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Materials Letters 59 (2

Mechanical properties and microstructure of AZ31 Mg alloy processed

by two-step equal channel angular extrusion

Li Jina,b, Dongliang Lina,b,T, Dali Maoa,b, Xiaoqing Zenga,c, Wenjiang Dinga,c

aSchool of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, PR ChinabOpen Laboratory of the Educational Ministry for High Temperature Materials and Tests, Shanghai Jiao Tong University, Shanghai 200030, PR China

cNational Engineering Research Center of Light Alloys Net Forming, Shanghai Jiao Tong, University, Shanghai 200030, PR China

Received 16 September 2004; accepted 17 September 2004

Available online 28 March 2005

Abstract

A new equal channel angular extrusion (ECAE) processing, two-step ECAE, was applied to control the microstructure and mechanical

properties of AZ31 Mg alloy. The ultra-fine grain size of 0.5 Am has been produced, and both the ductility and yield stress of the alloy were

significantly increased after the processing, which is ascribed to grain refinement as well as incomplete dynamic recovery and

recrystallization during the second-step ECAE at 453 K.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Mechanical properties; Microstructure; Equal channel angular extrusion (ECAE); AZ31 Mg alloy

1. Introduction

Magnesium alloys have high potential as structural

materials due to their high specific properties. However,

magnesium alloys have poor formability and limited

ductility at room temperature ascribed to their hexagonal

close-packed (HCP) crystal structure. Grain refinement is

an important practice used to improve the mechanical

properties of magnesium alloy. While equal channel

angular extrusion (ECAE) provides a technique for

producing ultra-fine grain sizes in the submicrometer or

nanometer range in bulk materials [1,2]. Mabuchi et al. [3]

reported that fine-grain Mg alloy with a grain size of 1 Amhas been obtained by ECAE. The microstructure and

tensile properties of ECAE AZ31 Mg alloy were studied in

some articles [4–6], and the following common results

were obtained, which were that the grains were refined

effectively, the elongation was improved, but the yield

0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.matlet.2004.09.061

T Corresponding author. School of Materials Science and Engineering,

Shanghai Jiao Tong University, Shanghai 200030, PR China. Tel.: +86 21

62932544; fax: +86 21 62820892.

E-mail address: dllin@sjtu.edu.cn (D. Lin).

stress decreased. Similar results about the mechanical

properties of ECAE AZ61 Mg alloy have also been

obtained [7]. Nevertheless, it is difficult to improve both

the strength and the ductility of the ECAE Mg alloy,

although the grain size can be refined significantly by

ECAE processing.

Zan et al. [4] reported that the grain size of ECAE

AZ31 Mg alloy decreased, the 0.2% proof stress increased,

but the elongation decreased when ECAE processing

temperature decreased. It is suggested that both the

ductility and the yield stress of ECAE AZ31 alloy could

be improved by lowering the processing temperature.

However, it is limited to lowering the ECAE processing

temperature for the Mg alloy. For example, AZ91 [3],

AZ61 [7,8], AZ31B [9], and ZK60 [10] Mg alloys could

only be ECAE-processed at the lowest temperatures of 473

K, 548 K, 473 K, and 433 K, respectively. ECAE has been

investigated by Yoshida et al. [11] for AZ61 at 473 K for

two passes and at 448 K for four passes to lower the

processing temperature and to obtain a finer grain size of

less than 1 Am. However, there was no report on the

mechanical property of subsequent processing at lower

temperature.

005) 2267–2270

(a) As-received

10µm

(b) 523K/4pass (c) 498K/4pass

10µm 10µm

Fig. 1. The optical photographs of as-received AZ31 alloy and ECAE AZ31 alloy. (a) As-received; (b) 523 K/4 pass; (c) 498 K/4 pass.

L. Jin et al. / Materials Letters 59 (2005) 2267–22702268

In this study, a new ECAE processing procedure, named

two-step ECAE, was applied to improve both the strength

and the ductility of AZ31 Mg alloy. The processing

temperature of the second step could be lower than that of

first step in the two-step ECAE. Emphasis was placed on the

relationship between microstructure evolution and mechan-

ical properties.

-5 0 5 10 15 20 25 30 35 40

0

50

100

150

200

250

300

C BA

stre

ss (

MP

a)

strain (%)

A As-received alloy

B ECAE 4 pass at 523k

C ECAE 4 pass at 498k

Fig. 2. Engineering stress–strain relations for the AZ31 Mg alloy.

2. Experimental

The material used in the present study was a commercial

Mg–Al–Zn alloy, AZ31, and was received as a commer-

cially extruded bar with a grain size of 20 Am. The ECAE

specimen was cut to 10 mm�10 mm�65 mm. ECAE was

carried out on the as-received material through a die made

of H13 steel with an internal angle of 908 between the

vertical and horizontal channels. The strain intensity encould be calculated by the equation: en=1.15Ncotan (//2)

[12], where N is the pass number, so the strain of each pass

is 1.15. Graphite was used as lubricant. The ECAE

processing was carried out at the temperature ranging from

453 K to 573 K, with a constant processing rate of 16.8

mm/min, All processings were conducted by rotating each

sample about the longitudinal axis by 908 in the same

direction between consecutive passes, designated as route

Bc [13]. Repetitive pressings of the same sample were

performed up to four or five passes, with a strain equivalent

to 4.6 or 5.75. The flat tensile specimens with a gauge

section of 10 mm�3 mm�1.5 mm were cut from the

sample produced by ECAE with electro-discharge machine.

Tensile tests were carried out in 5�10� 4 s� 1 at room

temperature. Microstructures of the samples were examined

by optical microscope (OM) after mechanical polishing and

etching at room temperature using a solution of 1% HNO3,

24% C2H6O2, and 75% water. For transmission electron

microscopy (TEM), disks were thinned at 233 K with a

twin-jet polisher under conditions of 10 mA and 75 V using

a solution of 1% HClO4 in ethanol. The thinned specimens

were examined using a JEM-100 electron microscope

operating at 120 kV.

3. Results and discussion

3.1. The microstructure and mechanical properties of ECAE

AZ31 alloy

ECAE processing was carried out at the temperature

ranging from 473 K to 573 K for the as-received alloy in our

study. However, cracks appeared when the processing

temperature was lower than 498 K. The samples processed

by ECAE at a constant temperature were named ECAE

alloy in this study. Fig. 1 shows the optical photographs of

as-received alloy, four-pass ECAE specimen processed at

523 K and at 498 K, respectively. The average grain sizes of

these AZ31 alloys were determined to be 20, 5, and 2 Am,

respectively. The refinement of the grain size after ECAE

processing proves that the processing temperature is an

important factor used to control the microstructure of AZ31

alloy, lower processing temperature, and finer grain size,

which is similar with the results in the previous reports

[4–6]. The grains in the ECAE AZ31 alloy are equiaxed and

homogeneously distributed, which suggests that recrystalli-

zation took place during ECAE processing.

Fig. 2 shows the stress–strain curves of as-received alloy,

four-pass ECAE specimen processed at 523 K and at 498 K.

(a) 498K/5pass (b) 498K/4pass-453K/1pass

5µm 5µm

Fig. 3. The optical photographs of ECAE AZ31 Mg alloy by two-step ECAE. (a) 498 K/5 pass; (b) 498 K/4 pass–453 K/1 pass.

L. Jin et al. / Materials Letters 59 (2005) 2267–2270 2269

The elongation of the four-pass ECAE specimen processed

at 523 K and 498 K increased to 35% and 32%, respectively,

from 21% of as-received alloy, but their yield strength

decreased to 113 MPa and 136 MPa from 153 MPa,

respectively. The plots indicate that the ductility of the

AZ31 alloy is enhanced effectively, but its yield strength is

decreased markedly by ECAE processing even with the

grain refined effectively. So the ECAE temperature is an

important factor to control the mechanical properties of the

AZ31 Mg alloy at a given strain intensity en. The higher is

the processing temperature, the higher is the elongation but

the lower is the yield stress.

3.2. The microstructure and mechanical properties of two-

step ECAE AZ31 alloy

The results in Figs. 1 and 2 indicate that the grain

refinement and the yield stress of ECAE AZ31 alloy have

been improved by lowering the processing temperature. In

this study, a new two-step ECAE was designed to lower the

processing temperature for AZ31 alloy. Repetitive extru-

sions of the sample were performed up to four passes at 498

K and continually performed one more pass at 453 K, and

the sample processed by the procedure was named two-step

ECAE alloy.

(a) (b)

200nm

Fig. 4. The TEM photographs of (a) as-received alloy; (b) AZ31 alloy after ECAE:

K/1 pass.

Fig. 3 shows the optical photographs of five-pass ECAE

specimen processed at 498 K and two-step ECAE alloy at a

given strain of 5.75. Their grain sizes were determined to be

2 and 0.5 Am, respectively. So ultra-fine grains in

submicrometer range can be produced by the two-step

ECAE by lower processing temperature. It should be noted

that the grains in the two-step ECAE AZ31 are not fully

resolved by OM, indicating that it may not be fully

recrystallized. Further observations by TEM were carried

out to show microstructure evolution of the alloy during the

two-step processing.

Fig. 4 shows the TEM micrographs of as-received ECAE

and two-step ECAE alloys. In Fig. 4(a), there are a few

dislocations in the interior of large grains and careful

observation reveals that the grain boundaries are high angle

grain boundaries. In Fig. 4(b), there were also a few

dislocations in the interior of grains in spite of large

straining by ECAE and effective grain refining. Most of

the grain boundaries are revealed to be high angle grain

boundaries and fully defined subgrain boundaries, which

suggests that dynamic recovery and recrystallization took

place during ECAE at 498 K. In Fig. 4(c), within grains and

on the grain boundaries, many dislocations are visible, and

there are incompletely developed subgrains and cell

structure formed by the tangled dislocations. It suggests

(c)

200nm 200nm

498 K/5 passes; and (c) AZ31 alloy after two-step ECAE: 498 K/4 pass–453

-5 0 5 10 15 20 25 30 35-50

0

50

100

150

200

250

300

350

CBA

Str

ess

(MP

a)

Strain (%)

A: As-receivedB: ECAEed AZ31 alloy (498k/5pass)C: Two-step ECAEed AZ31 alloy (498k/4pass-453k/1pass)

Fig. 5. Engineering stress–strain relations for the two-step ECAE AZ31 Mg

alloy.

L. Jin et al. / Materials Letters 59 (2005) 2267–22702270

that dynamic recovery and recrystallization do not fully take

place when the processing temperature decreases to 453 K

for the two-step ECAE AZ31 Mg alloy.

Fig. 5 shows the stress–strain curves of as-received alloy,

ECAE alloy, and two-step ECAE alloy. For the ECAE alloy,

the elongation increases to 34% from 21% of as-received

alloy; however, the yield stress decreases to 104 MPa from

152 MPa. For the two-step ECAE alloy, the elongation

increases to 29% from 21% of as-received alloy, and the

yield stress increases to 231 MPa from 152 MPa. It can be

concluded that for the AZ31 alloy, both the ductility and

yield stress can be improved significantly by two-step

ECAE processing.

The mechanical properties of ECAE Mg alloy were

controlled by the microstructure of the alloy, including grain

size, grain boundary structure, dislocation structure, and

texture. At present, a well-accepted idea is that texture

modification plays an important role in increasing the

ductility of ECAE Mg alloy [5,14,15]. However, the factors

affecting the strength of ECAE Mg alloys are not clear and

should be clarified. According to the results in Figs. 4 and 5,

the mechanical properties of AZ31 Mg alloy were affected

by the grain boundary structure and dislocation distribution.

For ECAE AZ31, after five ECAE passes at 498 K, the

ductility was increased but the yield stress decreased

effectively with grain refinement. However, for two-step

ECAE AZ31 alloy, after four ECAE passes at 498 K and

then one pass at 453 K, the grain size was finer than that of

ECAE alloy after four ECAE passes at 498 K, but the yield

stress was improved markedly with a slight decrease of

ductility. Moreover, a lot of incompletely developed

subgrains and cell structure formed in the AZ31 alloy by

two-step ECAE, which probably results in increasing yield

stress of AZ31 Mg alloy. It indicates that the grain boundary

structure and dislocation substructure, besides grain size and

texture, play an important role on the mechanical properties

of AZ31 Mg alloy. It can be concluded that the two-step

ECAE processing could optimize the microstructure and

improve the strength and ductility of the alloy.

4. Conclusion

The processing temperature is an important factor that

affects the microstructure and mechanical properties of the

Mg alloy by ECAE. The two-step ECAE processing was

successfully applied to control the microstructure and

mechanical properties of the AZ31 alloy by lowering the

processing temperature to 453 K. The ultra-fine grain of 0.5

Am has been obtained, and both the ductility and strength of

the alloy were improved significantly after the processing,

which can be explained by grain refinement as well as

incomplete dynamic recovery and recrystallization during

the processing.

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