THERMALLY ACTIVATED DOUBLE KINK GENERATION STUDIED BY ULTRASONIC MEASUREMENTS DURING A CYCLIC DEFORMATION

Abstract
In a new measurement technique, attenuation α and propagation time t of ultrasonic waves have been measured on an aluminium sample under a low frequency sinusoidal applied stress (1). During heating, a lot of such Ɗα(σ) and Ɗt/t(σ) curves (~300) , each measured during one cycle of the stress, have been drawn. These closed curves have some typical shapes very different from one mechanism to another controlling the mobility of the dislocations (2). Bordoni peak has been recently studied in detail by Esnouf and Stadelmann (3) by low frequency internal friction measurements and by Gotthardt in electron microscopy (4). These studies show that the double kink generation (DKG) model is the best one to explain the characteristics of this peak. In this paper a new model is presented to calculate the shape and the evolution of the Ɗα(σ) and Ɗt/t(σ) curves during the thermally activated generation of double kink induced by the low frequency applied stress. The theoretical Ɗα(σ) and Ɗt/t(σ) curves obtained in the presence of this mechanism are completely different from those due to an interaction between point defects and dislocations (5). More precisely they show a typical "assymetry" between Ɗα(σ) and Ɗt/t(σ) curves. Experimentally the Ɗα(σ) and Ɗt/t(σ) curves observed in the temperature range of the Bordoni relaxations B₁ and B₂ in aluminium are very similar to those predicted by the model.