41

Nucleation and Texture Development During Dynamic Recrystallization of Copper

D. T. McDonalda, F. J. Humphreysb and P. S. Batec

Manchester Materials Science Centre, Grosvenor Street, Manchester M1 7HS, U.K.

ad.mcdonald@postgrad.manchester.ac.uk

bjohn.Humphreys@manchester.ac.uk

cpete.bate@manchester.ac.uk

Keywords: Dynamic Recrystallization; Texture; Copper;

Abstract

Dynamic recrystallization and texture development in polycrystalline copper have been

investigated. Specimens were deformed in channel-die plane strain compression to true strains

from 0.1 to 0.7 within the temperature range 200°C to 600°C, and the resulting microstructures

were investigated with the use of high resolution electron backscatter diffraction (EBSD). Dynamic

recrystallization in copper was initiated by the bulging of pre-existing high angle grain boundaries

(HAGB), and occurred primarily by strain induced boundary migration (SIBM). Increasing

misorientations from parent to dynamically recrystallizing grains indicated the occurrence of lattice

rotations within the bulges, leading, in some cases to the formation of a HAGB behind the bulge.

Discrimination between recrystallized and deformed components in material which had partially

undergone dynamic recrystallization was carried out, followed by texture analysis. This revealed

most of the recrystallized material to have orientations close to that of the deformed material,

however, some remote orientations were observed which could not be related to the deformation

texture by twin or 40° <111> relationships.

Introduction

Dynamic recrystallization is a softening or restoration process that occurs during deformation at

elevated temperatures in metals with a low to intermediate stacking fault energy [e.g.1]. It is of

considerable importance during the thermomechanical processing of metals, since an understanding

of this phenomenon would enable improved microstructural control during hot deformation.

Dynamically recrystallized grains tend to nucleate at existing HAGBs and at areas of high

misorientation gradient and dislocation density, such as at deformation bands and triple junctions.

Dynamic recrystallization generally originates at grain boundary bulges, which appear at relatively

low strains and are assumed to be a form of SIBM. Recent investigations into the nucleation of

dynamic recrystallization [2,3] have emphasized the role of twinning, and also the mechanism

proposed by Drury and Humphreys [4] involving HAGB bulging and lattice rotations within the

bulges, in the nucleation of dynamically recrystallized grains.

The evolution of texture during dynamic recrystallization is, at present, not fully understood.

Previous investigations into the development of texture during the dynamic recrystallization of

austenitic stainless steels [5] and the intermetallic TiAl [6] have resulted in the discovery of sharp

recrystallization textures. The dynamically recrystallized textures observed in copper, however,

have proven to be generally relatively weak and random, with only moderately intense deformation

components surviving [7,8]. The deformation conditions, notably the strain rate and deformation

temperature, have been shown to have a profound effect on the dynamically recrystallized textures

in copper [6,9]. The dynamically recrystallized textures of copper samples deformed in torsion at

low temperatures have recently been characterised in detail by Toth and Jonas [10,11] who found

that strong shear deformation components were observed at low temperatures, but at higher

Materials Science Forum Vols. 495-497 (2005) pp 1195-1200Online available since 2005/Sep/15 at www.scientific.net© (2005) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.495-497.1195

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 130.207.50.37, Georgia Tech Library, Atlanta, USA-13/11/14,06:50:03)

41

temperatures these weakened as other components including that of rotated cube strengthened.

Wusatowska-Sarnek et al. [2] similarly report the presence of a <101> fibre compression texture at

low temperatures, and although this component remained throughout a range of temperature, it was

found to weaken as the temperature increased. An explanation of the texture weakening in copper

at higher temperatures has been given by Hasegawa et al. [9], who found that an increase in

temperature was accompanied by an increase in the frequency of twinning during grain boundary

migration, and at very high temperatures, when the frequency of twins was large, the texture

became random.

The application of high resolution EBSD has enabled the discrimination between

recrystallized and unrecrystallized regions in materials which have undergone partial dynamic

recrystallization. This enables an investigation of texture relationships between dynamically

recrystallized grains and the parent grains from which they originate.

Experimental Procedure

Hot Deformation.

Polycrystalline oxygen free high conductivity (OFHC) copper specimens with dimensions 10mm x

10mm x 10mm were prepared with an initial grain size of 180µm from hot-rolled plate. The

specimens were deformed in channel-die plane strain compression to true strains from 0.1 to 0.7 at a

strain rate of 10-3

s-1

within the temperature range 200°C to 600°C. Each specimen was water-

quenched within 2 seconds of the load being removed in order to minimise metadynamic

recrystallization which has been shown to cause problems during studies of this nature [8]. The

deformed samples were sectioned on the ND-RD plane, and metallographically prepared. High

resolution EBSD was carried out in a Philips XL30 FEGSEM using the HKL Channel 5 acquisition

system, and by appropriate selection of the step size was used for both microstructural

characterisation and for evaluating bulk textures.

Identifying the Dynamically Recrystallizing Grains and Texture Measurement.

In samples which had undergone partial dynamic recrystallization, the recrystallized grains were

detected and quantified from EBSD maps, using the method developed for measuring static

recrystallization [12]. The microstructure was reconstructed into units of similar orientation

(cells/subgrains/grains) from the EBSD data, and recrystallized regions were defined in terms of the

following three parameters:- a) the size of the unit relative to the mean unit size, b) the pattern quality

of the unit and c) the percentage of the boundary of the unit which was high angle boundary. The

values of a-c were set by comparing the results with maps of orientation and pattern quality, and values

of a=3, b=0.8, c=10 were found to give consistent results. The size and orientations of the

recrystallized grains and the fraction recrystallized were calculated in this routine, and an example of a

map in which the dynamically recrystallized grains are identified is given in fig. 3. When the

recrystallized pixels in the map had been identified, it was then possible to construct pole figures or

ODFs of either the recrystallized or unrecrystallized material, as well as for the overall texture of the

material.

Results and Discussion

Nucleation of Dynamic Recrystallization.

Optical and SEM investigations of the microstructures of material deformed at 400°C showed a

range of features related to the nucleation of dynamic recrystallization. Figure 1a shows an

orientation contrast map of a bulging HAGB in a specimen deformed to a true strain of 0.22 at a

temperature of 400°C. The pre-existing HAGB appears to be bulging into regions of high stored

energy, and in the wake of these bulges a lack of substructure is observed, typical of SIBM. The

1196 Textures of Materials - ICOTOM 14

41

cumulative misorientation from a parent grain interior into a bulged region was found to increase, as

shown by Fig. 1b. It has been suggested [4] that boundary bulging may lead to non-uniform

deformation, leading to the development of misorientations within the bulges of the type observed.

Figure 2a shows an orientation contrast map of dynamic recrystallization in a slightly

more advanced stage. The original grains 1 and 2 had a twin relationship, and the small

dynamically recrystallized grain X, which has no significant substructure, and which has itself

twinned, has originated from grain 1. Figure 2b shows the cumulative misorientation from grain 1

into grain X along the line A. It is seen that there is a steadily increasing misorientation through

grain 1 until there is a discontinuous increase of 10-15o

at the interface of grain X.

Three smaller bulges are seen to the right of grain X. These are of orientations within 10o of grain

X, with misorientations of ~5o to grain 1 and are clearly undergoing a similar mechanism. More

detailed examination of grain 1 (fig. 2c) shows that it contains a diffuse deformation band, and that

the orientations of grain X and the smaller bulges are within the orientation spread of grain 1. It

appears that the recrystallization mechanism in this case is a combination of SIBM and nucleation

of dynamic recrystallization in the region bordering a deformation band.

Dynamic Recrystallization Textures.

In material which has started to dynamically recrystallize- but before the recrystallized grains have

strained to a significant extent- it is possible to automatically discriminate between the deformed

0

2

4

6

8

10

12

0 5 10 15 20 25

Distance (µm)

Mis

ori

en

tatio

n (

o)

A B

HAGB

Figure 1 EBSD map of copper

deformed to a true strain of 0.22

at 400°C.

a) Orientation contrast map.

HAGBs are in black and low

angle grain boundaries (LAGB)

in white.

b) Cumulative misorientation

data corresponding to line AB in

fig. 1a. Misorientations are

relative to grains A and B

respectively.

(a)

(b)

BB

AA

ND

RD

ND

RD

BB

AA

ND

RD

ND

RD

ND

RD

ND

RD

Materials Science Forum Vols. 495-497 1197

41

matrix and recrystallized grains. Overall texture data can then be derived which can, in principle,

give statistically significant information about relationships between recrystallized and precursor

orientations.

The immediate problems with this procedure are setting up the discrimination procedure and

dealing with the large amount of data involved. Data must be obtained at small step sizes to allow

discrimination and these step sizes must be smaller than the substructural scale, whereas they can be

larger than the grain size in conventional EBSD texture determination. Because of this, only a

limited amount of information is available at this time. This does begin to show the possibilities of

the method, however.

(a) (b)

(c)

Figure 3 shows one of four maps used for this analysis. The recrystallized grains were identified

using the method described above, and continuous orientation distributions were derived from the

point set data using harmonic series, truncated at lmax= 22 and without any Gaussian smoothing.

Sections of the ODFs from the two discriminated fractions are also shown in fig. 3. The deformed

fraction shows major components near Goss and brass orientations, and Goss is also present in the

recrystallized fraction along with a component near {012}<342>, on the edge of the brass

component in the deformed fraction.

The recrystallized orientations in that example were close to the deformed texture. The same

was found in other maps, although a component remote from the deformation texture also occurred.

Figure 2 EBSD of specimen deformed to

a true strain of 0.34 at 400°C.

a) Orientation contrast map with LAGBs

represented in white and HAGBs in

black.

b) Cumulative misorientation plot along

the line A.

c) Detail of map using relative

orientation contrast, showing the

deformation banding in grain 1.

ND

RD

ND

RD

ND

RD

Grain 1Grain 1

Grain 2Grain 2

AA

XX

Grain 1Grain 1

Grain 2Grain 2

AA

XX

0

2

4

6

8

10

12

14

16

18

20

0 5 10 15

Distance (µm)

Mis

orienta

tion (

o)

Grain XGrain 1

New HAGB

1198 Textures of Materials - ICOTOM 14

41

This was near {233}<311> and is shown in fig. 4, which gives relevant ODF sections for the

combined data sets. No consistent orientation relationship between the deformed and recrystallized

textures has been identified, but a 40º rotation about <111> can be ruled out even with the limited

statistics involved. It may be that the component near {233}<311> has resulted from twinning of a

component closer to the deformation texture, though this cannot be certain. It is quite reasonable

that recrystallized components would be close to the deformation texture in the early stages of

dynamic recrystallization when a boundary bulging mechanism is involved, as demonstrated in figs.

1 and 2, and a significant amount of growth twinning occurs in this material in both static and

dynamic recrystallization.

Figure 3 a) EBSD map of specimen deformed to a true strain of 0.34 at 400°C, which has been

coded to show the recrystallized (dark grey) and deformed (light grey) fractions. High angle

boundaries are shown black, low angle boundaries white. b) Sections of the ODFs of the two

fractions showing the dominant texture components.

Figure 4 ODF sections from the combined, discriminated data sets shown as overlays of the

recrystallized texture- with 3 and 5 x random contour lines- on the deformed texture with >5 x

random shaded. In the sections at ϕ2= 75º and 90º, recrystallized components close to the deformed

texture are discernable, where at ϕ2= 55º, the recrystallized component (A) is remote from the

deformed texture.

ϕ2= 55º ϕ2= 75º ϕ2= 90º

A

4

8

12

16

20

24

28

90º

90º

0º ϕ1

ϕ2= 0º Φ

Deformed

Recrystallized

(a) (b)

ND

RD

ND

RD

Materials Science Forum Vols. 495-497 1199

41

Summary

Dynamic recrystallization and texture development in OFHC polycrystalline copper have been

investigated. The main results are as follows:

• HAGB bulging has been observed as a precursor to dynamic recrystallization and appears to

be directly related to the substructure.

• Dynamic recrystallization in copper at 400oC occurs primarily by SIBM, although

subsequent twinning of the recrystallized grains is common.

• Increasing cumulative misorientation from a parent grain into a dynamically recrystallizing

grain indicates the occurrence of lattice rotations within the bulges, leading, in some cases to

the formation of a HAGB behind the bulge.

• There is evidence that the misorientations associated with the bulged regions may either

form before or after the bulging, and further investigation of this is required.

• Discrimination of the recrystallized components in the early stages of dynamic

recrystallization has been carried out. The derived textures show that most recrystallized

material had orientations close to that of deformed material, but some remote orientations

were observed. These were not related to the deformation textures by either twin or 40°

<111> relationships.

Acknowledgements

The support of EPSRC for this project through provision of a research studentship for D. T.

McDonald is gratefully acknowledged.

References

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Textures of Materials - ICOTOM 14 10.4028/www.scientific.net/MSF.495-497 Nucleation and Texture Development during Dynamic Recrystallization of Copper 10.4028/www.scientific.net/MSF.495-497.1195

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