**3.4 Comparison with other fcc single crystals**

276 Recent Trends in Processing and Degradation of Aluminium Alloys

a high-resolution scanning microscope using SEM-FEG/EBSD techniques. The development of dislocation structure observed in TEM is presented in Fig. 13 for each crystallite and for two deformation degrees. The advance of structure refinement together with the increase of deformation was observed in the crystallites with the Goss orientation {100}<011>. In crystallites of shear orientation {100}<011>, two pairs of coplanar slip systems were active what led to the decomposition of the crystal resulting in the change of initial crystallite orientation to two symmetrically situated positions of {112}<111> orientations. The areas, in

The texture-structure examinations were performed using the SEM-FEG/EBSD system at a mezzo-scale, which allows reproducing the "electron" image of structure as regards the

(a)

(b)

Fig. 14. Orientation maps (shown as a "function" of IPF colors) for the part of bi-crystal of Goss (a) and shear (b) orientation and corresponding {111} pole figure. Measurement step

RD

RD

TD

TD

which different slip systems dominated were separated by intermediate bands.

**3.3.2 SEM examinations of local texture changes** 

crystallographic orientation changes.

100nm. Reduction z≅20%

In order to document better the correlation between the AE behavior and the localization of deformation connected with twinning processes and the formation of shear bands, experiments at temperature of liquid nitrogen (77 K) were carried out on single crystals of Ag and Cu. The selected results for the Ag single crystals are presented in Fig. 15, and for copper in Fig. 16. The results refer to the same orientation {112}<111> and two subsequent

Fig. 15. Courses of AE and external force and corresponding microstructures of Ag single crystals of orientation {112}<111> channel-die compressed at T=77K: (a) – reduction z≅33% and (b) – reduction z≅63.4%

Mechanical Behavior and Plastic Instabilities of Compressed Al Metals

**3.5 Acoustic emission vs. twinning in Al crystals** 

and Alloys Investigated with Intensive Strain and Acoustic Emission Methods 279

The presented results helped to establish a scheme of the microstructure evolution and mechanisms of deformation during channel-die compression of single crystals of fcc metals. The substantial element of the model is, that in the range of **intermediate reductions** (from about 30% to 65%) in the initial stages of compression, a change of the deformation mechanism from intensive twinning resulting in high AE of big activity of AE sources into the generation and localization of primary family of shear bands takes place. In the range of **small reductions** (up to about 35%) the dislocation mechanisms of ordinary slip dominate and processes of twinning can be initiated, while in the range of **high reductions** (above about 65%) the formation of another family of shear bands begins in the secondary slip

Based on the above considerations it can be stated, that the presented results directly indicated the correlation of the following four elements: high peak of AE event rate, abrupt decrease of external force, the formation of twin lamella or the nucleation of shear band as

However, the twins were not observed neither in the Al crystals nor Al bi-crystals using the accessible methods of optical and electron microscopy. On the other hand the presented pole figures in Figs. 9 and 10 surely do not exclude the possibility of twinning. Moreover, they become a kind of proof that the process of deformation twinning in fact has occurred. It should be stressed that in this kind of discussion an argument is often raised, that the existence of twin orientations itself is not a proof that the process of deformation twinning has taken place. Similarly, the microstructures obtained using the TEM technique (Paul et al., 2001) may certify the fact that the deformation twinning occur also in single crystals of

There is also another kind of confirmation of such a statement: it is the audible effect. In many cases, during the compression tests knocks typical for twinning were heard in the frequency range audible for the human ear. Hence, it is probable, that the difficulties in the documenting the twins in microstructure images are due to very high stacking fault energy of Al. Very fast processes of recovery or even recrystallization taking place in the sample being moved from the liquid nitrogen to the ambient temperature "blurr" the possible twins formed during deformation. In general the problem has not been definitely solved so far,

The observed correlations between AE and the mechanisms of deformation can be explained in terms of highly synchronized and collective behavior of groups of many dislocations, particularly in reference to the processes of dislocation annihilation at the free surface of the sample. Moreover, the description of dislocation annihilation based on the soliton properties of dislocation (Pawełek, 1988a; Pawełek et al., 2001) is closer to the reality than the description resulting from the application of the theory of continuous media (& Burkhanov,

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

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

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

systems, not coplanar with respect to the primary systems (Pawełek et al., 1997).

well as the appearance of a step at the surface of deformed crystal.

Al, although – it should be impartially said – they are not too convincing.

although the results obtained may contribute, to some extent, to its full solution.

1972; Natsik & Chishko, 1972, 1975).

**operations** 

**method** 

stages of compression, comprising conventionally intermediate reductions in the range from about 30% to 65%. It is visible, that the behavior of AE and its correlations with external loads are qualitatively very similar. It confirms that the deformation mechanism changes from an ordinary slip through strong twinning (Fig. 15a and 16a and appropriate optical microstructures) to the mechanism of shear band formation (Fig. 15b, 16b and corresponding optical images).

The considerable drop of AE event rate is a characteristic feature of twinning → shear bands transition, while corresponding high AE peaks are distinctly correlated with abrupt drops of the external load, which is the most evidently caused by the appearance and development of individual shear bands, which belong to the same primary family. For example, the last high AE peak visible in Fig. 16a at about 2200s may originate from an already forming shear band.

Comparing the courses of force and AE and the microstructure (Fig. 8) for the Al crystal of {112}<111> orientation with respective plots and images for the Ag and Cu single crystals of the same orientation (Fig. 15 and 16, respectively) and analyzing the courses of force and AE for the {531}<231> orientation in the Al single crystals (Figs.9a and 10a) a similarity to a large extent can be noticed, which lets us state, that also in the Al single crystals compressed at low temperatures, the transition of the type twinning → shear bands after initial slip deformation is quite probable.

Fig. 16. Courses of AE and external force and corresponding microstructures of Cu single crystals of orientation {112}<111> channel-die compressed at T=77K: (a) – reduction z≅41% and (b) – reduction z≅53.4%
