Temperature Dependence of the Dynamics of Portevin-Le Chatelier Effect in Al-2.5%Mg alloy

Tensile tests were carried out by deforming polycrystalline samples of Al-2.5%Mg alloy at four different temperatures in an intermediate strain rate regime of 2x10-4s-1 to 2x10-3s-1. The Portevin-Le Chatelier (PLC) effect was observed throughout the strain rate and temperature region. The mean cumulative stress drop magnitude and the mean reloading time exhibit an increasing trend with temperature which is attributed to the enhanced solute diffusion at higher temperature. The observed stress-time series data were analyzed using the nonlinear dynamical methods. From the analyses, we could establish the presence of deterministic chaos in the PLC effect throughout the temperature regime. The dynamics goes to higher dimension at a sufficiently high temperature of 425K but the complexity of the dynamics is not affected by the temperature.Comment: 18 pages, 8 figures; accepted in Met. Mater. Trans.

BALL MILLING AMORPHIZATION IN A VIBRATING FRAME GRINDER

On propose un modèle simple du transfert d'énergie par chocs au matériau en cours de broyage dans un dispositif à plateau vibrant : l'énergie par choc varie comme la masse de la bille et le carré de l'amplitude et de la fréquence de vibration du plateau. Le tout est réduit d'un facteur d'efficacité du transfert qui dépend, entre autres, de la rigidité relative de la bille par rapport au matériau traité . La fréquence des collisions entre la bille et le matériau varie comme 1'amplitude et la fréquence de vibration du plateau et comme l'inverse de la gravité terrestre. Pour tester l'influence de ces paramètres sur la stabilisation par broyage de phases métastables, un dispositif original a été construit, qui permet entre autres, de procéder à des broyages prolongés sous bon vide. Les résultats préliminaires obtenus sur Ni10Zr7 montrent l'importance du facteur d'efficacité pour atteindre une amorphisation complète du matériau. Sous bon vide, il est possible d'obtenir une feuille continue de matériau amorphe.We propose a simple model of energy transfer in the course of ball milling in a vibrating frame device : the energy transferred per impact goes as the mass of the ball and the square of the amplitude and frequency of vibration of the frame. A reduction efficiency factor must be introduced, which depends on the ball rigidity with respect to that of the material being processed. The frequency of impact between the ball and the material goes as the amplitude and frequency of vibration of the frame and as the inverse of the terrestrial gravitation constant. For assessing the role of the above parameters in stabilizing metastable phases by ball milling, a new device has been built which allows to keep good vacuum during long time milling treatments. Preliminary results on Ni10Zr7 point to the crucial role of the efficiency factor in reaching full amorphization. Under good vacuum, continuous foils of amorphous material can be obtained