Acoustic emission and deformation bands in Al-2.5% Mg and Cu-30% Zn

Abstract

The acoustic emission (AE) generated during Luders and Portevin-Le Chatelier (PLC) deformation bands has been studied in Al-2.5% Mg and Cu-30% Zn. It has been observed that the cumulative AE detected during the Luders deformation increases at decreasing strain rates while Luders bands become shorter and are increasingly serrated, especially in AlMg. Once PLC bands develop, AE appears in a stepwise fashion. The acoustic activity concentrates mainly on the beginning of every new band, but preceding the first load drop. Once nucleated, the PLC bands propagate with relatively low AE. Some burst activity is recorded during the stress raise after the load drops, while a continuous type burst is emitted during the load drop. Total AE increases with decreasing strain rate during serrated flow, as during the Luders regime. The increase in total AE correlates very well either with the shortening of Luders bands or the increase in serration amplitude of PLC bands at decreasing imposed deformation rates. A close correspondence can be established between the phenomenology of AE and TEM and in situ HVTEM observations concerning dislocation dynamics and dislocation breeding in localized slip bands. The relatively low AE recorded at high imposed deformation rates when the bands (Luders or PLC) move smoothly can be ascribed to the multislip mechanism of coarse slip bands involved in the bands movement, which clearly may preclude the emission of large acoustic bursts. On the other hand, at low deformation rates stress relaxation and static ageing during discontinuous band movement immobilizes previously operative sources, so that activation of fresh sources become necessary in order to resume deformation. Dislocation movements in undeformed slip systems is presumably very fast so that a high level AE should be possible when fresh sources are put into operation. As a consequence, AE increases at decreasing strain rates. At increasing strain rates the available time for stress relaxation and pile up stabilization at the band front after load drops is progressively reduced. Previously operative sources can now be reactivated to a larger extent, decreasing the need for fresh sources activation. Reactivation of previously operative sources presumably occurs with low level AE due to the high dislocation density in the slip planes. In this way the strain per active plane increases with strain rate while acoustic activity decreases. The correspondence between stress amplitude of serrations and AE can be rationalized in the same terms. A complementary explanation in terms of the crystallographic microbands involved in the macro shear bands nucleation seems possible in the case of PLC bands. When the band is smooth at high strain rates the stress level at the band front is continuously high and the number of unsuccessful microbands needed to originate every localized shear band is presumably lower than at low strain rates when the stress level decreases after the load drops. AE is therefore higher at the lower strain rates than at the higher ones. Something essentially similar must occur when every new PLC band is nucleated. No stress concentrations nor mobile dislocations are available after a band traverses the entire gauge length and the deformation must be started by copious activation of new sources (microbands), with high emission preceding the first load drop. Once the band is formed the deformation mode involved reduces the acoustic activity during the band movement, so that a step-wise AE is observed.