Mechanical Properties of an Al-5.4%Mg-0.5%Mn-0.1%Zr Alloy Subjected to ECAP and Rolling

Sergey Malopheyeva, Alla Kipelovab, Ilya Nikulinc, Rustam Kaibyshevd

Laboratory of Mechanical Properties of Nanostructured Materials and Superalloys, Belgorod State University, Pobeda 85, Belgorod 308015, Russia

amalofeev@bsu.edu.ru, bkipelova@bsu.edu.ru, cnikulin_ilya@bsu.edu.ru, drustam_kaibyshev@bsu.edu.ru

Keywords: aluminum alloy, fine-grain structure, equal channel angular pressing, superplastisity

Abstract. Superplasticity and microstructural evolution of a commercial Al-5.4%Mg-0.5%

Mn-0.1%Zr alloy subjected to severe plastic deformation through equal-channel angular pressing

(ECAP) and subsequent rolling was studied in tension at strain rates ranging from 1.4×10-4

to

5.6×10-2

s-1

in the temperature interval 400-550°C. The alloy had an unrecrystallized microstructure

with an average crystallite size less than 5 µm. The alloy exhibited the yield strength of ~370 MPa,

ultimate strength of ~450 MPa and elongation-to-failure of ~15% at ambient temperature. In spite of

small crystallite size the alloy shows moderate superplastic properties. The highest

elongation-to-failures of ~450% appeared at a temperature of ~500°C and an initial strain rate of

~1.4×10-3

s-1

, where the strain rate sensitivity coefficient, m, is of about 0.57. The relationship

between superplastic ductilities and microstructure is discussed.

Introduction

The commercial Al-Mg-Mn alloys are widely used in aircraft, shipbuilding and automobile

industry due to excellent combination of strength, corrosion resistance, weldability and low cost. It is

now well established that high mechanical properties and superplastic ductilities at high strain rates

may be achieved in alloys with grain sizes of ~ 1-10 µm [1]. A substantial reduction in the grain size

of aluminum alloys can be attained through severe plastic deformation (SPD) [2,3]. Recently, it has

been shown that in numerous aluminum alloys the grain size may be significantly reduced by

imposing an severe plastic deformation through the process of equal-channel angular pressing

(ECAP) [4-7]. This technique is highly suitable for achieving superplasticity in bulk billets of

aluminum alloys. Aluminum alloys having such ultrafine grained or even submicrocrystalline

structures are capable to exhibit superplastic ductilities at higher strain rates or lower temperatures

[4-7]. Usually, samples produced by ECAP are generally in the form of rods having square or circular

cross-sections, which cannot be used directly in industrial SPF forming operations where the material

is required to be in the form of thin sheets. Thus, application of rolling after ECAP is highly suitable

to fabricate superplastic sheets.

The present work is dedicated to examine a commercial Russian Al-5.4%Mg-0.5%Mn-0.1%Zr

alloy (denoted as 1561 Al). The aim of this study is to consider the potential for using cold rolling

following ECAP to achieve high strength in superplastic sheets of the 1561 Al.

Material and Experimental Procedures

The commercial 1561 Al with chemical composition of Al–5.43%Mg–0.52%Mn–0.1%

Zr–0.12%Si–0.014%Fe (in weight %) was manufactured by direct chill casting and then subjected to

two step homogenization at 440 °C for 4 hours and 500 °C for 8 hours. Next, the 1561 Al was

machined to rods with dimension of 20× 20× 100 mm3. These rods were deformed by ECAE at

temperature of 250 °C using an isothermal die with a square cross-section of 20× 20 mm. The channel

had an L-shaped configuration with an angle of intersection of 90° and that at the outer arc curvature

of 1°. Deformation through this die produced a strain of ~1.15 in each passage [8]. Both the die and

samples were coated with lubricant containing graphite. The specimens were subjected to 8 passes of

Materials Science Forum Vols. 667-669 (2011) pp 815-820Online available since 2010/Dec/30 at www.scientific.net© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.667-669.815

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: 128.120.175.166, University of California Davis, Davis, United States of America-12/05/14,10:49:10)

ECAP using route BC. A cumulative strain of 9.2 was imposed. Finally, the samples were water

quenched. Then rods were subjected by rolling with a reduction of ~0.86at room temperature. Total

true strain was about 10. The X, Y and Z planes correspond to the plane to the rolling (RD), tension

(TD) and normal (ND) directions, respectively, as shown in Fig. 1.

The tensile specimens with a gauge length of 6

mm and cross-section of 1.5×3 mm2

were cut

parallel rolling direction. Tensile tests were

performed at room temperature and at strain rate of

1.4×10-3

s-1

. Also, tensile samples were tested at a

strain rate of ~1.4×10-3

s-1

and temperatures ranging

from 400 to 550°C. Temperature accuracy was

within ±1 °C. Each sample was held at testing

temperature for about 10 min in order to reach

thermal equilibrium. The values of the strain rate

sensitivity m were determined by strain jump tests

[9,10].

Electron-backscattering diffraction (EBSD)

analysis was used to examine rolling samples and

test samples. The specimens were slightly

electropolished in 1:3 HNO3/C2H5OH solution to

give a strain-free surface. The EBSD analysis was performed using a FEI Quanta 600FEG SEM

equipped with a high-resolution electron-backscatter analyzer. In the data presented, high angle

grains boundaries (HAGBs) were defined as θ≥15° in misorientation and low angle grains boundaries

(LAGBs) as 3°<θ<15°. HAGBs and LAGBs are depicted in EBSD maps as black and white lines,

respectively. Grain size was measured in the longitudinal and transverse directions using EBSD maps

and optical metallographic images by the mean linear intercept method.

Experimental results

Microstructure after ECAP followed by CR. It is seen in Fig. 2 that non- uniform structures are

formed after CR following ECAP. Two types of crystallites are distinctly distinguished. The first

structural type is coarse original grains subdivided by low angle boundaries on subgrains. The

average size of such grains is ~4.5 µm in the rolling direction and ~1.9 µm in the transverse direction.

The second type is newly developed grains with

the average size of about 0.6 µm. The fraction of

newly developed grains is ~15%.The fraction of

HAGBs is ~55%.

Notably the size of these subgrains is similar to

one of the new recrystallized grains belonging to

second type. Thus, a partly recrystallized stricture

with high fraction of subgrains was produced in

sheets of 1561 Al by ECAP followed by cold

rolling.

Mechanical properties at room temperature.

Tensile samples of the 1561 Al alloy subjected to

CR following ECAP were pulled to failure at an

initial strain rate of ~1.4×10-3

s-1

. It is seen (Table

1) that 1561 Al alloy with microstructure produced

by SPD exhibits high mechanical properties. For

example the strain hardened AA5083-H111 alloy

exhibits yield strength of ~225 MPa, ultimate

strength of ~317 MPa and elongate-to-failure of ~14% [11]. On the other hand the present alloy

Fig. 1. Schematic illustration of a one pass of

ECAP.

Fig.2 Microstructure of the 1561 Al after CR

following ECAP at 250 °C. Total strain is ~10.

816 Nanomaterials by Severe Plastic Deformation: NanoSPD5

subjected to SPD demonstrates values of yield

strength of ~370 MPa, ultimate strength of ~450 MPa

and elongation-to-failure of ~15%. Thereby, CR

following ECAP improves the yield strength on 60%

and ultimate strength on 70% with retained plasticity.

Superplastic behaviour. The typical stress – strain

curves in the temperature interval 400 – 550°C at an

initial strain rate of 1.4 × 10-3

s-1

, are shown in Figs. 3.

Extensive strain hardening takes place initially during

superplastic deformation (Fig. 3). The apparent

steady-state flow takes place after 200% deformation

at 500 °C. After reaching the maximum, the flow

stress continuously decreases until failure at other

temperatures. All fractured samples showed strain

localization and, therefore, an apparent softening after

the stress peak can be attributed to extensive necking

in gauge region (Fig. 4).

The maximum value of elongation-to-failure of

about 450 % was found at 500 °C (Fig. 5). A decrease

or increase in deformation temperature results in

gradual decreasing of elongation-to-failure.

Fig. 6 show plots of flow stress, σ, taken at strain

~50%, and the coefficient of strain rate sensitivity, m,

as a function of initial strain rate. It is clearly seen that

at all examined temperature the strain-strain rate

curves show an evidence for a sigmoidal shape which

is typical for superplastic materials [9,10]. The values

of the coefficient of strain rate sensitivity have a

maximum at strain rates close to 5×10-4

s-1

. The maximum ductility of 450% with the corresponding

strain rate sensitivity coefficient of about 0.57 was obtained at strain rate of ~1.4×10-3

s-1

and 525 °C.

Typical dependencies of the strain rate sensitivity coefficient, m, against strain, for the 1561 Al

alloy subjected to ECAP followed by CR is presented in Fig. 7. It is seen that values of the strain rate

sensitivity tends to decrease with increasing strain at all examined temperature. However this value is

higher than 0.4 except for temperature of 400 and 525°C. Decreasing m value with strain indicates

instability of plastic flow in the present alloy.

Microstructural evolution. The microstructural evolution of the 1561 Al alloy subjected to

ECAP+CR during static and dynamic annealing was studied in grip and gauge sections, respectively

(Fig. 8, Table 2). It was found that uniform grained structure are formed in grip section of the 1561

aluminum alloy (Fig. 8a and 8c). Initial grains tend to grow under static annealing in the temperature

range of 400-550°C (Table 2). As a result, the grain size in grip sections increase in ~3-5 times

comparing to initial structure. The maximal grain size was achieved at 525 °C.

Under dynamic annealing conductions, substantial grain growth takes place at temperature higher

than 475°C (Fig. 8b and 8d, Table 2). In addition, grains tend to elongate along tension direction

(Figs. 8b and 8d). Non-equiaxed grains is suggested a significant contribution of dislocation glide to

Fig. 4. Appearance of samples tensioned to

failure at strain rate 1.4×10-3

s-1

and

different temperatures

Table 1. Mechanical properties strain hardened AA5083-H111 and 1561 Al subjected to

ECAP+CR

Yield strength

[MPa]

Ultimate strength [MPa] Elongate-to-failure

[%]

AA5083-H111 225 317 14

1561 Al

(ECAE+CR) 370 450 15

Materials Science Forum Vols. 667-669 817

total deformation [9,10]. It may be indicative for decreased contribution of grain boundary sliding

(GBS) to total elongation which is confirmed by a reduction in the m value with strain (Fig. 7.). It is

known that the plastic stability is mainly provided by high values of m [9,10]. In our case the values of

m continuously decrease with strain, providing unstable plastic flow and necking in gauge length.

Cavitation under superplastic deformation in the 1561 Al alloy was measured in the samples

pulled to failure at an initial strain rate of ~1.4×10-3

s-1

and various temperatures (Table 2). In the temperature

range 400-550°C the 1561 Al alloy exhibited

considerable cavitation. Cavities have a jagged shape

suggesting the plasticity-controlled cavity growth

mechanism [8,9]. It was observed that the cavities

form stringers toward the tension direction.

Observation of the specimen cross-section near

fracture surface showed that the formation of bridges

between cavity stringers in the transverse direction

takes place. Thus, nucleation and growth of the voids

plays a significant role in failure of the 1561 Al alloy

subjected to ECAP+CR processing.

Fig. 5. Temperature dependence of

elongation-to-failure

Fig. 7. Variation of strain rate sensitivity, m,

with strain.

Fig. 3. Typical stress – strain curves of

1561 Al subjected to ECAP+CR.

Fig. 6. Strain rate dependence of flow stress, σ, (a) and coefficient of strain rate sensitivity, m, (b).

818 Nanomaterials by Severe Plastic Deformation: NanoSPD5

Table 2. Average grain size, Ls and Ld, and the grain aspect ratios, ARs and ARd, after static annealing

and superplastic deformation, respectively, and porosity volume fracture.

T [°C]

400 450 475 500 525 550

Local strain in gauge section (the

equivalent time of static annealing

in grip section [min])

120

(25)

275

(45)

340

(51)

450

(67)

180

(33)

90

(22)

Ls [µm]* 10/7.4 11.6/7.8 11.4/10.8 13.9/12.2 25.4/21.4 16.3/15

ARs 1.3 1.2 1.1 1.1 1.2 1.1

Ld [µm]* 9.7/6 10.9/9.5 15.1/10.4 19.5/13.2 30.6/21.8 25.4/17.1

ARd 1.6 1.5 1.4 1.5 1.4 1.5

Porosity volume fracture in gauge

section [%] 3 26 25 28 19 4

*Numerator and denominator are shown grain sizes in the longitudinal and transverse directions,

respectively.

Discussion

The present study demonstrates the feasibility to produce SMC structure in sheets of 1561 Al alloy

through extensive grain refinement by ECAP followed by cold rolling. The alloy subjected to SPD

exhibits high mechanical properties at ambient temperature and moderate superplastic properties at

Fig. 8. Microstructural evolution during superplastic deformation at a strain rate of ~ 1.4×10

-3 s

-1:

(a) and (c) grip section and (b) and (d) gauge section at 400 and 500 °C, respectively. Tension

direction is horizontal.

Materials Science Forum Vols. 667-669 819

high temperature deformation. It was found that SP elongations are mainly attributed to high porosity

and extensive grain growth under superplastic deformation. It is clearly seen that partly recrystallized

structure is highly unstable against grain growth [3]. At high temperatures, the grain growth may lead

to decreasing contribution of GBS to total deformation. In addition, GBS of coarse grains cannot be

well accommodated; stress concentration at boundaries causes extensive strain-induced cavitations

[9,10]. Thus, present alloy shows moderate elongations, because of the rate of cavity growth in this

material is promoted due to the coarsening of microstructure produced by static annealing.

It can be expected that the optimization of the parameters of the initial heat treatment as well as the

ECAP processing route, such as temperature, path or strain rate, can lead to uniform microstructure

providing enhanced superplastic ductility in the 1561 Al alloy.

Conclusions

1. It was found that CR following ECAP is suitable for the formation of submicron structure in the

sheet of 1561 Al.

2. The sheets processed by ECAP+CR exhibits yield strength of ~370 MPa, ultimate strength of

~450 MPa and elongate-to-failure of ~15%.

3. The highest elongation of 450% was recorded at atemperature of 500 °C and a strain rate of

1.4 × 10-3

s-1

with corresponding strain rate sensitivity coefficient, m, of ~0.57.

Acknowlegement

This study was supported by Department of Education and Science, Russian Federation, under

grant No. Р977. Authors are grateful to staff of Joint Research Center, Belgorod State University, for

their assistance with computational and instrumental analysis.

References

[1] A. H. Chohshi, A. K. Mukherjee and T. G. Langdon: Mater. Sci. Eng R10 (1996), p. 237.

[2] A. Kipelova, I. Nikulin, S. Malopheyev and R. Kaibyshev: Proceedings of the 12th International

Conference on Aluminium Alloys, Japan Institute of Light Metals, Japan (2010), p. 2174.

[3] F. J. Humphreys, P. B. Prangnell, J. R. Bowen, A. Gholinia and C. Harris: Phil. Trans. R. Soc.

Lond. A 357 (1999), p. 1663.

[4] S. Komura, Z. Horita, M. Furukawa, M. Nemoto and T.G. Langdon: Metall.Trans. A, Vol. 32A

(2001), p. 707.

[5] S. Lee, P. B. Berbon, M. Furukawa, Z. Horita, M. Nemoto, N. K. Tsenev, R. Z. Valiev and T. G.

Langdon: Mater.Sci.Eng., Vol. A272(1999), p. 63.

[6]. R. Z. Valiev, D. A. Salimonenko, N. K. Tsenev, P. B. Berbon and T. G. Langdon: Scr. Mater.,

Vol. 37 (1997), p.1945.

[7] F. F. Musin, R. O. Kaibyshev, Y. Motohashi, T. Sakuma and G. Itoh: Mater. Trans., Vol. 43

(2002), p. 2370.

[8] Z. Horita, M. Furukawa, M. Nemoto and T. G. Langdon: Mater. Sci. Tech. 16 (2002), p.1239.

[9] J. Pilling and N. Ridley: Superplasticity in Crystalline Solids, The Institute of Metals, London,

1989.

[10] O. A. Kaibyshev: Superplasticity of Alloy, Intermetallics and Ceramics, Springer-Verlag,

Berlin, 1992.

[11] R. C. Rice, J. L. Jackson, J. Bakuckas and S. Thompson:Metallic Materials Properties

Development and Standartization (MMPDS) (2003), p.1728.

820 Nanomaterials by Severe Plastic Deformation: NanoSPD5

Nanomaterials by Severe Plastic Deformation: NanoSPD5 10.4028/www.scientific.net/MSF.667-669 Mechanical Properties of an Al-5.4%Mg-0.5%Mn-0.1%Zr Alloy Subjected to ECAP and Rolling 10.4028/www.scientific.net/MSF.667-669.815

DOI References

[2] A. Kipelova, I. Nikulin, S. Malopheyev and R. Kaibyshev: Proceedings of the 12th International

onference on Aluminium Alloys, Japan Institute of Light Metals, Japan (2010), p. 2174.

doi:10.4028/www.scientific.net/MSF.667-669.815 [3] F. J. Humphreys, P. B. Prangnell, J. R. Bowen, A. Gholinia and C. Harris: Phil. Trans. R. Soc. ond. A 357

(1999), p. 1663.

doi:10.1098/rsta.1999.0395 [4] S. Komura, Z. Horita, M. Furukawa, M. Nemoto and T.G. Langdon: Metall.Trans. A, Vol. 32A 2001), p.

707.

doi:10.1007/s11661-001-0087-9 [6] . R. Z. Valiev, D. A. Salimonenko, N. K. Tsenev, P. B. Berbon and T. G. Langdon: Scr. Mater., ol. 37

(1997), p.1945.

doi:10.1016/S1359-6462(97)00387-4 [7] F. F. Musin, R. O. Kaibyshev, Y. Motohashi, T. Sakuma and G. Itoh: Mater. Trans., Vol. 43 2002), p.

2370.

doi:10.2320/matertrans.43.2370 [2] A. Kipelova, I. Nikulin, S. Malopheyev and R. Kaibyshev: Proceedings of the 12th International

Conference on Aluminium Alloys, Japan Institute of Light Metals, Japan (2010), p. 2174.

doi:10.4028/www.scientific.net/MSF.667-669.815 [3] F. J. Humphreys, P. B. Prangnell, J. R. Bowen, A. Gholinia and C. Harris: Phil. Trans. R. Soc. Lond. A

357 (1999), p. 1663.

doi:10.1098/rsta.1999.0395 [4] S. Komura, Z. Horita, M. Furukawa, M. Nemoto and T.G. Langdon: Metall.Trans. A, Vol. 32A (2001), p.

707.

doi:10.1007/s11661-001-0087-9 [7] F. F. Musin, R. O. Kaibyshev, Y. Motohashi, T. Sakuma and G. Itoh: Mater. Trans., Vol. 43 (2002), p.

2370.

doi:10.2320/matertrans.43.2370 [9] J. Pilling and N. Ridley: Superplasticity in Crystalline Solids, The Institute of Metals, London, 1989.

doi:10.1016/0036-9748(89)90062-8