ULTRAFINEGRAINED MATERIALS

Long-term self-annealing of copper and aluminium processedby high-pressure torsion

Alexander P. Zhilyaev • Terence G. Langdon

Received: 29 January 2014 / Accepted: 27 March 2014

� Springer Science+Business Media New York 2014

Abstract High-pressure torsion (HPT) is recognized as

the most effective method for producing ultrafine-grained

and even nanocrystalline structures in metallic systems.

Although there are many reports on microstructural

refinement of pure metals and metallic alloys, several

important problems remain unresolved. For example, more

information is needed on the homogeneity of the processed

microstructure and on the heat release and temperature rise

during HPT processing. Recently, there were reports of

hardening in HPT-processed pure metals under conditions

of self-annealing. This report presents new experimental

data on the relaxation processes in HPT copper and alu-

minium processed at room temperature under loads of 6.0

and 1.0 GPa for zero and one turn. The experimental

results were obtained using X-ray diffraction and by

measuring the average Vickers microhardness after ageing.

Introduction

The thermal stability of materials processed by severe

plastic deformation (SPD) was a significant concern from

the first experiments in SPD processing, but it becomes of

paramount importance if the materials are tested at

elevated temperatures. These possible limitations on SPD

processing are addressed most readily by examining the

thermal stabilities of the ultrafine-grained (UFG) micro-

structures after processing using high-pressure torsion

(HPT) [1, 2] where HPT was selected because it is the

optimum procedure for achieving exceptional grain

refinement to the submicrometer or even the nanometer

level [3, 4].

An early report on thermostability described experi-

ments on Cu processed by HPT to give grain sizes of

*114 nm [5]. In these experiments, the densities of the

lattice dislocations were estimated from X-ray diffraction

(XRD) as *1014 m-2 and a series of annealing experi-

ments showed that the onset of grain growth began at

temperatures above *175 �C. These results were sup-

ported by differential scanning calorimetry using a heating

rate of 10 K min-1 where the onset of an exothermic peak

in the UFG Cu was detected at *120 �C [5]. The enthalpy

values were equal to *0.26 for the Cu sample, and the

annealing results were further confirmed by taking mea-

surements of the electrical resistance and the microhard-

ness where thermal stability was also evident up to

temperatures of *175 �C for HPT Cu. Later, the thermo-

stability of HPT copper was investigated as a function of

the numbers of whole revolutions applied to the material,

and in these experiments the temperatures for the onset of

recovery and grain growth were estimated as *180 �C [6].

Similar observations were also reported for HPT copper

using a pressure of 8 GPa and rotation through three turns

where there was a microhardness level of *150 MPa and a

crystallite size of *0.2 lm with high dislocation density of

*4 9 1015 m-2 [7]. Annealing at temperatures of 134,

269 and 405 �C for 1 h resulted in a monotonic decrease in

the microhardness and the dislocation density, and a cor-

responding increase in the crystallite size.

A. P. Zhilyaev (&) � T. G. Langdon

Materials Research Group, Faculty of Engineering and the

Environment, University of Southampton,

Southampton SO17 1BJ, UK

e-mail: AlexZ@anrb.ru; A.Zhilyaev@soton.ac.uk

A. P. Zhilyaev

Institute for Metals Superplasticity Problems, Khalturina 39,

Ufa 450001, Russia

123

J Mater Sci

DOI 10.1007/s10853-014-8208-1

At the same time, there exist some controversies con-

cerning the microhardness evolution during low-tempera-

ture annealing of HPT copper. Thus, there is a report of a

slight increase in the microhardness of HPT Cu after

annealing at low temperatures, and this was explained by the

formation of di-vacancies during low-temperature annealing

because of the abundance of vacancies in HPT copper [8].

Thus, the concentration of vacancies may be as high as the

value at the melting point (C1V & 1.5–3.0 9 10-4 [9]) or

even higher. The slight increase in the microhardness during

low-temperature annealing is supported by recent data where

there was a 5 % increase of Hv in commercial-purity copper

deformed by HPT and annealed at 100 �C for 1 h [10].

Room-temperature annealing of copper by electron back-

scattered diffraction after cryo-rolling has revealed abnor-

mal grain growth which was detected after natural ageing

over a period of 1.5 years [11]. Even in high melting point

materials, such as Ni–20 % Cr alloy, natural ageing over

7.5 years yielded a mean grain size which was reduced by

*30 % [12].

It is apparent from this brief overview that natural

ageing of the heavily deformed pure metals may invoke

relaxation processes including vacancy and dislocation

annihilation plus recrystallization and grain growth.

Accordingly, the aim of the present investigation was to

monitor the evolution in the Vickers microhardness, Hv,

and to use XRD to measure the evolution of appropriate

microstructural parameters including the lattice parameter,

a: the crystallite size, d: and the microstrain, he2i1/2. The

experiments were conducted on copper and aluminium of

commercial purity subjected either to compression under

an applied pressure for 1 min without torsional straining,

equivalent to N = 0 where N is the number of turns, or to

torsional straining under an applied pressure through

N = 1 turn. These parameters were monitored for periods

of time corresponding to 1, 12, 24, 48 and 96 h after HPT

processing.

Experimental materials and procedures

Disks of commercial-purity aluminium and copper were

obtained from Goodfellow Cambridge Ltd. (Huntingdon,

UK). The typical chemical compositions were given as (in

ppm) for copper: Ag 500, Bi \10, Pb \50, O 400, and

other metals \300; and for aluminium: Cu \500, Fe

\4000, Mn \500, Si \3000, and Zn \1000. Before HPT

processing, the aluminium and copper were annealed for

1 h at 650 and 950 �C, respectively. Measurements showed

that the initial Vickers microhardness values were

Hv & 18 for the annealed aluminium, and Hv & 40 for the

annealed Cu, and these values are in good agreement with

those reported in the literature. As in Fig. 1, X-ray analysis

showed that the full width at the half maximum (FWHM)

of the (111) peaks of the annealed Al and Cu specimens

are very close to that determined from the standard which

was used for measuring an instrumental broadening of

*0.06�.

The HPT specimens were cut in the form of disks having

diameters of 10 mm and thicknesses of about 1 mm. These

disks were processed at room temperature by HPT under

two different conditions. First, disks were held for 1 min

without torsional straining at an applied pressure, P, of

either 1.0 or 6.0 GPa for the aluminium and copper sam-

ples, respectively: this condition is henceforth designated

the samples where N = 0. Second, disks were held under

the same applied pressures and subjected to torsional

straining for one turn using a rotation speed of 1 rpm: these

samples are henceforth designated N = 1. The processing

was conducted under quasi-constrained conditions where

there is a small outflow of material between the two anvils

around the periphery of each disk during the processing

[13, 14]. In order to check on the increase in the temper-

ature during HPT, the temperature was monitored for

copper strained to a total of N = 10 turns under a pressure

of P = 6.0 GPa, and the result is shown in Fig. 2. It is

readily apparent from this plot that the temperature for the

HPT copper samples used in this investigation was below

30 �C after one revolution.

The microhardness was measured along two diameters

of all HPT disks with distances of 0.5 mm between each

indentation. These measurements were taken using a Ma-

tsuzawa Seiki MHT-1 microhardness tester equipped with

a Vickers indenter. The Hv values were recorded under

loads of 50 and 100 gf for aluminium and copper,

respectively, with the same dwell time of 15 s used for

both materials. A typical Hv profile for HPT copper pro-

cessed through one turn is shown in Fig. 3 where the

dashed line denotes the average microhardness value

obtained by averaging all the points along the two

diameters.

The analyses by XRD were undertaken using

a Bruker D2 Phaser instrument with Cu radiation

(Ka1 = 1.54060 A) and a Ni monochromator with a 1D

LYNXEYE detector. The scan step was 0.02�, and the

delay time was 2.5 s. A Rietveld analysis using MAUD

software [12] was performed in order to monitor the lattice

parameter, a; the microcrystallite size, d; and the micro-

strain he2i1/2. An example of fitted experimental and cal-

culated X-ray profiles is given in Fig. 4 where it is evident

that the calculated curve is in good agreement with the

experimental points.

Microhardness measurements and XRD analysis were

performed on the Al and Cu specimens after keeping the

disks at room temperature for periods of time of 1, 12, 24,

36, 48 and 96 h after HPT processing.

J Mater Sci

123

Experimental results

In order to investigate the effect of room-temperature

ageing, Fig. 5 shows the evolution of the average mi-

crohardness for (a) aluminium and (b) copper as a function

of the ageing time after HPT processing. In Fig. 5a for

aluminium, the errors bars are large, but there is a trend for

a small increase in the microhardness for Al specimens

after ageing for 24 h for the HPT disk processed by N = 1

turn and for 36 h for the disk subjected to compression

without torsional straining. It is necessary to note, however,

that this tendency lies within the range of the experimental

error. The situation is more complicated with the HPT

copper samples shown in Fig. 5b where there is an increase

in hardness after 12 h for the disk processed through one

turn. By contrast, there is no similar increase for the sample

processed in compression without torsional straining, and

instead the microhardness values decrease smoothly as a

function of the time of ageing. In the case of the copper

samples, the experimental data appear to be more reliable

because the error bars do not overlap over the whole range

of data.

0

100

200

300

400

500

Inte

nsity

(C

ount

s), 1

03

Inte

nsity

(C

ount

s), 1

03

2θ (°)38.2 38.4 38.6 38.8 39.0 43.2 43.4 43.6 43.8 44.0

0

50

100

150

200

2θ (°)

(a) (b)

Fig. 1 (111) Peak of annealed samples: a aluminium (FWHM = 0.07�), and b copper (FWHM = 0.08�)

0 2 4 6 8 1020

25

30

35

40

45

50

55

60

T (

°C)

Time (min) or N

P=6 GPa1 rpm

294

300

306

312

318

324

330

336

342

348

T (

K)

Fig. 2 Temperature as a function of number revolution for copper

processed at load of 6 GPa. Note: readings were taken from the

thermocouple located at 7 mm above the surface of upper anvil

-6 -4 -2 0 2 4 610

20

30

40

50

60

70

Average Hv=52.3±1.9

Hv

Distance (mm)

Fig. 3 Typical microhardness profile for HPT aluminium. Dashed

line denotes average Hv value

Fig. 4 An example of Rietveld refinement for copper samples

(P = 6 GPa, N = 0, 1 min)

J Mater Sci

123

Figure 6 depicts the values of the lattice parameters for

HPT aluminium in Fig. 6a and for HPT copper in Fig. 6b.

For the aluminium specimens, there are no significant

changes in the lattice constant with time. As shown in

Fig. 6a, the values of the lattice constant for the compressed

sample with N = 0 and the HPT specimen with N = 1 turn

both lie above the lattice parameter recorded in the litera-

ture where a = 4.0494 A [16]: this value is shown as a red

line in Fig. 6a. For the copper specimens, the situation is

again more complicated. For the compressed sample where

N = 0, the lattice constant is initially deflected but ulti-

mately comes to a final value of *3.63 A. By contrast, the

lattice parameter for the sample with N = 1 has a small

minimum after 36 h of ageing, and the relevant value is then

very close to the lattice constant given in the literature

where a = 3.6150 A [17]. In practice, however, the values

of the lattice parameters for both the samples are consis-

tently above this reported value, and this is due to the use of

a material having only commercial purity.

Similar variations are found also for the crystallite size

and the microstrain. Figure 7 shows the crystallite sizes of

(a) HPT aluminium and (b) HPT copper as a function of the

ageing time. For the compressed Al disk where N = 0, the

measurements show rapid deflections, and these changes

appear to have little or no correlation with the

microhardness evolution shown in Fig. 5a. Nevertheless,

the crystallite size of the HPT aluminium where N = 1 turn

appears to reasonably follow the microhardness trend, and

it slightly changes after 13 h of ageing. For the compressed

copper sample shown in Fig. 7b for N = 0, the crystallite

size correlates reasonably with the change in the lattice

parameter in Fig. 6b, but there appears to be no similar

correlation with the microhardness evolution in Fig. 5b.

Conversely, the Cu sample strained for N = 1 turn shows a

constant crystallite size of *120 nm which correlates

directly with the lattice constant in Fig. 6b but not with the

average microhardness in Fig. 5b.

The values of the measured microstrains are shown in

Fig. 8 for (a) HPT aluminium and (b) HPT copper. For the

compressed Al disk with N = 0, the evolution is the

inverse of the crystallite size, but there is a clear correlation

with the average microhardness in Fig. 5a. For the alu-

minium sample strained to one whole turn, the microstrain

initially increases but thereafter it decreases after about

36 h of ageing. For the copper samples in Fig. 8b, the

microstrain for the N = 0 sample increases with the

increasing ageing time, but for the HPT processed sample

where N = 1 turn there is a gradual decrease in the mi-

crostrain which correlates well with the average microh-

ardness in Fig. 5b.

0 10 20 30 40 50 60 70 80 90 10040

45

50

55

60

N=0N=1

Ave

rage

Hv

Time (h)

P=1 GPa

0 10 20 30 40 50 60 70 80 90 100100

110

120

130

140

P=6 GPaN=0N=1

Ave

rage

Hv

Time (h)

(a) (b)

Fig. 5 Average microhardness, Hv, for HPT samples: a aluminium, and b copper

0 20 40 60 80 1004.03

4.04

4.05

4.06

4.07

4.08

Latti

ce p

aram

eter

(Å

)

Time (h)

N=0N=1

P= 1 GPa

0 20 40 60 80 1003.60

3.61

3.62

3.63

3.64

3.65

Latti

ce p

aram

eter

(Å

)Time (h)

N=0N=1

P= 6 GPa

(a) (b)Fig. 6 Lattice parameter of

HPT samples: a aluminium, and

b copper

J Mater Sci

123

Discussion

All the above results, when considered together, show that

there are significant changes both in the microhardness

values and in the values of various microstructural

parameters in samples of aluminium and copper during

self-annealing at room temperature. It is also apparent that

more definitive studies are now required in order to obtain

closer correlations between the various effects that have

been recorded over relatively short self-annealing times of

up to a maximum of 96 h. It is appropriate to consider the

effect of self-annealing on each measured parameter.

Effect of self-annealing on microhardness

To some extent, there is a trend of a small increase in the

average microhardness after ageing for 12–24 h for alu-

minium, and copper samples strained for one whole turn

(Fig. 5). For the specimens subjected only to compression

with N = 0, there is a little or no increase in the average

microhardness. An unambiguous interpretation comes from

the fact that different areas of the HPT disks are strained to

different amounts, and therefore they have distinctive

recovery rates. This is evident from a set of Hv profiles for the

Al samples strained for one whole turn under a pressure of

1.0 GPa as shown in Fig. 9, where the profiles were taken

after ageing times of 1, 12, 24, 36 and 96 h. It is clear that the

recovery rate is the highest in the centre of the disk. Spe-

cifically, the microhardness in the centre measured after 96 h

of ageing is two times lower than that in the freshly processed

sample. This is an interesting result because in high-purity

aluminium subjected to HPT it was shown that the recovery

process is the highest at the periphery of the disk [18]. The

0 20 40 60 80 1000.2

0.4

0.6

0.8

Cry

stal

lite

size

(µ

m)

Time (h)

N=0N=1

P= 1 GPa

0 20 40 60 80 1000.1

0.2

0.3

0.4

Cry

stal

lite

size

(µ

m)

Time (h)

N=0N=1

P= 6 GPa

(a) (b)Fig. 7 Crystallite size of HPT

samples: a aluminium, and

b copper

0 20 40 60 80 1000.1

0.2

0.3

0.4

0.5

0.6

Mic

rost

rain

, 10

-3

Time (h)

N=0N=1

P=1 GPa

0 20 40 60 80 1000.2

0.3

0.4

0.5

0.6

Mic

rost

rain

, 10

-3

Time (h)

N=0N=1

P=6 GPa

(a) (b)Fig. 8 Microstrain of HPT

samples: a aluminium, and

b copper

-6 -4 -2 0 2 4 630

40

50

60

70

80

Time 96 h: Hv=52.8±1.0

Time 36 h: Hv=53.8±0.7Time 24 h: Hv=55.7±0.5

Time 12 h: Hv=55.3±0.7

Ave

rage

Hv

Distance (mm)

Time 1 h: Hv=53.4±0.7

Fig. 9 Detailed Hv profiles for HPT-strained aluminium (N = 1) as a

function of ageing time

J Mater Sci

123

difference in these results demonstrates the effect of the

purity level in controlling the recovery behaviour.

Effect of self-annealing on lattice parameters

The evolution in the lattice constant may reflect the level of

impurities or the presence of bi- and tri- vacancies in the

metal matrix. The level of vacancies estimated in HPT

copper by measuring the retained electroresistivity can be

comparable with or even higher than the vacancy concen-

tration near the melting point. The change in lattice

parameter as a function of vacancy and interstitials con-

centration is given as [19]

Da

a0

¼ C � ðv1 þ viÞ3 � X ;

where Da = (a - a0) is the change in the lattice parameter

in which a0 is due to the presence of vacancies and inter-

stitials in the matrix, C is their concentration, v1 and vi are

the vacancy and interstitial relaxation volumes, respec-

tively, and X is the atomic volume. The relaxation volume

of vacancies is always negative (thus, -6.6 A3 for alu-

minium and -3.5 A3 for copper [20]), and the sign of the

relaxation volume of the interstitials depends on the ionic

radius of the elements. This means that the change in the

lattice constant represents an interplay between the con-

centrations of vacancies and the concentration of intersti-

tials in the lattice. If the concentration of interstitials

remains unchanged then the lattice constant should

decrease with the annealing time, but this was not observed

for the strained metals where N = 1 turn. In practice, the

lattice constants of aluminium and copper determined in

this investigation are higher than those in the literature [16,

17] due to the presence of impurities in these commercial-

purity materials.

Effect of self-annealing on crystallite size

and microstrain

As indicated above, there is a small increase in microh-

ardness of HPT aluminium with N = 1 turn after ageing

for 12–24 h. However, the crystallite size in Fig. 7a and the

microstrain in Fig. 8a have corresponding maxima for the

same ageing condition. In order to explain the small

increase in the microhardness, it is possible to estimate the

dislocation density using the expression:

q ¼2ffiffiffi

3p� e2� �1=2

b � d ;

where b = 2.86 A is the Burgers vector [21] and d and

he2i1/2 are the crystallite size and microstrain, respectively.

The calculated dislocation density for HPT aluminium with

N = 1 as a function of ageing time is presented in Fig. 10.

Thus, the dislocation density is not high (*1013 m-2) and

decreases with ageing time such that there is a minimum at

a time of 36 h. More experiments will be needed to provide

a detailed explanation of these trends.

Summary and conclusions

(1) Samples of commercial-purity aluminium and cop-

per were processed by HPT and then aged for vari-

ous times at room temperature. A detailed analysis

was conducted using integrated XRD methods and

measurements of the average microhardness values.

(2) Aluminium and copper samples subjected only to

compression with no torsional straining (N = 0)

show no significant hardening, whereas for copper,

there is a continuously decreasing average microh-

ardness. After torsional straining through one turn,

there is noticeable hardening after ageing for

12–24 h. Integrated XRD analysis revealed no

significant correlation between microstructural

parameters (lattice constant, crystallite size and

microstrain) and increasing microhardness.

(3) Careful inspection of the microhardness profiles

along diameters of the disks showed that the

recovery rates are different in different positions on

the HPT disks. Specifically, the recovery was higher

at the centre of the disk than at the edge.

Acknowledgements This work was supported by the European

Research Council under ERC Grant Agreement number

267464-SPDMETALS.

0 20 40 60 80 1000.8

0.9

1.0

1.1

1.2

Dis

loca

tion

dens

ity (

m-2),

×10

13

Time (h)

Fig. 10 Calculated dislocation density for HPT Al strained for one

turn as a function of ageing time

J Mater Sci

123

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