**3.1 Twinning and shear band formation in Al single crystals**

The examinations of AE in Al mono- and bi-crystals were aimed at the identification of unstable plastic flow mechanisms connected with the deformation twinning and the formation of shear bands. The compression test were carried out mainly at temperature of liquid nitrogen (77K). Moreover, the still controversial problem of deformation twinning in Al crystals is also considered here.

The course of AE and the external force in the Al crystal of Goss orientation {110}<001> is shown in Fig. 6a whereas Fig. 7 comprises, for the purpose of comparison, the results of AE and the course of external force for the crystal of {112}<111> orientation.

Fig. 6. Course of AE and compressive force in Al single crystal of Goss orientation (a) together with corresponding acoustogram (b)

**3. AE in Al mono- and bi-crystals compressed at liquid nitrogen temperature** 

The examinations of AE in Al mono- and bi-crystals were aimed at the identification of unstable plastic flow mechanisms connected with the deformation twinning and the formation of shear bands. The compression test were carried out mainly at temperature of liquid nitrogen (77K). Moreover, the still controversial problem of deformation twinning in

The course of AE and the external force in the Al crystal of Goss orientation {110}<001> is shown in Fig. 6a whereas Fig. 7 comprises, for the purpose of comparison, the results of AE

0 200 400 600 800 1000 1200 1400 1600 1800 2000 **time [s]**

(a)

(b)

Fig. 6. Course of AE and compressive force in Al single crystal of Goss orientation (a)

together with corresponding acoustogram (b)

**Force [N] X 12**

**3.1 Twinning and shear band formation in Al single crystals** 

and the course of external force for the crystal of {112}<111> orientation.

Al crystals is also considered here.

**AE Events [1/s]**

When analyzing Fig. 6, it can be stated, that evident correlation exists between the course of force and behavior of AE. All the local decreases of the force curve are accompanied by more or less distinct areas of elevated AE activity. It seems that these strong plastic instabilities on the compression curve correspond to the occurrence of shear bands. Vertical areas in the acoustogram containing a broad range of frequency spectrum visible in Fig. 6b.

On the other hand, low temperature courses of AE impulses together with external compressive force in dependence on time are presented in Figs. 8 and 9a and 10a for the Al single crystals of two selected orientations: {112}<111> (Fig. 8) and {531}<231> (Fig. 9a for reduction z≅27.1% and Fig. 10a for reduction z≅51.4%). Attention should be drawn to some characteristic features of the recorded courses.

Moreover, the experimental {111} pole figures (EXP) presented in Figs.9b and 10b, as well as the calculated orientation distribution functions (ODF) referred to in Figs.9c and 10c, illustrate explicitly the existence of twin orientation after compression to z≅51.4% (Fig. 10b and 10c), and suggest strongly the possibility of deformation twinning also in the Al single crystals channel-die compressed at the liquid nitrogen temperature. In {111} pole figure (Fig. 10c), the component of twin orientation ( 4 4 1) [ 1 3 8], appearing after reduction z≅51.4% (initial matrix orientation ( 1 3 5)[ 1 3 2 ], Figs. 9b and 9c) is now given by orientation ( 2 2 5)[ 3 7 4 ] (Fig. 10c) – corresponds to the twinning on the active co-planar slip system.

Fig. 7. Acoustic emission and compressive force in Al single crystal of orientation type C≡{112}<111>. The arrows indicate the correlations between AE and drops of force

Based on the dislocation dynamics and the AE model (Jasieński et al., 2010; Pawełek, 1988a; Pawełek et al., 2001; Ranachowski et al., 2006) a number of AE impulses, which were generated due to the appearance of an individual twin lamella can be estimated. It was assumed, that the twins formed as a result of the pole mechanism action. It was also accepted, that an individual AE impulse occurred, when a partial twinning dislocation, which moved in the area of a single atomic plane, approached the surface. This suggestion is in agreement with the results and concepts reported by Boiko et al. (1973, 1974, 1975) for the

Mechanical Behavior and Plastic Instabilities of Compressed Al Metals

{111} pole figure experimental EXP (b) and recalculated ODF (c)

performed using TEM and CBED and SEM-FEG/EBSD techniques.

**3.3 Development of Al bi-crystal dislocation structure** 

**AE Events [1/s]**

**3.2 AE during channel-die compression of Al bi-crystals** 

and Alloys Investigated with Intensive Strain and Acoustic Emission Methods 273

Fig. 10. Courses of AE and external force (a) of Al single crystal of initial orientation {531}<231> channel die-compressed at T=77K up to reduction z=51.4% and corresponding

The examinations of AE tests in Al bi-crystals subjected to low-temperature channel-die compression were also performed. Fig. 11 show the behavior of AE and force together with corresponding acoustogram for the Al bi-crystals of {110}<100>/{110}<011 hard/Goss orientation. After the first stage of deformation (z≅30%), the sample was trimmed and compressed again until it was reduced by z≅50%. Courses of AE and external force for a bicrystal of {110}<001>/{100}<011> Goss/shear orientation are illustrated in Fig. 12 for the first stage of deformation (z≅20%). For such an orientation also structural examinations were

**EXP ODF** 

0 500 1000 1500 2000 **time [s]**

(a)

0

2000

4000

6000

8000

10000

**Force [N]**

12000

14000

16000

18000

**MATRIX (- 2 2 5)[-3 7 - 4]** 

**TWIN (- 4 4 1)[-1 -3 8]** 

relationship between AE and elastic twins generated in calcite crystals. It was also assumed for simplification that distance *a* between the atomic planes was equal to the value of Burgers vector of dislocation i.e. *a*≅*b*≅1.0x10-4μm.

Fig. 8. Courses of AE and external force of Al single crystal of orientation {112}<111> channel-die compressed at T=77 K up to reduction z=61.6%; microstructure inserted nearby illustrates shear band

Fig. 9. Courses of AE and external force (a) of Al single crystal of initial orientation {531}<231> channel-die compressed at T=77K up to reduction z=27.1% and corresponding {111} pole figures: experimental EXP (b) and recalculated ODF (c)

The thickness of twin lamella, estimated visually from microstructure images, was in the range from 0.1μm up to 1.0x103μm, which, at magnifications of order 10x÷100x, used the most frequently, gave the thickness of a real twin lamella, at first approximation, in the range from 1 up to 100μm. Hence the number of atomic planes engaged and the number of elementary impulses completing the AE peak from an individual twin was of order 104÷106, which was in satisfactory agreement, as far as the order of value was concerned, with the value observed.

relationship between AE and elastic twins generated in calcite crystals. It was also assumed for simplification that distance *a* between the atomic planes was equal to the value of

Fig. 8. Courses of AE and external force of Al single crystal of orientation {112}<111> channel-die compressed at T=77 K up to reduction z=61.6%; microstructure inserted nearby

 **2 mm** 

**ODF EXP** 

**initial orientation** 

(a) (b) (c)

{111} pole figures: experimental EXP (b) and recalculated ODF (c)

Fig. 9. Courses of AE and external force (a) of Al single crystal of initial orientation {531}<231> channel-die compressed at T=77K up to reduction z=27.1% and corresponding

The thickness of twin lamella, estimated visually from microstructure images, was in the range from 0.1μm up to 1.0x103μm, which, at magnifications of order 10x÷100x, used the most frequently, gave the thickness of a real twin lamella, at first approximation, in the range from 1 up to 100μm. Hence the number of atomic planes engaged and the number of elementary impulses completing the AE peak from an individual twin was of order 104÷106, which was in satisfactory agreement, as far as the order of value was concerned, with the

Burgers vector of dislocation i.e. *a*≅*b*≅1.0x10-4μm.

illustrates shear band

value observed.

Fig. 10. Courses of AE and external force (a) of Al single crystal of initial orientation {531}<231> channel die-compressed at T=77K up to reduction z=51.4% and corresponding {111} pole figure experimental EXP (b) and recalculated ODF (c)

### **3.2 AE during channel-die compression of Al bi-crystals**

The examinations of AE tests in Al bi-crystals subjected to low-temperature channel-die compression were also performed. Fig. 11 show the behavior of AE and force together with corresponding acoustogram for the Al bi-crystals of {110}<100>/{110}<011 hard/Goss orientation. After the first stage of deformation (z≅30%), the sample was trimmed and compressed again until it was reduced by z≅50%. Courses of AE and external force for a bicrystal of {110}<001>/{100}<011> Goss/shear orientation are illustrated in Fig. 12 for the first stage of deformation (z≅20%). For such an orientation also structural examinations were performed using TEM and CBED and SEM-FEG/EBSD techniques.
