**4. AE in polycrystalline Al alloys deformed before and after intensive strain operations**

### **4.1 AE in AA6060 and AA2014 alloys compressed before and after using the ECAP method**

The measurement of AE were carried out for Al alloys of AA6060 and AA2014 type subjected to compression in a channel-die after the ECAP processing in a channel of circular cross-section. The AE behavior and the courses of compressive force of Al alloy of AA6060

Mechanical Behavior and Plastic Instabilities of Compressed Al Metals

sections observed after 4-fold ECAP operation in the circular channel.

(a) (b)

0

100

200

300

**AE events [1/s]**

400

500

600

Fig. 18. TEM microstructure of Al alloy of AA6060 type after 4-fold processing in the ECAP

0 50 100 150 200 250 300 350 400 **time [s]**

(a)

0

500

1000

1500

**force [N]**

2000

2500

angular channel of circular cross section: (a) - horizontal section, (b) - cross section

the AA6060 alloy, compressed also after 2-fold ECAP operation.

and Alloys Investigated with Intensive Strain and Acoustic Emission Methods 281

tendency of AE to decrease reported earlier (Kúdela et al., 2011; Kuśnierz et al., 2008) in the samples of Mg-Li and Mg-Li-Al alloys compressed after processing with intensive deformation, and for comparison, presented here in the section 4.3 of this chapter. The next Figs. 18a and 18b show a TEM microstructure of the AA6060 alloy on horizontal and cross

The results of AE examinations of the Al AA2014 alloy during compression tests are shown in Fig. 19 in which the courses of AE and external force in the sample after 2-fold processing in the angular circular channel ECAP (Fig. 19a) as well as the corresponding TEM microstructure (Fig. 19b) allow the conclusion, that the average level of AE is lower than in

type subjected to compression tests after 2- and 4-fold processing in the ECAP circular channel are presented in Figs. 17a and 17b, respectively, in which a significant decrease of AE level measured with the RMS parameter was observed. The observation confirms the

Fig. 17. AE and compressive force in Al alloy of AA6060 type subjected to tests of compression after two-fold (a) and four-fold (b) processing in the ECAP angular channel of circular cross section

type subjected to compression tests after 2- and 4-fold processing in the ECAP circular channel are presented in Figs. 17a and 17b, respectively, in which a significant decrease of AE level measured with the RMS parameter was observed. The observation confirms the

> 1 801 1601 2401 time [s]

> > (a)

1 801 1601 24 01 time [s]

(b)

compression after two-fold (a) and four-fold (b) processing in the ECAP angular channel of

Fig. 17. AE and compressive force in Al alloy of AA6060 type subjected to tests of

0

20

80

140

RMSEA [mV]

200

1000

2000

RMS EA [mV]

3000

4000

0

0.E+00

circular cross section

2.E+04

3.E+04

force [N]

5.E+04

20000

force [N]

40000

60000

tendency of AE to decrease reported earlier (Kúdela et al., 2011; Kuśnierz et al., 2008) in the samples of Mg-Li and Mg-Li-Al alloys compressed after processing with intensive deformation, and for comparison, presented here in the section 4.3 of this chapter. The next Figs. 18a and 18b show a TEM microstructure of the AA6060 alloy on horizontal and cross sections observed after 4-fold ECAP operation in the circular channel.

The results of AE examinations of the Al AA2014 alloy during compression tests are shown in Fig. 19 in which the courses of AE and external force in the sample after 2-fold processing in the angular circular channel ECAP (Fig. 19a) as well as the corresponding TEM microstructure (Fig. 19b) allow the conclusion, that the average level of AE is lower than in the AA6060 alloy, compressed also after 2-fold ECAP operation.

(a) (b)

Fig. 18. TEM microstructure of Al alloy of AA6060 type after 4-fold processing in the ECAP angular channel of circular cross section: (a) - horizontal section, (b) - cross section

Mechanical Behavior and Plastic Instabilities of Compressed Al Metals

0

0

20000

40000

force [N]

60000

20000

40000

force [N]

60000

and Alloys Investigated with Intensive Strain and Acoustic Emission Methods 283

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200

time [s]

(a)

0 400 800 time [s] 1200 1600 2000

(b)

Fig. 20. AE and compressive force in dependence on time in Al alloy of AA6060 type

subjected to tests of compression after one (a) and two (b) HPT processes

0

0

10

20

30

40

50

AE events [1/s]

60

70

80

90

100

10

20

30

40

**AE events [1/s]**

50

60

70

80

Fig. 19. Courses of AE and compressive force (a) in AA2014 Al alloy compressed after 2-fold ECAP processing of circular cross section and its corresponding TEM microstructure (b)

(b)

### **4.2 AE in Al AA6060 and AA2014 alloys compressed before and after HPT treatment**

The AE and compressive force courses were examined on the Al AA6060 alloys subjected to compression tests before the HPT operation, after one and two HPT operations (Figs. 20a and 20b, respectively). It is visible, that the AE distinctly decreases after two-fold HPT processing compared with the AE level after one HTP operation. However, from Fig. 21, an evident decrease of the AE rate in the sample already after one HPT rotation (Fig. 21b) compared with the initial state of AA2014 alloy sample (Fig. 21a) can be observed.

#### **4.3 Comparison to Mg-Li and Mg-Li-Al alloys**

Fig. 22 presents one of more important results obtained so far on the Mg8Li alloy compressed before and after the application of ECAP technique in squared cross section channel (Kuśnierz et al., 2008). These results are even more pronounced in the case of Mg10Li and Mg10Li5Al alloys compressed after the application of HPT technique (Kúdela et al., 2011). The result of the applied HPT process, is presented in Fig. 23 for the Mg10Li alloy after a three-fold, whereas the relation of AE activity versus compression force for the Mg10Li5Al alloy is shown in Fig. 24 also after a three-fold HPT treatment.

The decrease of AE activity of compressed samples subjected to the both ECAP and HPT processes is closely connected with the size of grains. The initial microstructure of Mg8Li alloys (Fig. 22a) consists of size grains of order of several hundreds of micrometers (102μm), whereas the microstructure after a large plastic deformation by ECAP is visible in Fig. 22b. After the ECAP and/or HPT processes, applying only a few rotations, the grain size decreased three orders of magnitude to hundreds of nanometers (10-1μm, Fig. 22b, 23b and 24b). These microstructures of Mg-Li and Mg-Li-Al alloys, presented in are the most pronounced examples of the refining effect of intensive strain processes arriving at UFG and/or nanocrystalline structure.

On the other hand the decrease of the AE level in Mg10Li and Mg10Li5Al (Figs. 23a and 24a) is of about two order of magnitude, as in the case before HPT operation, not presented here, since it is about 104/s, similarly as in the case of Mg8Li alloys (Fig. 22a).

The similar statement we can refer to the microstructures of Al alloys of AA6060 and AA2014 type, presented in Figs. 18 and 19, respectively. The phenomenon of the decrease of

(b) Fig. 19. Courses of AE and compressive force (a) in AA2014 Al alloy compressed after 2-fold ECAP processing of circular cross section and its corresponding TEM microstructure (b)

**4.2 AE in Al AA6060 and AA2014 alloys compressed before and after HPT treatment**  The AE and compressive force courses were examined on the Al AA6060 alloys subjected to compression tests before the HPT operation, after one and two HPT operations (Figs. 20a and 20b, respectively). It is visible, that the AE distinctly decreases after two-fold HPT processing compared with the AE level after one HTP operation. However, from Fig. 21, an evident decrease of the AE rate in the sample already after one HPT rotation (Fig. 21b)

Fig. 22 presents one of more important results obtained so far on the Mg8Li alloy compressed before and after the application of ECAP technique in squared cross section channel (Kuśnierz et al., 2008). These results are even more pronounced in the case of Mg10Li and Mg10Li5Al alloys compressed after the application of HPT technique (Kúdela et al., 2011). The result of the applied HPT process, is presented in Fig. 23 for the Mg10Li alloy after a three-fold, whereas the relation of AE activity versus compression force for the

The decrease of AE activity of compressed samples subjected to the both ECAP and HPT processes is closely connected with the size of grains. The initial microstructure of Mg8Li alloys (Fig. 22a) consists of size grains of order of several hundreds of micrometers (102μm), whereas the microstructure after a large plastic deformation by ECAP is visible in Fig. 22b. After the ECAP and/or HPT processes, applying only a few rotations, the grain size decreased three orders of magnitude to hundreds of nanometers (10-1μm, Fig. 22b, 23b and 24b). These microstructures of Mg-Li and Mg-Li-Al alloys, presented in are the most pronounced examples of the refining effect of intensive strain processes arriving at UFG

On the other hand the decrease of the AE level in Mg10Li and Mg10Li5Al (Figs. 23a and 24a) is of about two order of magnitude, as in the case before HPT operation, not presented here,

The similar statement we can refer to the microstructures of Al alloys of AA6060 and AA2014 type, presented in Figs. 18 and 19, respectively. The phenomenon of the decrease of

compared with the initial state of AA2014 alloy sample (Fig. 21a) can be observed.

Mg10Li5Al alloy is shown in Fig. 24 also after a three-fold HPT treatment.

since it is about 104/s, similarly as in the case of Mg8Li alloys (Fig. 22a).

**4.3 Comparison to Mg-Li and Mg-Li-Al alloys** 

0.5μm

and/or nanocrystalline structure.

Fig. 20. AE and compressive force in dependence on time in Al alloy of AA6060 type subjected to tests of compression after one (a) and two (b) HPT processes

Mechanical Behavior and Plastic Instabilities of Compressed Al Metals

annihilation of many dislocations.

Mg8Li

F

0 2000 4000 TIME, s

0

AE

1E+5

AE EVENTS RATE, dNz/dt [1/4s]

2E+5

and Alloys Investigated with Intensive Strain and Acoustic Emission Methods 285

intensity and activity of AE in the materials subjected to intensive strain processing, may be explained here based on the consideration of two vital processes. The first one is connected with the strengthening mechanism resulting from the intensive deformation, because a significant growth of dislocation density compared with the initial state takes place after the processing. In this way a collective motion of dislocations generated during the compression is strongly limited due to intensive interaction of mobile dislocations, e.g. with the forest dislocations or precipitate particles and solute atoms. Another process is bound with the tendency to the growth of plasticity (or even superplasticity) in intensively deformed materials. The contribution in the AE decrease after the intensive processing occurs, when on the expense of typical dislocation slips along the favored planes of the crystalline lattice within individual grains, the start of the grain boundary slips begins, which is probably less acoustically effective compared with the effective mechanism of collective and synchronized

0

**600μm 300μm** 

Fig. 22. AE and external force in two-phase Mg8Li alloys before (a) and after (b) four-fold

(a) (b)

ECAP processing. At the bottom the corresponding optical microstructures

20

FORCE, kN

40

Fig. 21. Behavior of AE and compressive force in dependence on time in Al alloy of AA2014 type subjected to tests of compression before (a) and after one HPT process (b)

1 401 801 time [s] 1201 1601

(a)

1 401 801 1201 1601 2001 2401 2801 **time [s]**

(b) Fig. 21. Behavior of AE and compressive force in dependence on time in Al alloy of AA2014

type subjected to tests of compression before (a) and after one HPT process (b)

0

10000

0

10000

20000

**force [N]**

30000

20000

30000

force [N]

40000

50000

0

0

500

1000

1500

**AE events [1/s]**

2000

2500

3000

2000

4000

AE events [1/s]

6000

8000

intensity and activity of AE in the materials subjected to intensive strain processing, may be explained here based on the consideration of two vital processes. The first one is connected with the strengthening mechanism resulting from the intensive deformation, because a significant growth of dislocation density compared with the initial state takes place after the processing. In this way a collective motion of dislocations generated during the compression is strongly limited due to intensive interaction of mobile dislocations, e.g. with the forest dislocations or precipitate particles and solute atoms. Another process is bound with the tendency to the growth of plasticity (or even superplasticity) in intensively deformed materials. The contribution in the AE decrease after the intensive processing occurs, when on the expense of typical dislocation slips along the favored planes of the crystalline lattice within individual grains, the start of the grain boundary slips begins, which is probably less acoustically effective compared with the effective mechanism of collective and synchronized annihilation of many dislocations.

Fig. 22. AE and external force in two-phase Mg8Li alloys before (a) and after (b) four-fold ECAP processing. At the bottom the corresponding optical microstructures

Mechanical Behavior and Plastic Instabilities of Compressed Al Metals

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

deformation techniques (Pawełek et al., 2007, 2009).

tests only for three samples of Al alloys of AA5754 type.

orientation

**AE Events [1/s]**

total number of events

and Alloys Investigated with Intensive Strain and Acoustic Emission Methods 287

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

**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

(cut out angle *β)* 0° 22.5° 45° 67.5° 90°

ΣC for AA5754 alloy 3400 3500 8020 2520 4500 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

**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

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

(a)

**Force [N]**

in Table 1, where maximum Σc (red color) and minimum ones (blue) are given.

counts recorded during the tensile test are showed in the form of histogram.

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)

Fig. 24. Courses of AE events rate and force versus time during compression of Mg10Li5Al alloy after three-fold HPT operation (a) and corresponding TEM microstructure (b)

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

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 inhomogenous deformation are typical for the PL phenomenon.

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