates the presence of two distinct phases in the steel.

Fig. 2. (left) EBSD-phase map of the steel, indexed against an Fe BCC. Regions of poorest pattern quality are not indexed, shown in black.

Fig. 3 (right) Reclassified phase map and associated phases statistics from the dual phase steel.

To separate these phases the three parameters: band slope, band contrast and pattern quality were used. By manually selecting regions in the map the brighter regions are defined as ferrite and darker regions defined as martensite. Then using the phase training tool the software can accurately distinguish between the two phases, shown in Fig. 3.

2: Advanced High Strength Transformation-Induced Plasticity (TRIP) Steel

The second example is a more complex TRIP steel. These advanced high strength transformation-induced plasticity (TRIP) steels were mainly developed for automotive applications as they possess high strength–ductility ratios, formability and energy absorption properties [1]. They have a complex multiphase microstructure containing retained austenite, martensite, ferrite and bainite.

As shown in Fig. 4, TRIP steel microstructures are typically indexed during EBSD acquisition with Fe FCC (austenite) and Fe BCC match units. However, the phase indexed as Fe BCC includes martensite, ferrite and bainite. To get an accurate differentiation of these phases, they can be separated using Reclassify Phase.

Fig. 4. (left) EBSD-phase map of the TRIP steel indexed as Fe BBC and Fe FCC.

Fig. 5 (right) Band contrast map of the TRIP steel.

The significant contrast difference in the band contrast map (Fig. 5) indicates the four different phases. As a result of the varying dislocation density and local stress concentration in these different phases, the pattern quality is better in ferrite and austenite but worse in bainite. In the band contrast map, the darkest grey is bainite, brightest is both ferrite and austenite, while the mid-grey level is martensite.

Using the same method described in the first example the different BCC phases are identified based on pattern quality. The result is an accurate differentiation of the four phases and the phase statistics, as shown in Fig. 6.

Fig. 6. Reclassified phase map and associated phase stastics of the four phases in the TRIP steel.

Conclusion

A new innovative method has been developed to discriminate phases based on EBSD pattern quality. This solution has been demonstrated on both ferritic-martensitic steel and TRIP steel samples. The identification and quantification of these phases are important in understanding the steel performance and optimisation of steel design.

Reference

[1] K.-I. Sugimoto, M. Kobayashi, S.-I. Hashimoto, Ductility and strain-induced transformation in a high-strength transformation-induced plasticity-aided dual-phase steel, Metall. Trans. A 23 (1992) 3085–3091.