**1. Introduction**

262 Recent Trends in Processing and Degradation of Aluminium Alloys

Williams, G. and H. N. McMurray (2003). "Anion-exchange inhibition of filiform corrosion

Winkelman, G. B., K. Raviprasad, et al. (2007). "Orientation relationships and lattice matching for the S phase in Al-Cu-Mg alloys." Acta Materialia 55(9): 3213-3228. Wloka, J. and S. Virtanen (2008). "Detection of nanoscale eta-MgZn2 phase dissolution from

Wu, D. Y., S. Meure, et al. (2008). "Self-healing polymeric materials: A review of recent

Yin, T., M. Z. Rong, et al. (2007). "Self-healing epoxy composites - Preparation and effect of

Zamin, M. (1981). "The Role of Mn in the Corrosion Behavior of Al-Mn Alloys." Corrosion

Zhang, W. L. and G. S. Frankel (2003). "Transitions between pitting and intergranular

Zhao, X. Y. and G. S. Frankel (2007). "Quantitative study of exfoliation corrosion: Exfoliation

Zheludkevich, M. L., I. M. Salvado, et al. (2005). "Sol-gel coatings for corrosion protection of

Zhou, X., G. E. Thompson, et al. (2003). "The influence of surface treatment on filiform

Zhou, X. L., Y., Thompson, G.E., Scamans, G.M., Skeldon, P., Hunter, J.A. (2011). "Near-

Materials Transactions A: Physical Metallurgy and Materials Science. Zin, I. M., R. L. Howard, et al. (1998). "The mode of action of chromate inhibitor in epoxy primer on galvanized steel." Progress in Organic Coatings 33(3-4): 203-210.

corrosion resistance of painted aluminium alloy sheet." Corrosion Science 45(8):

Surface Deformed Layers on Rolled Aluminium Alloys." Metallurgical and

Electrochemical and Solid State Letters 6(3): B9-B11.

developments." Progress in Polymer Science 33(5): 479-522.

corrosion in AA2024." Electrochimica Acta 48(9): 1193-1210.

metals." Journal of Materials Chemistry 15(48): 5099-5111.

of slices in humidity technique." Corrosion Science 49(2): 920-938.

Composites Science and Technology 67(2): 201-212.

Analysis 40(8): 1219-1225.

37(11): 627-632.

1767-1777.

on organic coated AA2024-T3 aluminum alloy by hydrotalcite-like pigments."

an Al-Zn-Mg-Cu alloy by electrochemical microtransients." Surface and Interface

the healant consisting of microencapsulated epoxy and latent curing agent."

The aim of the present chapter has been to substantiate and explain the relation among the behavior of acoustic emission (AE) parameters, the course of external load, evolution of microstructure and the dislocation mechanisms of slip and the localization of deformation connected with twinning, formation of slip and shear bands. The problem is of fundamental meaning, when qualitative and quantitative relations between the rate of AE events, amplitude and energy of AE signals and other AE descriptors in relation to micro-processes occurring in a material are to be discussed.

It is commonly believed that twinning is the most efficient source of acoustic emission (Bidlingmaier et al., 1999; Boiko, 1973; El-Danaf et al., 1999; Heiple & Carpenter, 1987; Tanaka & Horiuchi, 1975) due to fast release of great amount of elastic energy. It is connected with the fact that the velocity of twinning dislocations is higher than this of slip dislocations (Boiko, 1973), which results in the increase of contribution of accelerating effects in the recorded AE impulses.

One of the first AE investigations concerned the tensile test of titanium and its alloys (Tanaka & Horiuchi, 1975), in which it was established, that the AE activity in Ti was bound with twinning from the beginning, while in Ti alloys the AE impulses from twinning appeared after a high degree of deformation. During compression of the γ-TiAl alloy, AE sources were identified as generally coming from slip, twinning and the propagation of microcracks. It was reported, however, that the detailed mechanisms by which moving dislocations create elastic waves are still not fully understood (Bidlingmaier et al., 1999).

Moreover, the problem of twinning in Al has still remained controversial. It is believed quite commonly, that at least in simple uniaxial strain state, like in a tensile test, twins do not appear in Al. One of the aim of this chapter is to demonstrate, that there are numerous proofs, that in a complex strain state, which occurs in the channel-die compression of single Al crystals at temperature of liquid nitrogen the twinning processes do occur.

The reasons for undertaking such a research are numerous. At first, there is lack in literature of the experimental data on the AE behavior during channel-die compression of single

Mechanical Behavior and Plastic Instabilities of Compressed Al Metals

description of the AE method is presented in the next section 2.2.

Fig. 1. Scheme of the device for the channel-die compression test

**2.2 Acoustic emission method** 

source of the displacement,

velocity of shear wave,

and Alloys Investigated with Intensive Strain and Acoustic Emission Methods 265

compressive force and AE descriptors. A broad-band piezoelectric sensor recorded AE signals in the range from 100 kHz up to 1 MHz. The contact of the sensor with the sample was maintained with the aid of steel rail which served as a washer in the channel-die, as well as an acoustic wave-guide. In order to eliminate unwanted effects of friction against the wall of the channel, each sample was covered with a Teflon foil. The more detailed

The AE phenomenon takes place during a rapid release of elastic energy, accumulated in the material as a result of acting external or internal conversions, which can be emitted in the form of elastic waves which the frequency is contained between several kilohertz and a few megahertz. In metals and alloys, in general, they are generated as the effect of plastic deformation and particular dislocation strain mechanisms. The AE method enables sensitive monitoring effects in real time, even in considerable volume of investigated elements. Considering simplifying assumptions, that the function of amplitude of strain field changes in the AE source has a form of elementary shock, the point of observation is in a distant area, and the elastic wave propagates in a homogeneous medium, the elementary equation of the signal propagation distance given in literature (Resnikoff & Wells, 1998) takes the form:

2

*Gxt tx ttrv v r*

π ρ

> π ρ

where: *G*ij *(r',t'-t;r)* is Green function for the displacement in directions xi

delta function equal +∞, for *x*=0 and equal 0 for the remaining values *x.*

ρ

γi, γ *p*

1 1 (, ' ; ) (' / ) <sup>4</sup>

− = − −

γγ

2

*s*

1 1 ( ) (' / ) <sup>4</sup>

 δ

*v r*

− −− −

γγ

time *t*', in the case, when a local disturbance of strain field in point *r* in time *t* becomes the

and receiver-source, *r* – distance between AE source and sensor, δij – Kronecker delta, δ(*x*) –

Apart from the AE signal the apparatus registers also a noise of acoustic background and that generated during the processing of the recorded signal. In the course of its processing from the analogue form into digital one, so called *quantization noise* occurs, resulting from

 δ

*i j ij s*

*–* medium density, *v*p *–* velocity of dilatation wave, *v*s *–* 

j - for i=1, 2, 3 and j=1, 2, 3 are directional cosines source-receiver

 δ *ttrv*

' , yi ' , zi '

(1)

in point *r*', in

*i j p*

crystals. Secondly, in the contemporary materials science, one of the basic problems of plastic deformation of metals are questions of strain localization due to the formation and development of slip lines and slip bands as well as shear banding and twinning processes.

In the last decade the methods of intensive strain have become more and more widely used to obtain microstructure refinement and finally ultra fine-grained (UFG), nanocrystalline materials which have the excellent mechanical properties, such as great strength and plasticity or even superplasticity occurring in the conditions of relatively not too high temperatures (Vinogradov, 1998). They allow obtaining massive samples of metals ready for a further treatment. This refers in particular to the packet rolling with bonding, so called ARB (Accumulative Roll-Bonding) method (Saito et al., 1999; Pawełek et al., 2007). There are also known products obtained on industrial scale by the method of channel compression ECAP (Equal Channel Angular Pressing), (Kuśnierz, 2001). The method of torsion under high pressure HPT (High Pressure Torsion), (Valiev et al., 2000) has been the least known since obtaining the high pressure itself is a difficult problem.

The subject concerns the Al alloys of AA6060, AA2014 and AA5182 type as well as AA5754 and AA5251 ones. The examinations of Al alloys of AA6060 and AA2014 types were carried out applying the HPT method as well as ECAP technique with circular cross section of the channel. The anisotropy of Portevin-Le Châtelier (PL) and AE effects was described and the relation between the PL and AE effects in UFG (nanocrystalline) Al alloy after intensive strain processing was reported here for the first time.

On the other hand the results of the examinations of Mg-Li and Mg-Li-Al alloys, presented here for comparison, were carried out applying HPT method (Kúdela et al., 2011) as well as ECAP technique (Kuśnierz et al., 2008) with squared cross section of the channel.

The aim of this chapter has been also an attempt to present the correlations between the mechanisms of plastic deformation and the AE phenomenon and the discussion of the connection of AE with the possible phenomenon of superplasticity in UFG (nanocrystalline) aluminium alloys.
