**4.4.1 Anisotropy of AE and PL effects in Al alloys**

286 Recent Trends in Processing and Degradation of Aluminium Alloys

0

Fig. 24. Courses of AE events rate and force versus time during compression of Mg10Li5Al

The AE effect, which accompany the Portevin–Le Châtelier one (PL effect – known also as discontinuous or serrated yielding or jerky flow), are quite well documented (van den Beukel, 1980; Caceres & Bertorello, 1983; Cottrell, 1953; Korbel et al., 1976; Pascual, 1974; Pawełek, 1989). Pascual (Pascual, 1974), as one of the first showed, that strong correlations occurred between the AE behavior and plastic flow instabilities, resulting from the in-

It was established that the local peaks of yielding corresponded to the increases of AE and that they resulted from the dislocation breakaway from the atmospheres of foreign atoms (Cottrell atmospheres) as well as the multiplication of dislocations at the front of propagating deformation band, similar to the well known Lüders' band. The results presented below will be shortly discussed further on the basis of a simple dislocation-

dynamic (DD) model of PL effect (Pawełek, 1989), described slightly in section 4.4.2.

alloy after three-fold HPT operation (a) and corresponding TEM microstructure (b)

Fig. 23. Courses of AE events rate and force versus time during compression of Mg10Li alloy after application of three-fold HPT rotations (a) and corresponding TEM microstructure (b)

5

10

15

20

25 30

Force [kN]

0 500 1000 1500 2000 t [s]

0 500 1000 1500 t [s]

**4.4 AE and the Portevin–Le Châtelier effects in Al alloys** 

homogenous deformation are typical for the PL phenomenon.

(a) (b)

(a) (b)

Force [kN]

AE events rate [s-1]

AE events rate [s-1]

0

0

10

20

30

40

50

60

10 20

30

40

50 60

The phenomenon of PL effect anisotropy was observed for the first time in works (Mizera & Kurzydłowski, 2001; Pawełek et al., 1998). The present research was carried out in order to confirm the anisotropy of the both AE and PL phenomena as well as to study the possibility of the occurrence of PL and/or AE effects also in materials processed with intensive deformation techniques (Pawełek et al., 2007, 2009).

**Al alloys of AA5754 type.** The examinations of PL and AE effects were performed in fact for 5 orientations of samples cut out at angles β=0°, 22.5°, 45°, 67.5° and 90° with respect to the rolling direction. Fig. 25, shows the AE rate and courses of external force during the tensile tests only for three samples of Al alloys of AA5754 type.


Table 1. The total sum of AE events in Al AA5754 alloy in dependence on cut out angle β

Moreover, when analyzing the plots in Fig. 25a-c, it can be found, that anisotropy of AE in AA5754 alloy is connected with the maximum quantities Σc (about 8000), which occur for cut out angles β=45° whereas the minimum of Σc (about 2500) is for β=67.5°. It is illustrated in Table 1, where maximum Σc (red color) and minimum ones (blue) are given.

**Al alloys of AA5182 type.** Cold rolled sheets of Al AA5182 alloy were the subject of plastic deformation anisotropy analysis connected with the PL effect. The samples were cut out of the rolled sheet along the rolling direction (RD), transverse direction (TD) and at angle 45° between them. The investigated sheets were subjected to uniaxial tension at ambient temperature using a static QTEST testing machine at constant strain rate 5.3x10-4s-1 to the moment of their failure. In Fig. 26, the corresponding collection of intensity of AE signal counts recorded during the tensile test are showed in the form of histogram.

(a)

Mechanical Behavior and Plastic Instabilities of Compressed Al Metals

<sup>0</sup> <sup>10</sup>

53

0

50

100

150

200

the next section.

**Hits**

and Alloys Investigated with Intensive Strain and Acoustic Emission Methods 289

microstructure of materials. As it was suggested in previous works (Pawełek et al., 1998) the different distribution of grain orientations, i.e. the differentiation of sample textures were found to be reasons for the anisotropy of AE and PL. Generally, it means that the maximum AE, for example in the Al AA5754 sample for β=45°, is the result of the fact that the number of privileged slip systems of {111} type is greater than in the sample for other values of β, and, in consequence the number of active dislocation sources generating the AE events is greater. Moreover, the reasons for the AE generation during the effect of PL are related with the collective behavior of dislocation groups generated by the sources formerly blocked by the Cottrell atmospheres. Based on Cottrell idea, an own model of PL effect was proposed in (Pawełek, 1989). This model is presented schematically in Fig. 30 and discussed in short in

64

R D 0 10 64 27 15 45 53 133 0 147 62 T D 170 182 162 167 115

(a)

0

12345

**Li b i ó**

133

Fig. 26. Intensities of AE signals during tensile test in the form of histogram

<sup>170</sup> <sup>182</sup>

<sup>27</sup> <sup>15</sup>

62

115

147

162 167

Fig. 25. Anisotropy of AE and PL effects in tensile test of Al AA5754 alloy. AE and external force for individual cut out angles: (a) – β=0°, (b) – β=45° and (c) – β=90°

The anisotropy of PL and AE effects in these alloys resulted from the fact, that the highest number of AE signal counts were recorded during the tension of samples, which were cut out in the TD direction perpendicular to the rolling direction RD.

The correlations of amplitude of AE signals with the tensile curves of samples in the rolling direction, transverse direction and inclined 45° to them are shown in Fig. 27. The analysis of the results showed that during plastic deformation of the Al AA5182 alloy the AE intensity bound with the motion of dislocation occurs at a defined level of load dependant on

0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 **time [s]**

(b)

0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 **time [s]**

(c) Fig. 25. Anisotropy of AE and PL effects in tensile test of Al AA5754 alloy. AE and external

The anisotropy of PL and AE effects in these alloys resulted from the fact, that the highest number of AE signal counts were recorded during the tension of samples, which were cut

The correlations of amplitude of AE signals with the tensile curves of samples in the rolling direction, transverse direction and inclined 45° to them are shown in Fig. 27. The analysis of the results showed that during plastic deformation of the Al AA5182 alloy the AE intensity bound with the motion of dislocation occurs at a defined level of load dependant on

force for individual cut out angles: (a) – β=0°, (b) – β=45° and (c) – β=90°

out in the TD direction perpendicular to the rolling direction RD.

**Force [N]**

**Force [N]**

**AE Events [1/s]**

**AE Events [1/s]**

microstructure of materials. As it was suggested in previous works (Pawełek et al., 1998) the different distribution of grain orientations, i.e. the differentiation of sample textures were found to be reasons for the anisotropy of AE and PL. Generally, it means that the maximum AE, for example in the Al AA5754 sample for β=45°, is the result of the fact that the number of privileged slip systems of {111} type is greater than in the sample for other values of β, and, in consequence the number of active dislocation sources generating the AE events is greater. Moreover, the reasons for the AE generation during the effect of PL are related with the collective behavior of dislocation groups generated by the sources formerly blocked by the Cottrell atmospheres. Based on Cottrell idea, an own model of PL effect was proposed in (Pawełek, 1989). This model is presented schematically in Fig. 30 and discussed in short in the next section.

**Li b i ó** Fig. 26. Intensities of AE signals during tensile test in the form of histogram

(a)

Mechanical Behavior and Plastic Instabilities of Compressed Al Metals

show the tendency to disappear.

0

0

range was considerably lower – most often below 8 kHz.

5

10

**EA events**

15

10

20

**EA events [1/s]**

30

40

and Alloys Investigated with Intensive Strain and Acoustic Emission Methods 291

after ARB process are presented in Fig. 28. It is shown that in the case of not pre-deformed alloy (Fig. 28a) more essential correlations between the AE and the PL effects appear than in the case of alloy, pre-deformed with the ARB method (Fig. 28b). The behavior of force and AE during tension of AA5251 alloy obtained after n=6 passes of ARB. It can be seen that the correlations between the PL and AE effects continue to occur: local drops of force, characteristic for the PL effect correspond to the peaks of the rate of AE events. However, both the activity and the intensity of AE as well as the values of the local drops of force are no longer so distinct as in the case of not pre-deformed samples. Thus, it can be said that both the PL and the AE effects in samples of more refined grain size (UFG, nanocrystalline)

0 400 800 1200 **time [s]**

(a)

0 100 200 300 400 500 **time [s]**

(b) Fig. 28. Correlations between the AE behavior and the course of force during the PL effect in a tensile test of AA5251 alloy before (a) and after (b) the application of ARB operation

The application of modern software enabled the spectral analysis of AE signals in the preparation of acoustic maps (acoustograms). Fig. 29 shows, by the way of example, such an acoustogram for a sample after n=6 passes by the ARB method. It should be especially noticed that it is clearly shown here that the correlations between the PL and AE effects occur in the frequency range of AE signals above 17kHz (except the line at about 360s), which seems to be a very characteristic, never noticed earlier, feature of the PL effect. In all cases where AE and PL effects were examined in not pre-deformed state, this frequency

0

0

50

100

150

**force [daN]**

200

250

50

100

**force [daN]**

150

200

Fig. 27. Correlations of amplitude of AE signals with the tension curve of samples in the rolling direction (a); transverse direction (b) and inclined 45° to them (c)

### **4.4.2 AE and PL effects in Al AA5251 alloys processed with the ARB method**

The results of the first investigations of the relations between the mechanical properties, PL effect and the AE signals generated in a tensile test of Al alloys of AA5251 type before and

(b)

(c)

The results of the first investigations of the relations between the mechanical properties, PL effect and the AE signals generated in a tensile test of Al alloys of AA5251 type before and

Fig. 27. Correlations of amplitude of AE signals with the tension curve of samples in the

**4.4.2 AE and PL effects in Al AA5251 alloys processed with the ARB method** 

rolling direction (a); transverse direction (b) and inclined 45° to them (c)

after ARB process are presented in Fig. 28. It is shown that in the case of not pre-deformed alloy (Fig. 28a) more essential correlations between the AE and the PL effects appear than in the case of alloy, pre-deformed with the ARB method (Fig. 28b). The behavior of force and AE during tension of AA5251 alloy obtained after n=6 passes of ARB. It can be seen that the correlations between the PL and AE effects continue to occur: local drops of force, characteristic for the PL effect correspond to the peaks of the rate of AE events. However, both the activity and the intensity of AE as well as the values of the local drops of force are no longer so distinct as in the case of not pre-deformed samples. Thus, it can be said that both the PL and the AE effects in samples of more refined grain size (UFG, nanocrystalline) show the tendency to disappear.

Fig. 28. Correlations between the AE behavior and the course of force during the PL effect in a tensile test of AA5251 alloy before (a) and after (b) the application of ARB operation

The application of modern software enabled the spectral analysis of AE signals in the preparation of acoustic maps (acoustograms). Fig. 29 shows, by the way of example, such an acoustogram for a sample after n=6 passes by the ARB method. It should be especially noticed that it is clearly shown here that the correlations between the PL and AE effects occur in the frequency range of AE signals above 17kHz (except the line at about 360s), which seems to be a very characteristic, never noticed earlier, feature of the PL effect. In all cases where AE and PL effects were examined in not pre-deformed state, this frequency range was considerably lower – most often below 8 kHz.

Mechanical Behavior and Plastic Instabilities of Compressed Al Metals

A

B

(b) (c)

LOW DD

SS

<sup>S</sup> <sup>S</sup>

S

S

STRESS

K is the coefficient of rigidity of the system of machine–sample.

CONCENTRATION

(b) localization and nucleation of a band and (c) propagation of a slip band

(a)

S

S

Jaworski, 1988; Pawełek et al., 2001).

τ

and Alloys Investigated with Intensive Strain and Acoustic Emission Methods 293

D

S S

S

S

S

<sup>S</sup> <sup>S</sup>

S

S

S

C

S S

LOCKED SOURCES

Fig. 30. Simple dislocation-dynamic (DD) model of the PL effect: (a) jump-like drop of force,

In accordance with a simple dislocation-dynamic (DD) model of the PL effect (Pawełek, 1989), each local drop of the external force on the work-hardening curve (Fig. 30a) is connected with unlocking of the dislocation sources in a certain localized area of the sample. The consequence is the formation of a slip band (Fig. 30b) which continues to propagate (Fig. 30c) until the waiting time tw reaches again the value of the aging time ta. The strain rate in the slip band dε is greater than the rate of the homogeneous strain ε due the high DD. Accordingly, on force-time curve a local drop must occur (Fig. 30a), since, according to the known equation of Penning: K-1dσ/dt+ <sup>d</sup>ε = ε there occurs relation dσ/dt<0 for dε > ε ;

The discussion was carried out in respect with collective properties of motion of many dislocation groups as well as the internal and surface synchronized annihilation of dislocations. The dislocation model of AE event generation was the starting point, which was based on soliton properties of dislocations (Pawełek, 1985, 1987, 1988a,b; Pawełek &

HIGH DD

t > t w a t

Most of the models of PL effect (e.g. van den Beukel, 1980; Král & Lukáč, 1997; Onodera et al., 1997) are of phenomenological character and none of them explain clearly the physical mechanisms of the formation and propagation of the related deformation bands and which would be coherent with the models of the sources of AE. The presented results are briefly discussed below in the context of the dislocation models of the PL effect reported in literature (e.g. Pawełek, 1989; Pascual, 1974) and the theoretical concepts concerning the source of AE generation during plastic deformation of metals (e.g. Kosevich, 1979; Natsik & Burkhanov, 1972; Natsik & Chishko, 1972, 1975; Pawełek, 1988a; Pawełek et al., 2001).

Fig. 29. Acoustic map of AE signals generated during PL effect in tensile test of AA5251 alloy after six repetitions of ARB operation

Most of the models of PL effect (e.g. van den Beukel, 1980; Král & Lukáč, 1997; Onodera et al., 1997) are of phenomenological character and none of them explain clearly the physical mechanisms of the formation and propagation of the related deformation bands and which would be coherent with the models of the sources of AE. The presented results are briefly discussed below in the context of the dislocation models of the PL effect reported in literature (e.g. Pawełek, 1989; Pascual, 1974) and the theoretical concepts concerning the source of AE generation during plastic deformation of metals (e.g. Kosevich, 1979; Natsik & Burkhanov, 1972; Natsik & Chishko, 1972, 1975; Pawełek, 1988a; Pawełek et al., 2001).

Fig. 29. Acoustic map of AE signals generated during PL effect in tensile test of AA5251

alloy after six repetitions of ARB operation

Fig. 30. Simple dislocation-dynamic (DD) model of the PL effect: (a) jump-like drop of force, (b) localization and nucleation of a band and (c) propagation of a slip band

In accordance with a simple dislocation-dynamic (DD) model of the PL effect (Pawełek, 1989), each local drop of the external force on the work-hardening curve (Fig. 30a) is connected with unlocking of the dislocation sources in a certain localized area of the sample. The consequence is the formation of a slip band (Fig. 30b) which continues to propagate (Fig. 30c) until the waiting time tw reaches again the value of the aging time ta. The strain rate in the slip band dε is greater than the rate of the homogeneous strain ε due the high DD. Accordingly, on force-time curve a local drop must occur (Fig. 30a), since, according to the known equation of Penning: K-1dσ/dt+ <sup>d</sup>ε = ε there occurs relation dσ/dt<0 for dε > ε ; K is the coefficient of rigidity of the system of machine–sample.

The discussion was carried out in respect with collective properties of motion of many dislocation groups as well as the internal and surface synchronized annihilation of dislocations. The dislocation model of AE event generation was the starting point, which was based on soliton properties of dislocations (Pawełek, 1985, 1987, 1988a,b; Pawełek & Jaworski, 1988; Pawełek et al., 2001).

Mechanical Behavior and Plastic Instabilities of Compressed Al Metals

AE events and maximum abrupt drops of external force.

contribution to the presented paper and valuable discussion.

Vol.90, No.8, pp.581-587, ISSN 0044-3093.

*B*, Vol.55, pp.477- 482, ISSN 0370-1972.

dynamic model of the PL phenomenon.

dislocations.

forward.

8kHz.

**7. References** 

0921-5093.

**6. Acknowledgement** 

and Alloys Investigated with Intensive Strain and Acoustic Emission Methods 295

• The correlations between the AE and the mechanisms of deformation may be considered in terms of collective synchronized acceleration and annihilation of many

• A hypothesis, that the decrease of AE in alloys compressed after intensive strain is due to strengthening processes and beginning of slip along grain boundaries was put

• The anisotropy of AE and PL effects is bound with the maximum value of total sum of

• Correlations between the PL and AE effects occur in the frequency range above 17kHz, whereas in metals, not generating the PL effect, they occur in a lower range – below

The studies were financially supported by the research projects of the Polish Ministry of Science and Higher Education No N N507 598038 and No N507 056 31/128 as well as by the

I would like to thank also my friends and coworkers: dr Andrzej Piątkowski, prof. Zbigniew Ranachowski, dr Stanislav Kúdela, prof. Henryk Paul and prof. Zdzisłav Jasieński, for their

Beukel, van den, A. (1975). Theory of the Effect of Dynamic Strain Aging on Mechanical Properties. *Physica Status Solidi A*, Vol.30, pp.197-206, ISSN 0031-8965. Bidlingmaier, T.; Wanner, A.; Dehm, G. & Clemens H. (1999). Acoustic Emission during

Boiko, V.S. (1973). Dislocation Description of Twin Dynamic Behavior. *Physica Status Solidi* 

Boiko, V.S.; Garber, R.I.; Krivenko, L.F. & Krivulya, S.S. (1973). *Fiz. tverd. Tela,* Vol.15, p.321.

Boiko, V.S.; Garber, R.I.; Kivshik, V.F. & Krivenko L.F. (1975). *Fiz. tverd. Tela,* Vol.17, p.1541. Caceres, C.H. & Bertorello H.R. (1983). Acoustic emission during non-homogeneous flow in

El-Danaf, E.; Kalidindi, S.R. & Doherty, R.D. (1999). Influence of Grain Size and Stacking-

*Transactions A,* Vol.30, (May 1999), pp.1223-1233, ISSN 1073-5623.

Cottrell, A.H. (1958). Dislocations and Plastic Flow in Crystals, Oxford University Press. Chmelík, F.; Trojanowá, Z.; Převorovský, Z. & Lukáč P. (1993). The Portevin-Le Châtelier

Boiko, V.S.; Garber, R.I. & Krivenko, L.F. (1974). *Fiz. tverd. Tela,* Vol.16, p.1233.

Al Mg alloys. *Scripta Metallurgica,* Vol.17, No.9, pp.1115-1120.

Room Temperature Deformation of a γ-TiAl Based Alloy. *Zeitschrift für Metallkunde,* 

effect in Al-2.92%Mg-0.38%Mn alloy and linear location of acoustic emission. *Materials Science & Engineering A,* Vol.164, Nos.1-2, (May 1993), pp.260-265, ISSN

Fault Energy on Deformation Twinning in Fcc Metals. *Metallurgical and Materials* 

research project of the Polish Committee for Scientific Research No 3 T08A 032 28.

• The PL and AE effects in alloy after ARB treatment reveal the tendency to disappear. • The relation of AE and PL effects is in good accordance with a simplified dislocation-

It was suggested, that the AE increases accompanying the local flow peaks were bound to highly dynamic sources of overstress, which acted in the state of strong overstress (Pawełek et al., 1985) due to an abrupt breakaway from the Cottrell atmospheres. It is thus very probable, that in this case, apart from the annihilation of dislocations, the contribution of the enhancement of dislocation dynamics, which, due to the effect of overstress of the dislocation sources (e.g. Frank-Read (FR) type), may be significantly higher that in the case of usual action of FR sources. Also the results of one of the latest works on the PL and AE effects (Chmelík et al., 1993) do confirm that the dominating factors binding both AE and PL phenomena result from the multiplication of dislocations during the action of FR sources and the breakaway of dislocations from the Cottrell atmospheres.

Simultaneously with the above process there takes place the generation of AE events both due to the acceleration as well as annihilation of dislocations. Dislocations generated from the FR sources may attain very great accelerations resulting from the interactions of the dislocation-dislocation type. However, there are more premises (Boiko et al., 1973, 1974, 1975) maintain that the contribution to AE signals due to annihilation is considerably higher than that resulting from acceleration. These authors carried out the calculations showing that the expression for AE included three terms related to the dislocation annihilation, the rate of dislocation generation and to the dislocation acceleration. However, at the same time the two last terms are always considerably less important than the dislocation annihilation term. Moreover, the contribution from the annihilation of the dislocation segments when the dislocation loops are bearing off from the FR source is intensified and dominated by the processes of the surface annihilation of dislocations, such as it takes place e.g. in the case of the formation of dislocation steps on the sample surface due to the formation of slip lines and slip bands or the shear microbands. This observation is in accordance with the results obtained in another work (Merson et al., 1997), in which the strong influence of the surface on the AE generated due to plastic deformation of metals was clearly demonstrated. Moreover, the anisotropy occurrence of PL and AE effects was confirmed on the example of AA5754 and AA5182 type of Al alloys. The tendency to the decrease of intensity of the AE and PL effects in UFG (nanocrystalline) Al alloy of AA5251 type was observed (Pawełek, 2009) for the first time.
