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

278 Recent Trends in Processing and Degradation of Aluminium Alloys

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

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

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 twins

microshear bands

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%

optical images).

deformation is quite probable.

and (b) – reduction z≅53.4%

band.

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

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

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

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

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, 1972; Natsik & Chishko, 1972, 1975).
