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H Jiang and H S Ubhi

Evolution of EBSD Patterns during Sample Preparation

Successful EBSD work for characterizing and quantifying microstructures requires good metallographic preparation procedures. This study is an attempt to illustrate how EBSD patterns evolve through the various metallographic preparation steps using duplex steel and alumina samples as examples.

MethodologySeveral specimens about 3 x 4mm were cut from the duplex steel and alumina sheet samples, and mounted in conductive bakelite. In order to observe the affects of intrinsic deformation one surface of the steel sample was shot peened. Samples were prepared using standard metallographic procedures.

After each stage of grinding or polishing steps given in the Table 1, the specimens were examined in a FEGSEM and typical images plus EBSD patterns were recorded. No attempt was made to either optimise any of the polishing variables like load, rotation, time, or follow any specific route prescribed by suppliers of metallographic consumables. In the case of the duplex steel after each mechanical polishing step, the specimens were further electro-polished. Electro-polishing was carried out using a mixture of 5% perchloric acid and 95% acetic acid at a voltage of 40V, a temperature of 5ºC and about 10s duration in an eletro-lytic cell.

Figure 1. Series of BEI, FSD and EBSD images at various steps of polishing for Alumina

ConclusionsThis study clearly shows that evolution of EBSD patterns is dependent not only on surface polishing but also on the phases and deformation present in the specimens for visualization of microstructures using the EBSD technique. Standard metallographic polishing followed by final colloidal silica polish results in strain free surfaces suitable for EBSD. Where electro-polishing is possible satisfactory patterns can be obtained after any stage during mechanical grinding or polishing.

Figure 1 shows a series of typical back-scattered , fore-scattered electron images and EBSD patterns the preparing the alumina sample. An IPF coloured EBSD map for the final step is also shown. Good quality patterns are possible from the un-prepared surface, however, for satisfactory EBSD characterization a flat surface is preferable. Satisfactory patterns result after final colloidal silica or vibratory polishing steps. Ceramics being insulators cannot be electro-polished.

Results for the duplex steel for the mechanical and electro-polishing steps respectively in terms of back scattered electron images, typical EBSD patterns from the ferritic and austenitic phases and raw phase and local mis-orientation maps plus a average mis-orientation vs distance plot in Figure 2. It is clear from these data that weak EBSD patterns are observed from the ferrite phase after 9µm diamond polishing, whereas austenitic phase starts to produce patterns after the 3µm polishing step. EBSD phase maps from the 3µm step clearly show that only the ferrite

phase is indexedable. After the 3µm step both phases can be indexed, with improved hit rate, although the fraction of the austenite is very low and the local mis-orientation due to surface deformation are still large. The best patterns were obtained for both phases after a final colloidal silica or vibratory polish. EBSD maps from this step give a much truer picture of the microstructure, due to the removal of surface deformation which leads to high indexing rates. In addition local mis-orientation is now observed to be higher in the regions that have been subject to shot peening Electro-polishing results in excellent quality patterns in both phases after the final colloidal silica polishing step. No amount of mechanical or electrolytic polishing can produce sharp patterns from the intrinsically deformed surface introduced by shot peening, the effects of which become visible after 3µm diamond polishing and persists up to the electrolytic step. The effect of surface shot peening is clearly shown in EBSD maps as well as the average mis-orientations versus distance plot.

Acknowledgements Dr J R Saithala, Fine Tubes Ltd, Plymouth, UK, for providing the duplex steel sheet, Mr Graham Hammersley, Metal Improvement Co., Derby, UK for shot peening the duplex steel and Dr P Holdway, QinetiQ plc, Farnborough, UK, for providing the alumina samples.

Figure 2. Series of back-scattered images, from the Duplex stainless steel both mechanically and electro-lytically polished, typical EBSD patterns EBSD phase maps and local orientation maps and a mis-orientation vs distance plot.

As received

9 µm diamond

6 µm diamond

3 µm diamond

1 µm diamond

Colloidal Sillica

Vibratory Colloidal Silica EBSD IPF mapEasily indexable EBSP’s

Easily indexable EBSP’s

Indexable EBSP’s

Just indexable EBSP’s

Just indexable EBSP’s

Un-indexable EBSP’s

Indexable EBSP’s

BSE image at 0º tilt FSD images Typical EBSP’s

Results and Discussion

Material Polishing Abrasive Lubricant Rotation Force Time Surface Grit size Speed (rpm) (N) (min)

Steel SiC 500 water 300 20 2

Steel SiC 800 water 300 20 4

Steel/Alumina Grinding disc Diamond 9µm water 150 20 5

Steel/Alumina Satin woven Cloth Diamond 6µm Alcohol based 150 20 7

Steel/Alumina Satin woven Cloth Diamond 3µm Alcohol based 150 20 7

Steel/Alumina Syn. Short Nap cloth Diamond 1µm Alcohol based 150 15 7

Steel/Alumina Porous neoprene cloth Colloidal Silica 150 15 30

Steel/Alumina Porous neoprene cloth Colloidal Silica Vibratory 120

Table 1: Details various mechanical polishing steps.

ferrite austenite

ferrite austenite

ferrite austenite

phase map blue=ferrite red= austenite

Local misorientation map

ferrite and austenite easily indexable

ferrite and austenite easily indexable

ferrite and austenite easily indexable

Electro-polish

Electro-polish

Electro-polish

Electro-polish

ferrite austenite

ferrite and austenite easily indexable

800 grit SiC

9 µm diamond

6 µm diamond

3 µm diamond

1 µm diamond

Colloidal Silica

Vibratory

Poor quality un-indexable patterns

Poor quality un-indexable patterns

Poor quality un-indexable patterns

bcc just indexable

ferrite and austenite just indexable

ferrite and austenite easily indexable

ferrite and austenite easily indexable

EBSD phase map Local misorientation map

ferrite austenite

ferrite austenite

ferrite austenite

austeniteferrite

20µm

20µm