Dynamic Recrystallization During High Temperature Deformation of Magnesium

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Materials Science and Engineering A 490 (2008) 411420

Dynamic recrystallization during high temperaturedeformation of magnesium

T. Al-Samman , G. Gottstein

Institut f ur Metallkunde und Metallphysik, RWTH Aachen, 52056 Aachen, Germany

Received 12 November 2007; received in revised form 16 January 2008; accepted 5 February 2008

Abstract

As a consequence of the high critical stresses required for the activation of non-basal slip systems, dynamic recrystallization plays a vital role in

the deformation of magnesium, particularly at a deformation temperature of 200

C, where a transition from brittle to ductile behavior is observed.Uniaxial compression tests were performed on an extruded commercial magnesium alloy AZ31 at different temperaturesand strain rates to examine

the influence of deformation conditions on the dynamic recrystallization (DRX) behavior and texture evolution. Furthermore, the role of the starting

texture in the development of the final DRX grain size was investigated. The recrystallized grain size, measured at large strains ( 1.4) seemed

to be more dependent on the deformation conditions than on the starting texture. In contrast to pure magnesium, AZ31 does not undergo grain

growth at elevated deformation temperatures, i.e. 400 C, even at a low strain rate of 104 s1. Certain deformation conditions gave rise to a desired

fully recrystallized microstructure with an average grain size of18m and an almost random crystallographic texture. For samples deformed at

200 C/102 s1, optical microscopy revealed DRX inside of deformation twins, which was further investigated by EBSD.

2008 Elsevier B.V. All rights reserved.

Keywords: DRX; Twinning; Texture; Deformation; Flow behavior; EBSD

1. Introduction

At elevated temperatures the workability of magnesium sub-

stantially increases as additional slip systems, i.e. non-basal and

c + a slip become sufficiently available by thermal activation.

This conveys excellent formability to the material and enables

sheet production by hot rolling. During hot forming the material

is liable to undergo recrystallization, i.e. dynamic recrystalliza-

tion (DRX) which affects the crystallographic texture and thus,

material anisotropy. Hence it is of great importance to reveal the

texture forming mechanisms during hot working with concur-

rent recrystallization since most commercial wrought Mg alloys

will be fabricated to semi-finished products by such processing.

Recrystallization is understoodto proceed by nucleationof strain

free grains and their subsequent growth until complete impinge-

ment. Dynamic recrystallization in magnesium andits alloyshas

been reported to occur by several mechanisms. Those recrystal-

lization mechanisms can be divided, according to the nature of

the recrystallization process into two groups: continuous and

Corresponding author. Tel.: +49 241 80 26861; fax: +49 241 80 22301.

E-mail address: [email protected](T. Al-Samman).

discontinuous recrystallization. A continuous DRX process is arecovery process and proceeds by continuous absorption of dis-

locations in subgrain boundaries (low angle boundaries) which

eventually will result in the formation of high angle bound-

aries and thus, new grains [1]. Although this mechanism of

forming new grains is considered by many authors as a recrys-

tallization mechanism, it is stressed that the very nature of this

mechanism is a strong recovery process rather than a classical

recrystallization phenomenon, also referred to as discontinuous

recrystallization characterized by nucleationand nucleus growth

by high angle boundary migration.

Regardless of which mechanism continuous dynamic recrys-

tallization (CDRX) or discontinuous dynamic recrystallization

(DDRX), it is usually observed that the recrystallization of Mg

and its alloys is not accompanied with an obvious change of

crystallographic texture, in contrast to fcc materials, where the

recrystallization texture is usually very different from the defor-

mation texture and in many cases even more pronounced [2].

In magnesium, it is difficult to investigate the recrystallization

texture by means of macrotexture analysis in particular, due to

the sixfold rotation symmetry in the basal plane. Depending on

the activation of slip systems in the basal plane (single or double

slip), either a 1 1 2 0 RD or 1 0 1 0 RD (RD = rolling direc-

0921-5093/$ see front matter 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.msea.2008.02.004
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412 T. Al-Samman, G. Gottstein / Materials Science and Engineering A 490 (2008) 411420

tion) texture component will form, both of which are related by a

30 0 0 0 1 rotation. In the case of a strong basal texture (basal

plane parallel to the rolling plane), which develops favorably by

basal slip and twinning no matter what the initial orientation, it

is impossible to recognize a texture change after recrystalliza-

tion in the (0 0 0 2) pole figure owing to the symmetry problem

mentioned above. It would be even more difficult in case of a

basal fiber texture, i.e. the two main deformation components

(0001) 1 1 2 0 and (0 0 0 1) 1 0 1 0 and their scatter generat-

ing a (0 0 0 1) fiber texture, because then it would be impossible

to recognize any texture change even in a {1 0 1 0} pole figure.

In a previous study we demonstrated that electron back scatter

diffraction (EBSD) analysis is indeed a convenient and useful

method for investigating the relationship between deformation

and recrystallization textures, and we reported a 30 0 0 0 1

preference during nucleation and growth of the dynamically

recrystallized grains during plane strain compression tests at

200 and 300 C [3]. Bacroix and co-workers [4] reported similar

observations for a Zr2Hf alloy deformed at room temperature

by plane strain compression (PSC) and subsequently annealedat selected temperatures.

This paper addresses the influence of DRX on the deforma-

tion behavior during hot deformation at selected deformation

conditions (T, , ). Uniaxial compression was chosen as the

deformation mode since it allows conducting experiments at

precisely defined deformation conditions and the samples can

be easily quenched immediately after the tests for microstruc-

ture characterization. Although uniaxial compression may create

Fig. 1. (a) Schematic illustration of the sample orientations used for uniaxial

compression: (1) CD0ED, (2) CD45ED and (3) CD90ED; (b) experimental

setup; (c) CD0ED specimens deformed at 400 C/104 s1 to different strains

ranging from 0.15 to 2.2.

certain difficulties concerning texture measurements due to the

cylindrical symmetry which may conceal orientation changes

with crystal rotations about the cylinder axis, it has the advan-

tage over channel-die PSC that there are no lateral constraints

for deformation, which would give rise to a different texture

evolution.

2. Experimental procedure

The material used in the present study was a commercially

extruded magnesium alloy AZ31B with the following chemi-

cal composition (wt.%): 2.92 Al, 0.84 Zn, 0.33 Mn, 0.02 Si,

0.004 Fe, 0.001 Cu, 0.001 Ni, Mg (balance). The extrusion

parameters were as follows: extrusion temperature (400 C),

extrusion velocity (2 m/min), extrusion ratio (d0/d1)2 = 9. Prior

to the deformation experiments the received material was sub-

jected to annealing at 350 C for 6 h. The extruded and annealed

material had a mean grain size of 35m and a typical extru-

Fig. 2. Side view of a CD0ED compressed specimen at 200 C/102 s1 to

= 1.4 and corresponding SEM images of the marked regions ((1) sheared

area); (2) lateral surface of the compressed sample).


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T. Al-Samman, G. Gottstein / Materials Science and Engineering A 490 (2008) 411420 413

sion fiber texture with the (0 0 0 2)-basal planes lying parallel to

the extrusion axis (i.e. fiber axis perpendicular to c-axes). For

measuring the grain size, the linear intercept method was used.

Each measurement used 16 horizontal lines at a magnification of

500 covering a statistic of 4 0 0 0 grains in total. For the uniax-

ial compression tests, cylindrical specimens( 15mm 25 mm)

were machined from the extruded rod in three different orien-

tations with the compression axis CD parallel, perpendicular

and 45 aligned to the initial extrusion direction ED, respec-

tively (Fig. 1a). Uniaxial compression tests were carried out

at nine different deformation conditions, i.e. different temper-

atures (200, 300 and 400 C) and strain rates (102, 103 and

104 s1), but always to the same final strain. For the inves-

tigation of texture and microstructure development at various

stages of deformation some tests were terminated at selected

strains ranging from 15% to 220% (Fig. 1c), and the sample

was immediately quenched in water to freeze the microstruc-

ture for subsequent measurements. The compression rods of

the testing machine were equipped with heating elements to

heat the specimens from top and bottom up to the desiredtemperature (Fig. 1b). The time required to raise the temper-

ature to the desired value depended on the test temperature

and ranged between 2 and 5 min followed by a 5 min soaking

time to establish thermal equilibrium in the specimen. During

the tests, the temperature difference between top and bottom

of the sample did not exceed 1 K. This was assured by pre-

ceding temperature calibration using an advanced temperature

controller (Eurotherm 2704) and three thermocouples built in

top, middle, and bottom of the specimen. Hexagonal boron

nitride (h-BN) powder was used to reduce friction between

sample and compression rods and to minimize the barrelling

effect caused by it. After completion of the tests, specimens

for optical microscopy, X-ray texture measurements and EBSD

analysis were cut from the center area of the mid-section of

the deformed specimens, shortly ground with a very fine SiC

paper (4000 grit), and subsequently mechanically polished with

diamond paste of particle sizes 3 and 1 m, respectively. Final

polishing was performed using a colloidal silica solution. For

microstructure observations, specimens were etched after pol-

ishing in acetic picral [10 ml acetic acid + 10 ml H2O+70ml

picral (4% picric acid in solution with ethanol)]. For EBSD anal-

ysis some selected samples were additionally electro-polished

in a 5:3 solution of ethanol and H3PO4 to achieve best index-

ing. The step size used in EBSD measurements was between

0.2mand0.5m (depending on the grain size of the measured

specimen).

3. Results

3.1. Low temperature DRX

Taking into account the melting temperature of pure magne-

sium is 650 C, recrystallization occurring duringdeformation

at 200 C is referred to as low temperature dynamic recrystal-

Fig. 3. (a) Microstructure of the interior region of the same sample shown in Fig. 2 revealing DRX along serrated grain boundaries; (b) magnified image indicating

DRXgrain sizes of1m; (c) EBSDimage showinga necklace-type DRX structure; (d) flowcurves during uniaxial compression at selected deformation conditions.

Peak behavior at low temperatures goes along with the observed low temperature DRX.


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lization (LTDRX). An optical micrograph of a sample deformed

at 200 C/102 s1 is shown in Fig. 2 (top). Obviously, defor-

mation at those conditions, i.e. a relatively high strain rate, and

a temperature that is below the typical ductile transition temper-

ature (225250 C) necessary for activating c + a-pyramidal

slip, caused mechanical failure. This was observed on the cylin-

drical surface of the compressed specimen. SEM images were

taken from the fractured areas 1 and 2 on the lateral surface

of the specimen. As apparent from the image of area 1, the

sample underwent massive strain localization which resulted in

failure by shearing. Area 2 showed a rough and poor surface

quality comprising voids. By contrast, the examined interior

region of the same specimen revealed quite different results

(Fig. 3ac) because the specimen underwent dynamic recrys-

tallization (Fig. 3a). The recrystallized grain size was about

1m (Fig. 3b). Flow curves at different deformation condi-

tions (Fig. 3d) rendered for a low deformation temperature

(200 C) and different strain rates a typical stressstrain behav-

ior of a material undergoing DRX: after initial work hardening

a peak stress was attained, followed by rapid work softening,which typically indicates the onset of DRX (although not nec-

essarily). DRX does not start at the peak stress but already

at lower strains (arrow in Fig. 3d) that can be usually associ-

ated with the inflection point on the = / vs. flow stress

curve [5,6]. For practical purposes, the strain at 80% of the

peak flow stress is defined as the critical strain for the initia-

tion of DRX [7]. It is noteworthy, that the flow strain cannot

be considered as a state parameter like for instance the flow

stress, but as stated above for practical purposes the term crit-

ical strain can be used to describe the onset of DRX. In the

present case of a deformation at 200 C/102 s1 the initia-

tion of DRX corresponds to a critical strain of 10%. The

EBSD image (Fig. 3c) reveals a necklace-type microstructure

of fine DRX grains surrounding coarse deformed grains. Neck-

lace formation in AZ31 has been also reported by other authors

[810].

3.2. DRX grain size

The influence of deformation conditions on the recrystallized

grain size at steady state conditions (= 1.2) was examined

(Figs. 4 and 5). With increasing deformation temperature Tand

decreasing strain rate , i.e. with decreasing ZenerHollomon

parameter Z[Z = exp(Q/RT), Q is the activation energy and

R the gas constant] the DRX structure became coarser. The grainsize evolution at higher temperatures (400 C) was more strain

rate sensitive than at lower ones (200 C) (Fig. 5). The onset of

DRX during compression at 200 C/102 s1 (highest Z) gave

rise to a very fine, partially recrystallized microstructure with

an average grain size of 12 m (Fig. 3b) whereas compression

Fig. 4. Optical micrographs (perpendicular to CD) of CD0ED specimens illustrating the DRX microstructure upon uniaxial compression at various deformation

conditions ranging from 200 to 400 C and 102 to 104 s1 at a steady state strain of 1.2.


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Fig. 5. Average DRX grain size (linear intercept method) of CD0ED specimens

at = 1.2 as a function of deformation temperature and deformation strain rate.

at 400 C/104 s1 (lowest Z) resulted in a coarser grain size

of

18m, and the examined area of the specimen was fullyrecrystallized.

To investigate the influence of the initial texture on the

final DRX grain size, three different starting textures obtained

from the extruded rod (Fig. 1a) were subjected to uniaxial

compression under the same deformation conditions, varying

temperature and strain rate between 200 and 400 C and 102

and 104 s1, respectively. For a given temperature and strain

rate the average DRX grain size did not vary significantly with

initial texture (Fig. 6). Although the influence of the initial tex-

ture on the recrystallized volume fraction was not examined for

each case, the kinetics of DRX was found to be more affected

by the deformation conditions than by the crystallographic

texture.

3.3. Texture randomization

Deformation at the lowest Z (400 C and 104 s1) caused

texture randomization (Fig. 7). This is illustrated for test series

CD0ED (Fig. 1a) in Fig. 7 in terms of ODF sections 2 = 0 and

2 = 30 for early (= 0.3), moderate (= 0.7) and advanced

Fig. 6. DRX grain size evolution at different deformation conditions for three

different starting orientations, i.e. CD0ED, CD45ED and CD90ED.

(= 1.6) stages of deformation. Prior to uniaxial compression

the specimen had a strong extrusion texture with the basal planes

oriented parallel to the compression axis. With increasing defor-

mation strain the basal planes gradually reoriented towards the

compression direction, eventually giving rise at = 1.6 to a

weak, almost random texture. By contrast, texture development

for the same specimen CD0ED at other deformation conditions

always ended up in the formation of a strong basal texture

(basal planes aligned parallel to the compression plane) even

at 400 C but at higher strain rates. Interestingly, other spec-

imen types CD45ED and CD90ED (Fig. 1a) exhibited sharp

textures, even when tested at the lowest Z, i.e. 400 C/104 s1

[11].

3.4. Dynamic recrystallization of twins

In the literature little information can be found on the influ-

ence of twins on DRX during hot deformation of magnesium.

Although nucleation of DRX at twin boundaries was reported

[12], DRX inside twins was seldom found. Twin domains pos-sess much higher stored deformation energy compared to the

matrixand aretherefore ought to be favorable nucleation sites for

DRX. This hypothesis was corroborated for high Zdeformation

conditions, i.e. 200 C/102 s1 where DRX occurred in regions

occupied by twins (Fig. 8). Fortunately, some parts of the twins

that did not undergo DRX were still visible in the microstruc-

ture after quenching and therefore gave good evidence of DRX

taking place inside twins although some neighboring twins were

left without DRX for unknown reasons.

4. Discussion

4.1. Role of DRX in the brittleductile transition

Uniaxial compression at the highest Z (200 C/102 s1)

showed contrasting deformation characteristics close to and far

from the center of the specimen. Microstructure observations of

the brittle lateral surface of the specimen showed extensive

cracking (Fig. 2), whereas the ductile inner part of the spec-

imen revealed a partially recrystallized microstructure (Fig. 3).

As evident from Fig. 3a, the development of serrated grain

boundaries of coarse grains led to the nucleation of DRX by

bulging. This discontinuous mechanism caused the formation of

a necklace structure of recrystallized grains along the original

boundaries, seen in Fig. 3c. At higher deformation tempera-tures consecutive necklaces eventually consumed the deformed

microstructure and caused full DRX. In spite of the relatively

low deformation temperature of 200 C, and a relatively high

strain rate of 102 s1, DRX took place in the center of the

deformed specimen and caused softening, indicated by the flow

curve peak in Fig. 3d. This also improved the ductility in the

interior region of the material, i.e. caused a transition from

a brittle to a ductile behavior. The difference in deformation

behavior of sample center and lateral surface, particularly the

absence of DRX in the latter is most likely attributed to an inho-

mogeneous strain distribution throughout the sample, which is

typical for uniaxial compression. Additionally, the effect of a


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Fig. 7. ODF sections at2 = 0 and 2 = 30

revealing the texture evolution of specimen type CD0ED upon uniaxial compression at 400 C/104 s1 during different

stages of deformation. Intensity levels: 2, 4, 7, 12, 20 and 25.

slight temperature gradient between center and lateral surface

should also be considered. Low temperature dynamic recrys-

tallization (LTDRX), i.e. DRX at 200 C is conceived to occur

in magnesium due to the lack of easily activated slip systemsat low temperatures. It is also promoted by the low stacking

fault energy of magnesium and high grain boundary diffusivity

[13].

4.2. Influence of Z on DRX grain size and texture evolution

The recrystallized grain size showed a strong dependence

on both the temperature and strain rate. In particular, at highertemperatures it was very strain rate sensitive. Similar ten-

dency was observed while investigating the influence of Z

on the texture evolution of extruded pure magnesium dur-

Fig. 8. Optical (a) and SEM micrograph (b) of a CD0ED specimen deformed at 200 C/102 s1 up to = 1.2 showing dynamic recrystallization inside of

deformation twins. The micrographs are perpendicular to CD.


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Fig. 9. Texture evolution of extruded pure Mg upon PSC ( = 1.5) at different deformation conditions. At 400 C, texture evolution is very strain rate sensitive

compared to the one at 200 C [14].

ing PSC [14]. At 200 C (top row in Fig. 9) no significant

change of the texture was observed by changing the strain

rate. At 400 C (bottom row in Fig. 9) different strain rates

led to a conspicuously different texture evolution. This corrob-

orates that at higher temperatures strain rate sensitivity does

not only affect the DRX grain size but also the texture evolu-

tion.

Contrary to pure Mg and some other binary Mg alloys,

AZ31 did not undergo significant grain growth during low-

est Z deformation (400 C/104 s1). This was evident in

Fig. 4 by the fine grain size of 18m. By contrast, the

microstructure of an initially extruded pure magnesium sample

revealed very coarse grains (D 200m) upon PSC defor-

mation at 400 C/104 s1. The grain size can be related to

the steady state flow stress by a power law with an expo-

nent of 0.85 (Fig. 10). This corresponds to observations onother materials [15,16]. At 400 C, a rapid and significant

increase of the grain size was observed from the microstruc-

tures, and the measured grain size revealed a noticeable

deviation from the low temperature relation between flow

stress and grain size (Fig. 10), most probably due to grain

growth.

4.3. Role of DRX in texture randomization

The texture evolution (Fig. 7) suggests that a texture random-

ization could be realized under certain deformation conditions,

i.e. 400

C/104

s1

. The initial texture of test series CD0ED

(Figs. 1a and 7) was designed to suppress basal slip under

compressive loading since the basal planes were essentially

parallel to the loading direction (Schmid factor zero).

Under these conditions non-basal slip modes need to be acti-

vated for deformation. The slip system easiest to be activated

in magnesium besides basal slip is prismatic slip. However,

although this slip system is favorably oriented in the used

configuration, it cannot accommodate strain along the c-axis

and therefore cannot compensate the imposed deformation.

For this, c + a-pyramidal slip is needed and in fact, is also

favorably oriented for the chosen deformation geometry (high

Schmid factor). The selected initial texture promotes also

the formation of tensile twins with {1 0 1 2}-twinning planes

since the compression direction is aligned parallel to the

basal planes. However, increasing temperatures and decreas-

ing strain rates render twinning less important for deformation[17]. These facts would lead to the conclusion that uniax-

ial compression of CD0ED specimen at high temperatures

and low strain rates should give rise to a particular tex-

ture, developed by a combination of c + a-pyramidal and

prismatic slip. However, the obtained texture development

(Fig. 7) demonstrated that with increasing strain the speci-

men progressively lost its texture sharpness. For this reason,

texture randomization cannot be attributed to slip activity tak-

ing place on the {1 1 2 2}-pyramidal and {1 0 1 0}-prismatic

planes, but rather to DRX. After all, at 400 C/104 s1 DRX

was very much evident and resulted in a fully recrystallized

microstructure.


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Fig. 10. Optical micrograph and steady state flow stress vs. recrystallized grain

size of pure Mg indicating grain growth during high-temperature deformation.

4.4. Mechanism of DRX in twins and its effect on the

overall texture

From the metallography of recrystallization twins (Fig. 8)

no conclusions can be drawn on the DRX mechanism inside

of twins. Therefore, a detailed EBSD analysis was performed to

gain information on theorientationof therecrystallizedareasin a

twin with respect to the neighboring twins and the parent grains

(matrix). A sample area with recrystallized twins is shown in

Fig. 11. The twin boundaries revealed a 86 1 1 2 0 misorienta-

tion relationship with the matrix andtherefore, were identifiedas

{1 0 1 2}-tensile twins. The presence of some low angle bound-

aries in the recrystallized twin structure is apparently due to

the dynamics of the process, i.e. low angle boundaries are first

formed inside the twin and with progressing deformation they

increasingly incorporate dislocations and eventually convert to

high angle boundaries. This will fragment the original twin and

create a new structure of fine grains. Neighboring twins that

showed no DRX (Fig. 11c) were free of low angle boundaries,

and the reason for why they did not undergo recrystallizationis still unknown. May be the twin was unfavorably oriented for

deformation so that no dislocation structure was formed.

The orientation relationship of the recrystallized twins with

respect to the parent grains and the neighboring twins is pre-

sented in Fig. 12a and b in terms of the angle between the c-axis

of the grains andthe compression direction CD.As shown, the c-

axes of the parent grains were scattered around the compression

axis within 020 peaking at 15. They essentially had a basal

orientation (basal planes parallel to the compression plane of the

specimen). Neighboring tensile twins with no DRX were rotated

90 away from the parent grains. The c-axes of the recrystallized

structure of former twins scattered between 30

and 70

from thecompression axis. It should be stated that the presence of tensile

twins within the basal-oriented matrix is quite uncommon. At

the beginning of deformation, the prism-oriented grains can eas-

Fig. 11. (a) EBSD-Kikuchi band contrast map of the same specimen shown in Fig. 8 revealing recrystallized twins and {1 0 1 2} tensile twins; (b) detailed view of

a recrystallized twin indicating the presence of some low angle boundaries (thin lines) within the recrystallized structure comprised of high angle boundaries (bold

lines); (c) neighboring tensile twin showing no recrystallization.


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Fig.12. (a and b) EBSDmap and correspondingmisorientation angle histogram

of the same measured sample from Fig. 11 showing the orientation (c-axis) of

parent grains, recrystallized former twins and tensile twins with respect to the

compression axis; (c) microtexture of the measured area. Intensity levels: 1, 2,

4, 7, 10, 15, 20 and 30. RD: radial direction.

ily twin and the whole microstructure can be converted to twin

orientation quite rapidly. Once a basal texture has been attained,

i.e. the c-axes of grains have been aligned with the compressiondirection it is rather unlikely that the grains would exhibit tensile

twinning while their c-axes are being compressed. On the other

hand, during unloading of the highly compressed specimen, the

c-axis of parent grains could experience little yet enough

tensile along the loading direction, which renders tensile twin-

ning possible. Tensile twinning taking place during unloading

has been readily reported [18,19]. This hypothesis would also

explain why these twins were not recrystallized (as they form

during unloading and not before). Apparently, the tensile twins

Fig. 13. Schematic diagram showing the grain rotations produced by different

tensile twinning variants.

in the basal-oriented matrix comprised three different variants.

These are shown in Fig. 13.

In order to examine the influence of the texture of the

recrystallized twins on the final compression texture, the total

microtexture (Fig. 12c) was decomposed into three texture

components comprising the parent grains (Fig. 14a), different

variants of the tensile twins with no DRX (Fig. 14b) and the

recrystallized twins (Fig. 14c), respectively. From the intensi-

Fig. 14. Microtexture components of (a) parent grains, (b) tensile twins of different variants and (c) recrystallized twins. The sum of these textures results in the

overall texture shown in Fig. 12c. RD: radial direction (continued from Figs. 12 and 13).


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ties of the three texture components presented in Fig. 14 the

total texture is given in Fig. 12c. It can be concluded that the

recrystallization texture of the former twins plays an important

role in weakening the overall texture by counteracting the strong

deformation texture of the parent grains.

5. Conclusions

(1) During uniaxial compression at 200 C/102 s1 a transient

deformation behavior was found. The lateral surface of the

specimen exhibited typical failure characteristics and was

considered brittle. The center of the specimen behaved duc-

tile and revealed dynamic recrystallization. The respective

softening was reflected by the stressstrain and is conceived

to restore ductility to the material. Thedriving force for such

low temperature DRX was ascribed to a lack of easily acti-

vated slip systems at the examined deformation conditions.

(2) Uniaxial compression at the lowest Z, i.e. 400 C/104 s1

achieved a remarkably high strain of 2.2.

(3) The steady state DRX grain size depended much moreon deformation temperature and strain rate, i.e. on the

ZenerHollomon parameter Z than on the initial texture of

the specimen. It increased with decreasingZ. At 400 C, the

DRX grain size was markedly strain rate sensitive.

(4) AZ31 showed virtually no grain growth at elevated tempera-

tures (400 C) and low strain rates (104 s1) and developed

an average recrystallized grain size of 18m. By con-

trast, at the same deformation conditions pure magnesium

with comparable initial grain size developed coarse grains

(>200m) indicating the occurrence of grain growth.

(5) There is a processing window where complete recrystal-

lization and a nearly random texture can be established(T=400 C, = 104 s1, = 1.6). This was attributed

besides the high deformation temperature to the initial crys-

tallographic orientations that inhibited the activation of

mechanical twinning and suppressed basal slip, both caused

the formation of strong textures at higher Z processing.

Texture randomization was apparently due to DRX.

(6) Besides necklace formation as a DRX mechanism, dynamic

recrystallization also took place inside of twins (observed

during highest Zdeformation). The texture of recrystallized

twins wasmuch weaker than the texture of the parent grains.

The mechanism of DRX in twins was found to be ofcontinu-

ous nature, involving the formation of low angle boundaries

and their conversion to high angle boundaries forming new,

fine grains. {1 0 1 2}-Tensile twins (probably formed during

unloading) surrounded the recrystallized twins, as identified

by EBSD.

Acknowledgments

Financial support of the Deutsche Forschungsgemeinschaft,

grant no. (GO 335/27) is gratefully acknowledged. The authors

would like to thank Otto-Fuchs AG for the kind donation of the

material. Thanks also to Bashir Ahmad for help with certain

figures and measurements.

References

[1] S.E. Ion, F.J. Humphreys, S.H. White, Acta Metall. 30 (1982) 19091919.

[2] I. Samajdar, R.D. Doherty, Acta Mater. 46 (1998) 31453158.[3] G. Gottstein, T. Al-Samman, Mater. Sci. Forum 495497 (2005) 623632.

[4] K.Y. Zhu, D. Chaubet, B. Bacroix, F. Brisset, Acta Mater. 53 (2005)

51315140.

[5] D.L. Yin,K.F. Zhang,G.F. Wang, W.B.Han, Mater. Sci. Eng. A 392(2005)

320325.

[6] E.I. Poliak, J.J. Jonas, Acta Mater. 44 (1996) 127136.

[7] M.R. Barnett, Mater. Sci. Forum 426432 (2003) 515520.

[8] S. Spigarelli, M. El Mehtedi, M. Cabibbo, E. Evangelista, J. Kaneko, A.

Jager, V. Gartnerova, Mater. Sci. Eng. A 462 (2007) 197201.

[9] A. Galiyev, R. Kaibyshev, G. Gottstein, Acta Mater. 49 (2001) 11991207.

[10] M.R.Barnett,A.G. Beer, D. Atwell, A. Oudin, Scr. Mater.51 (2004) 1924.

[11] T. Al-Samman, G. Gottstein, in: K.U. Kainer (Ed.), Proceedings of the 7th

International Conference on Magnesium Alloys and their Applications,

Wiley-VCH, 2006, pp. 553559.

[12] M.R. Barnett, Mater. Trans. 44 (2003) 571577.[13] J. Tan, M. Tan, Mater. Sci. Eng. A 339 (2003) 124132.

[14] T. Al-Samman, G. Gottstein, Mater. Sci. Forum 539543 (2007)

34013406.

[15] B. Derby, Acta Metall. Mater. 39 (1991) 955962.

[16] M.J. Luton, C.M. Sellars, Acta Metall. 17 (1969) 10331043.

[17] C.N. Tome, S.R. Agnew, W.R. Blumenthal, M.A.M. Bourke, D.W. Brown,

G.C. Kaschner, et al., Mater. Sci. Forum 408412 (2002) 263268.

[18] M.D. Nave, M.R. Barnett, Scr. Mater. 51 (2004) 881885.

[19] L. Jiang, J.J. Jonas, R.K. Mishra, A.A. Luo, A.K. Sachdev, S. Godet, Acta

Mater. 55 (2007) 38993910.

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