The aim of this work is to show, in the case of pure aluminum, how useful information about mobile dislocations involved in plastic deformation can be obtained from ultrasonic measurements and contribute to the understanding of cyclic plasticity.
The measurements have been carried out by using the coupling technique, that is, ultrasonic waves are superimposed to a tension-compression mechanical stress, according to three experimental procedures: 1. During the cyclic plastic deformation, the attenuation evolutions are followed continuously. 2. The cyclic deformation is interrupted: at first the load is kept constant and then it is reduced. The attenuation (Δα) and velocity (ΔV/V) changes are recorded during the stress decrements. 3. The cyclic deformation is interrupted and the sample is completely unloaded: after some rest, Δα and ΔV/V are recorded during a low-amplitude characterization cycle.
The various results thus obtained are discussed on the basis of the dislocation string model in relation with basic dislocation mechanisms to which the attenuation and velocity are very sensitive.
In this presentation a special emphasis is given to: 1. The analysis of the asymmetrical character of Δα versus stress response in tension with respect to compression (Procedure 3). It is shown that the long-range internal stresses that originate from the cellular structure of dislocations induced by the cyclic deformation is responsible for this asymmetrical ultrasonic behavior. In turn, this asymmetry phenomenon appears to be a good internal stress indicator enabling us to follow the internal stress evolution obtained by changing the point (on the plastic fatigue loop) from which the sample is unloaded. 2. The discussion of the attenuation changes induced by the stress decrement of Procedure These changes result from two opposite contributions that appear to evolve all along the plastic fatigue loop: (1) the breakaway of the dislocations immobilized at obstacles and (2) the dynamic recovery process. 3. The determination of the density and the free loop length of dislocations. The dislocation density deduced from ultrasonic measurements by using the string model appears to increase rapidly during the hardening stage and then it remains quasi-constant. Moreover, from the previous results, it is concluded that this constancy arises from a dynamic equilibrium between creation and annihilation of dislocations, especially at large plastic strain amplitude.