**6. Acknowledgement**

294 Recent Trends in Processing and Degradation of Aluminium Alloys

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

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,

The more important results obtained in this chapter may be formulated in the form of the

• The AE method shows, that low temperature transition of the twinning → shear bands

• The formation of an individual twin lamella, a micro shear band and step on the surface is related with the generation of AE peak due to surface annihilation of 104÷106

• The intensity and activity of AE in alloys subjected to compression tests after processing with the methods of intensive deformation (HPT, ECAP and ARB) distinctly fell in

• The decrease of AE in intensively processed alloys is connected with a significant increase of refinement of microstructure and the tendency to the plasticity growth.

observed in Cu and Ag single crystals is also observed in Al single crystals.

and the breakaway of dislocations from the Cottrell atmospheres.

2009) for the first time.

following, more detailed conclusions:

respect to the unprocessed alloys.

**5. Conclusions** 

dislocations.

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

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 contribution to the presented paper and valuable discussion.
