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ARTICLE IN PRESSMRTEC-97; No. of Pages 6

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Available online at www.sciencedirect.com

www. jmrt .com.br

riginal Article

lectron backscatter diffraction (EBSD)icrostructure evolution in HPT copper annealed

t a low temperature�

lexander P. Zhilyaeva,b,∗, Semyon N. Sergeevb, Terence G. Langdona

Materials Research Group, Faculty of Engineering and the Environment, University of Southampton, Southampton, United KingdomInstitute for Metals Superplasticity Problems, Khalturina, Ufa, Russia

r t i c l e i n f o

rticle history:

eceived 6 June 2014

ccepted 25 June 2014

vailable online xxx

eywords:

ardness

igh-pressure torsion

omogeneity

a b s t r a c t

Detailed EBSD analysis was performed on copper specimens processed by high-pressure

torsion at P = 6 GPa for one whole turn and subsequently annealed at a temperature of

100 ◦C for 15, 30 and 60 min. The basic microstructural parameters (mean grain size, GB

statistics, microtexture) were evaluated in the mid-radius areas of the HPT disks. Micro-

hardness of all samples was measured across the two diameters and interlinked to the

microstructures observed. Small but noticeable changes of microhardness in HPT copper

after annealing were detected. The changes were interlinked to microstructural parameters

acquired by EBSD. The relationships obtained are discussed in terms of the microstructure

and microtexture evolution during low temperature annealing.

evere plastic deformation © 2014 Brazilian Metallurgical, Materials and Mining Association. Published by Elsevier

Editora Ltda. All rights reserved.

. Introduction

he processing of ultrafine-grained (UFG) and nanostructuredetallic materials using severe plastic deformation (SPD) [1]

s a new and promising method of enhancing the propertiesf metals and alloys for advanced structural and functionalpplications [2,3]. Traditionally, there have been two mainechniques for producing UFG materials using either equal-

Please cite this article in press as: Zhilyaev AP, et al. Electron backscatter

at a low temperature. J Mater Res Technol. 2014. http://dx.doi.org/10.1016/

hannel angular pressing (ECAP) [4] or high-pressure torsionHPT) [5,6] but other techniques are now available such asccumulative roll bonding (ARB) [7], multiaxial forging [8],

� Paper presented in the form of an abstract as part of the Proceedin014.∗ Corresponding author.

E-mail addresses: AlexZ@anrb.ru, A.Zhilyaev@soton.ac.uk (A.P. Zhilttp://dx.doi.org/10.1016/j.jmrt.2014.06.008238-7854/© 2014 Brazilian Metallurgical, Materials and Mining Associa

twist extrusion [9], plain strain machining (PSM) [10] and oth-ers [11]. SPD processing is attractive because the straining ispractically unlimited due to the unchanging sample geometryand shape. However, there is a general tendency for a satura-tion in grain refinement for high melting temperature metals[12] and recovery (and even recrystallization) for low meltingmaterials [13] processed by continuing SPD methods.

There is a considerable interest in creating UFG structurein pure copper and its alloys in order to get the optimum

diffraction (EBSD) microstructure evolution in HPT copper annealedj.jmrt.2014.06.008

gs of the Pan American Materials Conference, July 21st to 25th,

yaev).

combination of low wear rate and high conductivity. How-ever, pure copper has some distinct drawbacks, such as lowstrength and low thermostability, which tends to restrict its

tion. Published by Elsevier Editora Ltda. All rights reserved.

ARTICLE IN PRESSJMRTEC-97; No. of Pages 6

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2 j m a t e r r e s t e c h n

applications. There are numerous reports on the applica-tion of SPD for grain refinement of pure copper by ECAPand HPT where the thermostability of ultrafine-grained cop-per was analyzed. Probably the first comprehensive reportson copper were published in 1999 for ECAP [14] and 2000for HPT [15]. In the latter report, the microhardness of anHPT disk of 98.5% purity Cu (P = 5 GPa, N = 5 turns) dropsdown at an annealing temperature of about 180–200 ◦C. FromDSC data with a heating rate of 40◦/min, the most pro-nounced peak was detected for the HPT sample strained forN = 1 turn at 220 ◦C, which corresponds to the microhardnessmeasurements [15]. A similar temperature (∼190 ◦C) of recov-ery was also reported for higher purity HPT copper (99.9%).High purity cooper (99.99%) [16] shows similar behavior whenannealing at 134, 269 and 405 ◦C (0.3, 0.4 and 0.5 of melt-ing point) for 1 h. The microhardness drops down from 130to 80 Hv as the annealing temperature increases from 134 to269 ◦C.

There are some reports on low thermostability of UFG cop-per at room temperature in which recovery processes andgrain growth have been detected in a period of one monthand one year [17]. Other reports [18] show no significant grainrefinement in copper subjected to rolling at liquid nitrogentemperature suggesting that recovery processes take placeduring rolling.

In practice, the simplest way to achieve UFG copper is byHPT, which permits the processing of disks suitable for exper-iments on highly strained materials. In an earlier report [19],it was demonstrated that there is a slight increase of Vickersmicrohardness in HPT copper subjected to low-temperatureannealing. Careful monitoring of the microhardness andmicrostructure in HPT aluminum and copper stored at roomtemperature for long periods of time [20] has not revealed any

Please cite this article in press as: Zhilyaev AP, et al. Electron backscatter

at a low temperature. J Mater Res Technol. 2014. http://dx.doi.org/10.1016/

significant changes of these parameters. Thus, the presentreport was undertaken to study microstructure evolution ofHPT copper specimens subjected to annealing at 100 ◦C for 15,30 and 60 min.

25 µm

a b

Fig. 1 – Microstructure (a) and micorotext

0 1 4;x x x(x x):xxx–xxx

2. Experimental materials and procedure

Disks of copper were used as the starting material. Thematerial was purchased from Goodfellow Cambridge Ltd.,Huntingdon, UK, and the typical chemical composition wasgiven as (in ppm): Ag 500, Bi < 10, Pb < 50, O 400, other metals<300. The HPT specimens were in the form of disks havingdiameters of 10 mm and thicknesses of about 1 mm. Thesedisks were processed at room temperature by HPT for atotal of N = 1 turn under an applied pressure of P = 6 GPa.The processing was conducted under quasi-constrained con-ditions where there is a small outflow of material aroundthe periphery of each disk during processing. Parts of theprocessed specimens were annealed at 100 ◦C for a period of15, 30 or 60 min. During annealing, the HPT disks were placedon the flat surface of a thermocouple and hence the temper-ature was stable to within ±2◦. Prior to all measurements,the specimens were mechanically polished on 1000 grit SiCpaper followed by a final polish using a diamond suspensioncontaining monocrystalline diamond with a size of 3 �m withsubsequent electro-chemical polishing at room temperatureusing an electrolyte of HNO3:CH3OH = 1:3 with a voltage of 10 V.These samples were employed for microhardness measure-ments and electron backscatter diffraction (EBSD) analysis.

The microhardness was measured with steps of 0.5 mmalong the diameters of the disks using an FM-300 testerequipped with a Vickers indenter using a load of 100 gf anda dwelling time of 15 s.

The EBSD analysis was performed using a TESCAN MIRA3LMH FEG scanning electron microscope equipped with anEBSD analyzer “CHANNEL 5”, and a rectangular grid with scanstep of 50 nm was used. The EBSD analysis was performedfor regions located near the center of the disk, near the mid-

diffraction (EBSD) microstructure evolution in HPT copper annealedj.jmrt.2014.06.008

radius (2.5 mm from the center) and near the edge (∼4.5 mmfrom the center.) The acquired data were subjected to standardclean-up procedures involving a grain tolerance angle of 5◦ and

001 001

TD TD

RDRD

111

RD

TD

Texture name: harmonic: L=16, HW=5.0Calculation method: harmonic series expansionSeries rank (1): 16Gaussian smoothing: 5.0єSample symmetry: triclinic

max=4.1933.3022.6002.0481.6131.2701.0000.787

ure (b) of Cu specimen prior to HPT.

ARTICLE IN PRESSJMRTEC-97; No. of Pages 6

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Misorientation angle

Num

ber

Fra

ctio

n

Misorientation angle [degrees]

0.00

0.02

0.04

0.06

0.08

0.10

10 20 30 40 50 602 µm

a b

ution (b) at mid-radius of Cu disk after HPT (P = 6 GPa, N = 1).

amh

3

FtTwgo

mltmowowmrH

140

150

130

120

110

Annealing time (min)

Inte

grat

ed H

v

0 10 20 30 40 50 60

Fig. 3 – Integrated microhardness at x = r/2 as a function of

Fig. 2 – Microstructure (a) and GB misorientation distrib

minimum grain size of three pixels. The grain sizes wereeasured using the linear intercept as the distances between

igh-angle boundaries with misorientations above 15◦.

. Experimental results

ig. 1 presents (a) an initial microstructure and (b) the micro-exture of a copper specimen prior to high-pressure torsion.he average grain size by EBSD was larger than 25 �m withell-defined grain boundaries and twins in the interior of

rains. The pole figures in Fig. 1(b) reflect the inherent texturef extruded rods of the primary material.

Fig. 2 shows the inverse pole figure map and grain boundaryisorientation distribution of HPT copper processed under a

oad of P = 6 GPa for one whole revolution where the EBSD wasaken at the mid-radius of the disk. Significant grain refine-

ent with grains elongated along the torsional direction isbserved. The mean grain size by EBSD was about 200–300 nmhich is a typical grain size for HPT copper. The fractionf low-angle boundaries was below 10%, which is consistent

Please cite this article in press as: Zhilyaev AP, et al. Electron backscatter

at a low temperature. J Mater Res Technol. 2014. http://dx.doi.org/10.1016/

ith earlier reports [3]. Fig. 3 represents the integrated Vickersicrohardness taken at a distance of r/2, where r is the mid-

adius of the HPT disk. A slight increase in Hv is observed forPT copper annealed for 15 min. Although it is in the range of

60.0 40.0

2-Theta

Cu

200.0

400.0

Inte

nsity

1/2 [C

ount

1/2 ]

Fig. 4 – X-ray profile

annealing time.

the experimental error, the gain in microhardness is about 4%and it was systematically detected.

diffraction (EBSD) microstructure evolution in HPT copper annealedj.jmrt.2014.06.008

An X-ray analysis of HPT-processed copper in Fig. 4 gives asize of coherent domains of 287.5 ± 6.3 nm, which is in goodagreement with the EBSD data. Fig. 5 shows a Taylor factor map

80.0 100.0

[degrees]

of HPT copper.

Please cite this article in press as: Zhilyaev AP, et al. Electron backscatter diffraction (EBSD) microstructure evolution in HPT copper annealedat a low temperature. J Mater Res Technol. 2014. http://dx.doi.org/10.1016/j.jmrt.2014.06.008

ARTICLE IN PRESSJMRTEC-97; No. of Pages 6

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2 µm

2 µm

2 µm5 µm

A B

C

D

Fig. 5 – Taylor factor map of HPT copper: (a) initial, and after annealing at T = 100 ◦C for (b) 15 min, (c) 30 min and (d) 60 min.

2 µm

2 µm

2 µm 2 µm

a b

c d

Fig. 6 – Kernel average misorientation of HPT copper: (a) initial and after annealing at T = 100 ◦C for (b) 15 min, (c) 30 min and(d) 60 min.

ARTICLE IN PRESSJMRTEC-97; No. of Pages 6

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Table 1 – Experimental parameters of HPT copper subjected to annealing at 100 ◦C for 15, 30 and 60 min.

Specimen d (�m) KAM (◦) Taylor factor LAB (%) d (nm) Microstrain‹�2›1/2 (×10−3)

Dislocation density(m−2, ×1014)

EBSD EBSD EBSD EBSD X-ray X-ray EBSD X-ray

HPT Cu 0.33 0.537 3.318 20.4 193.8 2.125 7.322 1.484HPT Cu + ann. 15′ 0.36 0.553 3.295 23.1 185.0 2.150 7.540 1.573HPT Cu + ann. 30′ 0.39 0.563 3.359 21.1 210.0 2.300 7.677 1.482

faFsaT3ot(a1a

mgatfc(m

4t

IihloDeotu(BtwsdefcoXs

r

HPT Cu + ann. 60′ 0.34 0.549 3.317 20.1

or HPT copper after processing in Fig. 5(a) and after annealingt 100 ◦C for 15 min in Fig. 5(b), 30 min in Fig. 5(c) and 60 min inig. 5(d). The average value of the Taylor factor (TF) decreaseslightly from 3.318 for HPT copper to 3.295 for the HPT disknnealed for 15 min. For the sample annealed for 30 min, theF increases to a value of 3.359 and then it decreases again to.317 for the specimen annealed for 60 min. These variationsf the Taylor factors are not in agreement with changes inhe Vickers microhardness. The Kernel average misorientationKAM) maps for copper specimens are shown in Fig. 6. Theverage KAM also increases for the specimens annealed for5 and 30 min and then slightly decreases for the HPT coppernnealed for 60 min.

Table 1 provides a comprehensive summary of all experi-ental data obtained in this investigation. The area weighted

rain size obtained from EBSD shows a gradual increase withnnealing time. Also, there is a noticeable increase in the frac-ion of low-angle boundaries (LAB) for the specimen annealedor 15 min. X-ray analysis revealed a small decrease in theoherent domain size from 193.8 nm (HPT copper) to 185 nmafter annealing for 15 min) and a gradual increase in the

icrostrain level.

. Microstructure evolution during lowemperature annealing

n pure metals, there are a limited number of factors influenc-ng hardening: the Hall–Petch relationship and the dislocationardening and texture (corresponding to a change in the Tay-

or factor). The texture can change during significant changingf the microstructure as in recrystallization and grain growth.uring low temperature, annealing at 100 ◦C it is difficult toxpect significant changes in texture and the average valuesf the Taylor factor for all specimens clearly support this. Onhe basis of the experimental parameters, it is possible to eval-ate the dislocation density either using the average KAM

� = �/(b·h), where � is the average angle in radians, b is theurgers vector and h is the step size of 50 nm in EBSD), or usinghe microstrain level measured by X-rays (� = 2

√3‹�2›1/2/(b·d)),

here ‹ε2›1/2 is the microstrain and d is the coherent domainize. The two final columns in Table 1 represent the dislocationensities calculated on the basis of X-ray analysis and EBSDxperiments, respectively. The dislocation density calculatedrom KAM corresponds to the geometrically necessary dislo-

Please cite this article in press as: Zhilyaev AP, et al. Electron backscatter

at a low temperature. J Mater Res Technol. 2014. http://dx.doi.org/10.1016/

ations (GND) and it is about 3 times higher than the densityf the statistically stored dislocations (SSD) estimated from-ray analysis. Both values show a slight increase in the Cupecimen annealed for 15 min.

200.0 2.250 7.486 1.522

5. Summary

Integrated microhardness measurements at the mid-radiusposition show a systematic increase for the HPT copper sub-jected to low temperature annealing at 100 ◦C for 15 min. Themost correlated parameters that may be responsible for thischange are the fraction of low-angle grain boundaries and thesize of the coherent domains.

Conflict of interest

The authors declare no conflicts of interest.

Funding

This work was supported in part by the European ResearchCouncil under ERC Grant Agreement No. 267464-SPDMETALS(APZ & TGL).

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