Using electron backscatter diffraction (EBSD) to measure misorientation between ‘parent’ and ‘daughter’ grains. Implications for

recrystallisation and nucleation.

Angela Halfpenny1, David J. Prior1 and John Wheeler1

1Department of Earth and Ocean Sciences, Jane Herdman Building, University of Liverpool,

Liverpool, UK, L69 3GP

Keywords: EBSD, misorientation, recrystallisation,

Abstract

Electron backscatter diffraction (EBSD) is an extremely valuable tool, as it measures full

crystallographic orientation information. This technique has been used to measure the statistics of

misorientations between original ‘parent’ grains and recrystallised ‘daughter’ grains in a mylonitic

quartzite. The angle of misorientation has implications on the controlling recrystallisation

mechanism.

The sample is a natural mylonitic quartzite collected from the stack of Glencoul, NW

Scotland. The sample exhibits a common partially recrystallised microstructure. The data shows

the average misorientations between the ‘parent’ and ‘daughter’ grains are 30º, this value seems too

high for only subgrain rotation recrystallisation to be taking place. Moreover there is no gradation

in the boundary misorientation from the internal substructure of the ‘parent’ grain to the ‘daughter’

grains. The internal substructure size of the ‘parent’ grain is bigger than the size of the ‘daughter’

grains. For subgrain rotation recrystallisation you may expect to see a core and mantle structure

and for the ‘daughter’ grains’ to be of similar size to the internal substructure of the ‘parent’ grain.

Another mechanism has either controlled the recrystallisation altogether or has become active after

subgrain rotation had taken place and modified the microstructure.

Introduction

Creep deformation of rocks to high strains is facilitated by the processes of recovery and

recrystallisation [1];[2]. Recovery involves the rearrangement of dislocations generated by

deformation to create strain free areas (subgrains) surrounded by dislocations arranged into coherent

walls. Recrystallisation involves the nucleation and growth of new grains. Without recovery and

recrystallisation, steady state flow would be impossible, as the dislocations would interfere with

each other. Although recovery is quite well understood, the precise mechanisms by which

recrystallisation occurs are less clear. Most particularly, we do not understand fully the

mechanisms by which recrystallised grains nucleate.

The individual mechanisms by which recrystallised grains nucleate and grow and other

processes that might operate in tandem, such as grain boundary sliding, have specific, predictable

effects on the crystallographic relationships between host and recrystallised grains or as we name

them ‘parent’ and ‘daughter’ grains [3]. We can then analysis this crystallographic relationship

using electron backscatter diffraction (EBSD) which is a scanning electron microscope (SEM)

based technique that enables us to measure the orientations of individual grains (as small as ~ 50nm

in minerals)[4].

When recrystallisation is synchronous with deformation, it is called dynamic

recrystallisation. In the absence of concurrent deformation it is called static recrystallisation.

Dynamic recrystallisation occurs by nucleation, grain boundary migration and/or subgrain rotation.

Static recrystallisation involves grain boundary area reduction, with minor subgrain rotation, grain

boundary recrystallisation and recovery, which leads to the removal of undulose extinction,

Materials Science Forum Vols. 467-470 (2004) pp. 573-578online at http://www.scientific.net© 2004 Trans Tech Publications, Switzerland

Licensed to John Wheeler (johnwh@liv.ac.uk) - University of Liverpool - UKAll rights reserved. No part of the contents of this paper may be reproduced or transmitted in any form or by any means without thewritten permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 138.253.112.107-05/11/04,13:27:54)

straightening of grain boundaries and grain growth. The remaining dislocations and the large

surface of grain boundaries drive the process, mainly after deformation [1].

Evidence for dynamic recrystallisation is usually more difficult to find than evidence for

deformation or recovery. Two types of characteristic microstructures can be distinguished, which

are partially and completely recrystallised fabrics. In a partially recrystallised fabric a bimodal

grain size distribution is characteristic, with aggregates of small grains of nearly uniform size

between large grains exhibiting undulose extinction. Undulose extinction is an irregular extinction

of a single crystal under crossed polars due to a distorted crystal lattice with a high concentration of

defects. The small grains are probably new grains or ‘daughter’ grains formed by dynamic

recrystallisation.

A completely recrystallised fabric can be difficult to distinguish from a non-recrystallised

equigranular fabric. However, in an aggregate of grains formed by complete dynamic

recrystallisation, the grains will show evidence of internal deformation, a crystallographic preferred

orientation (CPO) and a relatively uniform grain size. By analysing the misorientation between the

original ‘parent’ grain and the recrystallised ‘daughter’ grains we can map out statistically the

crystallographic characteristics of the microstructure. This can then be compared with the predicted

effects of known recrystallisation and nucleation mechanisms, to determine the controlling

mechanism(s). The mechanisms are outlined below.

Grain boundary migration recrystallisation occurs in many cases due to two neighbouring

deformed grains have different dislocation densities, one with high and one with low; the grain

boundary may start to bulge into the grain with the highest dislocation density and form new,

independent crystals, this process is known as bulging. The process reduces the internal free energy

of the crystalline aggregate involved and is known as grain boundary migration [1].

Recovery can drive dislocations to be continuously added to subgrain boundaries. This

process is known as climb accommodated dislocation creep. As more dislocations enter the

subgrain boundary the angle between the crystal lattice on both sides of the subgrain boundary

increases until, gradually, the subgrain can no longer be classified as part of the original ‘parent’

grain. The development of a new grain by the progressive misorientation of subgrains is known as

subgrain rotation recrystallisation [2].

Sample description

The sample used in this study is a natural mylonitic quartzite collected from the stack of

Glencoul, NW Scotland (Fig. 1a). Formation of these mylonites is associated with greenschist

facies metamorphism [5]. The sample exhibits a common partially recrystallised microstructure.

The thin section used represents an XZ section, which is parallel to lineation and perpendicular to

mylonitic foliation (Fig. 1b). The protolith was a fairly pure Cambrian quartzite [6].

Figure 1a Field

photograph of the sample

location

Figure 1b Cross polarised

light optical microscope

picture of the XZ thin

section

a b

Recrystallization and Grain Growth574

EBSD and OC- imaging techniques

The analytical work was carried out on a Camscan X500 Crystal Probe SEM at 20 kV, with

a beam current of about 20nA, a working distance of about 24 mm and a specimen tilt of 70º.

Orientation contrast (OC) imaging used three solid-state forescatter detectors. The OC images

provided maps of where crystallographic orientations change. EBSD was used to measure the

crystallographic orientation. The EBSD patterns where imaged on a phosphor screen, viewed by a

low light camera and indexed using the HKL software package Channel 5. The thin sections where

prepared by the University of Birmingham. The sections were then chemically-mechanically

polished using syton fluid [7] and carbon coated using an Emitech K950X coating machine. The

sample had to be carbon coated due to prevent charging.

Results and Interpretations

The ‘parent’ grains are identified due to being relatively large, stretched out ribbon grains of the

original protolith. The recrystallised ‘daughter’ grains are identified as the small grains around the

edge of the ‘parent’ grain (Fig. 2c). From figure 2b you can see that the average misorientation

between the ‘parent’ and ‘daughter’ grains is 30º and there is no gradation in the boundary size (i.e.

increasing misorientation towards the edge) from the internal substructure of the ‘parent’ grain to

the ‘daughter’ grain. If you take a value of 2º for the critical misorientation the average internal

Figure 2a A band contrast map of a

‘parent’ and ‘daughter’ grain

relationship.

Figure 2b A band contrast map with

grain boundaries identified. Upto 5º

= light grey, >30º = black. At 2º

critical misorientation the average

subgrain size inside the ‘parent’ grain

is 8.53µm. The average ‘daughter’

grain size is 3.89µm.

Figure 2c A grain size map

illustrating the ‘parent’ in mid grey

and the ‘daughters’ in dark grey. The

average ‘daughter’ grain size at 10º

critical misorientation is 5.14µm.

1a 1b 1c

Materials Science Forum Vols. 467-470 575

subgrain size of the ‘parent’ is 8.53µm whereas the average ‘daughter’ grain size is 3.89µm. If you

take a value of 10º for the critical misorientation the average ‘daughter’ grain size is 5.14µm.

If subgrain rotation were the controlling recrystallisation mechanism we would expect to see

a gradual transition of aggregates of subgrains to aggregates of new grains with approximately the

same size. We would also expect to see subgrain boundaries, which pass laterally into grain

boundaries [1]. From the data it can be seen that the subgrain size and the recrystallised ‘daughter’

grain size are not the same, the ‘daughter’ grains are significantly smaller than the subgrains. The

data shows that there is no transition from subgrains to recrystallised ‘daughter’ grains and the

angle of misorientation is too great to have been produced by subgrain rotation alone. This infers

that another mechanism has either controlled the recrystallisation altogether, has become active

after subgrain rotation has taken place and modified the microstructure or that there are more than

one recrystallisation mechanism active during the deformation.

Figure 3a shows the ‘parent’ and ‘daughter’ grains plotted on a stereonet. Figure 3b shows

just the ‘parent’ grain, which plots as one tight cluster of points showing a single, dominant, c-axis

orientation. Figure 3c shows just the recrystallised ‘daughter’ grains. These grains plot in two,

fairly dispersed clusters. One of the clusters plots in a similar position to the ‘parent’ grain (as

shown in figure 3a), but the other plots in an equal position but on the opposite side of the stereonet.

Figure 3a Stereonet of the

‘parent’ and ‘daughter’ grains in

greyscaled all euler full

orientation colours.

Figure 3b Stereonet of the

‘parent’ grain only in

greyscaled all euler full

orientation colours.

Figure 3c Stereonet of the

‘daughter’ grains only in

greyscaled all euler full

orientation colours.

Recrystallization and Grain Growth576

Parent

Parent

Daughters

from the

right hand

side of the

parent

grain

Daughters

from the

left hand

side of the

parent

grain

There are different possible explanations for why the recrystallised ‘daughter’ grains create

two clusters. One of the explanations is that one of the clusters of ‘daughter’ grain orientations

represents grains, which have recrystallised from that parent grain and the second cluster represents

‘daughter’ grains, which have recrystallised from a second ‘parent’ grain. Another possible

interpretation is that each cluster orientation is controlled by a different nucleation mechanism,

which causes the orientation of the recrystallised ‘daughter’ grains to form two clusters. An

alternative possible explanation is that the recrystallised ‘daughters’ on each side of the ‘parent’

grain show both orientations, so its not one side creating one clusters, but that each side creates a

part of each cluster.

Figure 4 shows the grain size map of the ‘parent’ and ‘daughter’ grains with located, crystal

orientations marked on the map. The four crystal orientations for the parent grain are showing

nearly exactly the same orientation, which is what we expect from the stereonet data. The four

crystal orientations taken from the right hand side ‘daughter’ grains do not show a common

orientation and neither do the four crystal orientations taken from the left hand side ‘daughter’

grains. The data is showing that the recrystallised ‘daughters’ on each side of the ‘parent’ grain

show both orientations, so its not one side creating one clusters, but that each side creates a part of

each cluster.

Figure 5 shows a texture component map, which illustrates the internal strain of the parent.

It is clear that there is higher strain in the boundary region of the ‘parent’ grain whereas the centre

of the ‘parent’ grain is exhibiting low strain. Where there is a sudden change from light to dark that

shows a significant change in the orientation and the internal strain. This strain partitioning may

have effected which recrystallisation mechanism controlled the deformation.

Figure 4 A grain

size map showing

the orientation of

certain located

recrystallised

‘daughter’ grains.

Figure 4

Figure 5

Figure 5 A texture

component map

showing the internal

deformation for the

parent grain only.

Light areas = 0º

orientation change,

dark areas = 20º

Materials Science Forum Vols. 467-470 577

Conclusions

The average ‘parent’ to ‘daughter’ misorientation angle is 30º. The recrystallised ‘daughter’

grain size is significantly smaller than the internal subgrain size of the ‘parent’ grain. Moreover

there is no gradation in misorientation angle size from the internal substructure of the ‘parent’ grain

to the ‘daughter’ grains. The two orientation clusters for the recrystallised ‘daughters’ can be

accounted for via recrystallisation off the one ‘parent’ grain. These microstructural characteristics

do not fit in with a recrystallisation mechanism controlled by subgrain rotation alone. Either

subgrain rotation was not the active recrystallisation mechanism or another mechanism became

active after subgrain rotation and modified the microstructure. The other possibility is that no one

mechanism controlled the recrystallisation but a combination of recrystallisation mechanisms

formed the microstructure.

Acknowledgments- Thanks to Dave Prior and John Wheeler for supervising this project. Thanks

to ongoing discussions with Sandra Piazolo, Pat Trimby and Geoff Lloyd, which have helped in the

development of this project. This project was funded by UK NERC grant ner/a/s/2001/01181.

References

[1] C.W. Passchier and R.A.J. Trouw, Microtectonics. 2nd ed. 1996, Berlin Heidelberg New

York: Springer-Verlag. 289.

[2] J.L. Urai, W.D. Means, and G.S. Lister, Dynamic recrystallisation of minerals. Geophysical

Monograph, 1986. 36(The Paterson Volume): p. 161-199.

[3] J. Wheeler, et al., The petrological significance of misorientations between grains.

Contributions to mineralogy and petrology, 2001. 141: p. 109-124.

[4] D.J. Prior, et al., The application of electron backscatter diffraction and orientation contrast

imaging in the SEM to textural problems in rocks. American Mineralogist, 1999. 84: p.

1741-1759.

[5] R.D. Law, Heterogenous deformation and quartz crystallographic fabric transitions:

natural examples from the moine thrust zone at the stack of Glencoul, northern Assynt.

Journal of Structural Geology, 1987. 9(2): p. 001-015.

[6] R.D. Law, M. Casey, and R.J. Knipe, Kinematic and tectonic significance of

microstructures and crystallographic fabrics within quartz mylonites from the Assynt and

Eriboll regions of the Moine thrust zone, NW Scotland. Transactions of the Royal Society of

Edinburgh, 1986. 77: p. 99-125.

[7] G.E. Lloyd, C.C. Ferguson, and R.D. Law, Discriminatory petrofabric analysis of quartz

rocks using SEM electron channelling. Tectonophysics, 1987. 135: p. 243-249.

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