In Situ Evaluation of Detwinning Behavior in Extruded AZ31 Mg Alloy by AE

In-Situ

Evaluation of Detwinning Behavior in Extruded AZ31 Mg Alloy by AE

Takashi Yasutomi

and Manabu Enoki

Department of Materials Engineering, School of Engineering, The University of Tokyo, Tokyo 113-8656, Japan

Tension twinning readily occurs under compressive loading along extrusion direction because extruded AZ31 Mg plate has the texture with the basal planes parallel to the extrusion direction. Detwinning occurs in the twinned areas during tensile loading in the opposite direction. In this study, acoustic emission (AE) during the tensile test was investigated for the evaluation of detwinning behavior. Different AE features depending on the strain level were observed and detwinning behavior was divided into three stages. In thefirst stage, many AE signals with higher peak frequency were generated, then very few AE signals were observed in the second stage, andfinally again some AE signals with lower peak frequency were observed in the third stage. In addition, EBSD analysis was conducted to identify phenomena in each stage. The results suggested that twin boundary starts to move in thefirst stage, then twin area shrinks in the second stage, and twin area disappears in the third stage. Deference of AE characteristics in each stage seems to be caused by these phenomena. It can be concluded that AE is an effective method for evaluation of detwinning behavior. [doi:10.2320/matertrans.MAW201218]

(Received April 25, 2012; Accepted June 12, 2012; Published July 25, 2012)

Keywords: acoustic emission, extruded magnesium alloy, detwinning, twinning

1. Introduction

In recent years, the utilization of magnesium alloy for lightweight structural components has significantly increased where considerable weight savings can be made by substituting magnesium for aluminum or steel components. Its deformation typically occurs through crystalline slipping, twinning and detwinning. Critical resolved shear stresses (CRSS) have been reported for three different slip systems in single crystal magnesium.1,2) _{According to these results, the}

CRSS for a basal slip system at room temperature is much lower than those of the non-basal slip system as well as the twinning modes. Therefore, plastic deformation in polycrys-talline alloys appears to occur almost entirely by basal slip. However, the basal slip systems provide only two independ-ent slip systems, while the uniform deformation needs at least

five independent slip systems. Therefore the other systems especially twinning or detwinning have to be activated.3)

Mg­3Al­1Zn alloy is currently the most common Mg alloy used for sheet applications. Since extruded AZ31 Mg plate has the texture with the basal planes parallel to the extrusion direction, the material demonstrates different mechanical responses when deformed along different direc-tions.4) _{Tension twinning readily occurs under compressive}

loading along extrusion direction. If the tensile loading is applied after compressive loading, detwinning occurs and the formed twins become either slightly narrower or shorter.

Acoustic emission (AE) is an elastic wave which is generated within a material due to sudden localized and irreversible structure changes. It has been reported that deformation mechanism of Mg can be effectively evaluated by AE method.5­9) Twinning and detwinning are important sources of AE and their dynamic internal structure evolution can be analyzed effectively by AE.10) Liet al.reported that psuedoelasticity of pure magnesium can be measured by AE method.6,7)The anelastic recovery strain which seems to be obtained by detwinning in unstable twin area was quantita-tively evaluated by analyzing the AE events generated during a certain period of time after high-speed unloading. A model

for relationship between applied strain and cumulative AE counts during the anelastic recovery process was proposed. Therefore, AE is a promising method to study dynamic detwinning mechanism.

The object of this study is to evaluate detwinning behavior during the tensile testing with twin-induced AZ31 Mg alloy by AE method. The texture evolution of the alloy during the tensile test was examined by electron backscatter diffraction (EBSD). The characteristics of the detwinning behaviors are discussed based on the results of AE analysis and micro-structural features.

2. Experimental Methods

2.1 Materials

The material used in this study was a commercial hot-extruded AZ31 Mg alloy plate with 3 mm thickness produced by Izumi Metal Corp. Tensile specimens (Fig. 1) were prepared from this plate with loading direction parallel to the extrusion direction (ED). The gauge length and width were both 6 mm.

2.2 Induction of twins by pre-deformation

Compression test was performed at room temperature using universal tensile machine (AG-5000C, Shimadzu Co., Japan) to induce twins in the grains. The specimen was carefully attached to the machine to prevent buckling. Then the compressive load was applied parallel to the extrusion direction until 100 MPa at a strain rate of 2.78©10¹4_s¹1_.

After testing, the microstructure was observed by EBSD to confirm the presence of twins.

45mm 12mm

6mm

6mm R 15mm

5mm

2mm 96mm

Fig. 1 Tensile specimen configuration and positions of AE sensors.

+_{Graduate Student, The University of Tokyo}

2.3 Tensile testing with AE measurement

Tensile testing was performed at room temperature with a strain rate of 2.78©10¹4_s¹1 _{using the same machine}

described in the previous section. In addition, AE measure-ment was simultaneously performed using Continuous Wave Memory (CWM) which was developed by our group.11) CWM memorizes all AE waveforms continuously at a sampling rate of 10 MHz. Four AE sensors (M304A, Fuji Ceramics, Japan) having a frequency band of 100­1000 kHz were used in all the experiments. These sensors were attached on the specimen in the positions as shown in Fig. 1.

2.4 AE analysis

One important issue in AE waveform analysis is to classify effective AE events from unconcerned signals. In the case of tensile testing, any signals generated outside sensors such as signals from gripping can be considered as noise. In order to exclude these signals, four-channel AE measurement is preferable because two outside AE sensors are used as guard sensors. Any signals which generated outside the sensors will be detected first by these guard sensors and excluded. Furthermore, signals from the gauge section were extracted by the location of each event identified using two-step Akaike’s information criterion (AIC)12) of arrival time. In addition, the threshold level was set to 300 mV and high pass

filter of 100 kHz was applied to get rid of the noises from environments.

Usually Fourier Transform (FT) technique is used to analyze signal properties in frequency domain. However, spectral analysis can give good resolution only in frequency domain and it losses some information in the time domain. Moreover AE signals are complicated characteristics com-prising reflections from local and distant sources as well as different mechanisms of acoustic noise formation. Therefore, in order to get rid of these problems continuous wavelet transform (CWT) was used for AE signal processing. The CWT can decompose any signal into frequency bands and then the local time information can be achieved with correlated resolution. There are various kinds of analyzing wavelets, such as Gabor wavelet, Mexican hat, Daubechies wavelet, etc. In this study, Gabor wavelet was used because it provides a small window in the time as well as in the frequency domain.13) _{Then AE energy of}

each event was estimated by calculating square value of the highest intensity of each event. Additionally, cumulative AE energy was calculated by summing up the AE energy of all events.

2.5 Microstructures observation

EBSD was performed to evaluate the texture during the tensile testing. Various specimens tested at different amounts of strain were cut and then polished by a machine called

“cross section polisher” (CP), which utilizes an argon iron beam to mill the sample. This method prevents any damages from mechanical polishing and therefore the suitable surface for EBSD is easily obtained. Automated EBSD scans were performed in the stage control mode with TSL data acquisition software with a step size of 1.5 µm. The observation area was approximately 0.53 mm2 _{for each}

specimen.

3. Results

3.1 Microstructure characterization of as-received

AZ31 Mg alloy

Figure 2 shows the orientation imaging micrographs obtained by EBSD of the as-received AZ31 Mg alloy. The inverse pole figure (IPF) map (Fig. 2(a)) showed that the material had grains with a mean grain size of approximately 60 µm and no twins were detected. The polefigure (PF) with its reflecting surface normal to the extrusion direction is given in Fig. 2(b). The texture can be considered as a ring

fiber texture with the basal planes parallel to the extrusion direction. This texture is often found in extruded Mg alloy.

3.2 Tensile deformation behavior

Figure 3(a) shows the stress­strain curves of the as-received and pre-deformed specimens and a summary of their tensile properties is given in Table 1. The effect of pre-deformation on tensile behavior is clearly observed. The stress­strain curve exhibited a skewed shape, which is unseen in that of specimen without twins. This is clearly observed on the change in strain-hardening rate with respect to the strain (Fig. 3(b)), where the strain hardening rate of the pre-deformed specimen rapidly increased from around strain level of 2%up to about 3% and then decreased. Moreover, the pre-deformed specimen showed lower yield stress and elongation while there was not significant change of ultimate tensile stress. The reduction of yield stress and elongation of the pre-deformed specimen were approximately 74% (from 188 to 49 MPa) and 44%(from 12.1 to 6.8%), respectively.

(a)

2110 0001

1010

6.000 4.750 3.500 2.250 1.000 Max=8.265

(b)

Fig. 2 Microstructural characteristics of as-extruded material: (a) inverse polefigure map (b) (0002) polefigure.

Table 1 Mechanical properties of extruded AZ31 magnesium alloy.

TYS (MPa) UTS (MPa) ¤(%)

As-received 188 252 12.1

3.3 Texture evolution during tensile test

As described in the previous section, the tensile behavior of pre-deformed specimen was significantly different from

that of the as-received one. Therefore, the observations of microstructure by EBSD during tensile test at several loading steps shown in Fig. 3(a) with arrows were carried out to trace the texture evolution. Figure 4(a) shows the IPF map of the specimen after pre-deformation by compression test (before tensile test). Extension twins which lead to an 86° reorientation of grains were observed in almost all of the grains and twins were successfully induced by pre-deformation. Figure 4(b) shows IPF map of the texture right after yielding point. Significant change of twin characteristics such as the size or the number of them cannot be seen at this strain level. Then Fig. 4(c) shows IPF map of the texture when the strain hardening rate started to rise. At this point, some of twins disappeared and twins in the grains became small. Finally, Fig. 4(d) shows the IPF map of texture after strain hardening rate became moderate. At this strain level, very few twins were observed.

3.4 AE results

The objective of AE test is to detect the presence of emission sources and to provide information of the source. These can be obtained by signal processing and source characterization of detected AE events. Generally AE event is an individual signal bursts produced by any microstructural changes that generate elastic waves in a material under stress. Figure 5(a) shows plots of WT maximum frequency during the tensile deformation as a function of strain. Stress­strain curve was also given in the same figure to clearly show when AE events happened. Additionally, the strain dependence of strain hardening rate and cumulative AE energy rate are shown in Fig. 5(b). AE energy rate in this

Nominal stress,

σ

/MP

a

300

250

200

150

100

50

0 1

2 3

4

Strain (%) 20

15

10

5

0

0 2 4 6 8 10 12 14 As-received

After compression

(a)

(b)

Strain hardening rate,

d

σ

/

d

ε

/GP

a

Fig. 3 (a) Stress­strain curves of as received specimen and specimen after compression (b) Strain hardening rate of as received specimen and specimen after compression.

(b)

(c)

(d)

2110

0001

1010

(a)

figure is calculated by differentiating the cumulative AE energy from the strain. AE energy rate in the strain increased up to the maximum value at around yielding point and significantly reduced, then it started to increase again from strain level of 1.25%and decreased after reaching maximum point at 2.45%.

AE behavior is divided into three stages from the AE energy activity. The first stage is from strain level of 0 to 0.7%, the second stage is from 0.7 to 1.5%, and the third stage is from 1.5 to 3.5%. In thefirst stage, large amount of AE energy was generated, then very few AE energy was observed in the second stage, and finally again some AE energy was observed in the third stage. The frequency distribution of AE signals was also calculated by summing up the AE energy of each event and shown in Fig. 6. Thefigures show that frequency characteristics of AE are different in each stage. In the first stage, cumulative AE energy has a peak at 545 kHz. In contrast, it has a peak at 305 kHz in the third stage.

4. Discussion

4.1 Effect of compressive pre-deformation

The stress­strain curve of the extruded AZ31 alloy during the tensile testing dramatically changed due to compressive pre-deformation as shown in Fig. 3. It is notable that AE events generated not only around yielding point but also before significant increase of strain hardening rate as shown in Fig. 5(a). This feature of AE behavior is a little bit different from that of previous study in compression test with as-extruded Mg alloy5,6) _{where dislocation slip and}

twinning are the main deformation mechanisms and most of AE events were observed around yielding point. It is presumed that another deformation mechanism was activated in this specimen. In addition, texture evolution during tensile test by EBSD demonstrates that twins induced by pre-deformation shrank and disappear during the tensile defor-mation. These results indicate that the significant change of the stress­strain curve was mainly caused by detwinning behavior and it gave good agreement with the previous study.14­17) This detwinning behavior was clearly found by AE analysis. Since extension twinning leads to a reorienta-tion of 86.3° of the crystal lattice, all the basal planes in twinned lattices lie nearly perpendicular to the extrusion direction after compressive deformation. These twinned regions are capable of detwinning during reloading in subsequent tension. Thus, all the strains caused by twinning during compressive pre-deformation are recovered by detwinning during subsequent early tensile deformation resulting in the convex nature of the curvature.

4.2 Identification of detwinning behaviors

Detwinning behavior seems to involve three kinds of motions, initiation of twin shrinking, stable twin shrinking, and twin disappearance. The characteristics of these motions will be discussed by considering the results of AE and EBSD analysis. The result of EBSD analysis in this experiment agrees with other in-situmeasurements.16­18)

In the first stage, AE energy rate shows its first peak at around yielding point, which is corresponding to the significant drop of strain hardening rate at the initial part of deformation. According to the results of EBSD analysis, twin shrinking starts around yielding. Then, AE energy rate decreased according to the exhaustion of twins favorable for activation of detwinning after yielding. In the second stage, AE energy rate kept small. The result of EBSD shows that stable twin shrinking is a dominant deformation mechanism around this strain level. In the third stage, AE energy rate reached its second peak around strain level of 2.5% when strain hardening rate started to increase again significantly. In addition, EBSD results indicate that most of twin disappearances happen around this strain level. Significant increase of strain hardening rate after the second peak of AE energy should be due to the saturation of detwinning behavior with twin disappearance. Because both basal slip and extension twinning are difficult to operate with the resulting grain orientations, further post-detwinning defor-mation needs to be accommodated by the harder non-basal

hai dislocation slip.18)

Relationship between AE energy and EBSD results could be reasonably explained by considering characteristics of each detwinning motion. Similar characteristics of AE activity and strain­stress curve in this experiment were also found during compression test where twinning is a dominant deformation mechanism.15)Louet al.concluded that thefirst peak of AE is associated with nucleation of twins of the principal type in Mg, f1012g twin, and activated twins continue to grow without an accompanying large AE signal as straining. In twinning deformation, the force required for twin growth once nucleated is much less than that for their nucleation,19­21) _{and the amplitudes of AE events are one}

Nominal stress,

σ

Intensity (a.u.)

WT Peak frequency , f /kHz Strain (%)

0 1 2 3 4 5

20 15 10 5 0 1016 1015 1014 1013 1012 Strain hardening rate AE energy rate (a)

(b) 0

Strain hardening rate,

d

σ

ε

AE energy rate,

dE

/

d

ε

(a.u.)

603.4 331.4 182.1 100.0 2000 1099

300

order of magnitude higher during the nucleation of mechanical twins than during their broadening.5,15)_{With the}

same reason as twinning, AE energy generated from initiation of twin shrinking is different with that of stable twin shrinking. It is acceptable that stable twin shrinking generates small AE energy like twin growth because stable twin shrinking is also a similar twin boundary movement. Twin disappearance can also need relatively higher force to operate because twin disappearance is a reversal process of twin nucleation. This should be the reason why the AE energy rate peak was observed in the third stage.

Frequency distribution of cumulative AE energy was calculated in each stage and shown in Fig. 6. Prior to the discussion, it should be mentioned that AE waveform detected by sensors is influenced by the specimen confi g-uration, sensor characteristic, and other experimental con-ditions. Although it is difficult to compare the absolute value of frequency of AE signals, those are compared to identify the source mechanism in this study. Initiation of twin shrinking and twin disappearance appear to have different features because the peak of frequency distribution of cumulative AE energy was observed at different frequency. Initiation of twin shrinking generated AE energy with higher peak frequency around 550 kHz, while twin disappearance generates AE energy with lower peak frequency around 300 kHz. It is well known that the frequency of AE signal is determined by the event rise time of AE sources.22) For instance, the frequency of twinning nucleation is higher than that of dislocation slip because twinning nucleation is reported to occur faster than dislocation slip.5) _{In the same}

way, it can be concluded that initiation of twin shrinking occurs with shorter event rise time compared to that of twin disappearance because peak frequency of initiation of twin shrinking is higher than that of twin disappearance.

According to the results mentioned above, dynamic change of detwinning behavior during the tensile testing can be effectively evaluated by AE method and divided into three stages. Firstly, initiation of twin shrinking was mainly activated in thefirst stage and plentiful AE energy with high frequency was observed. Secondly, stable twin shrinking mainly occurred in the second stage and very few AE events were detected. Finally, twin disappearance mainly occurred in the third stage, and some AE energy with low frequency was observed. Schematicfigures of the detwinning behavior in each stage are shown in Fig. 7.

5. Conclusions

Detwinning behavior during tensile testing with twin-induced specimen was evaluated in detail by AE method. The results obtained from the present study are as follows:

(1) Detwinning behavior could be divided into three stages by the results reasonably, initiation of twin shrinking, stable twin shrinking and twin disappearance.

(2) Yielding and significant increase of strain hardening rate during the tensile deformation are caused by initiation of twin shrinking and twin disappearance, respectively.

(3) Initiation of twin shrinking and twin disappearance are strong sources of AE during detwinning behavior. The event rise time of initiation of twin shrinking is shorter than that of twin disappearance.

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,

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Cumulative AE energy

,

E

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(a) 1.5 1.2 0.9 0.6 0.3 0

(c)

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Cumulative AE energy

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(×107₎ (×106₎ (×108₎

Fig. 6 Cumulative AE energy­WT peak frequency relations: (a) thefirst stage (b) the second stage (c) the third stage.

(a)

(b)

(c)

Fig. 7 Schematicfigures of the detwinning behavior in each stage: (a) the

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