アルミニウム合金における高温降伏現象

Abstract

In Al-Mg and Al-Cu alloys, a new type yielding accompanied by a remarkable yield drop was observed when they were pulled above about 350°C. In order to determine the characteristics of this high temperature yielding phenomenon, several binary aluminium alloys were investigated by tensile tests at various temperatures from room temperature to 500°C and various strain rates from 4×10−4 to 4×10−2 sec−1. The results were as follows: (1) This yielding were observed only in Al-Mg and Al-Cu alloys in which the size difference between solute and solvent atoms is relatively large, but not observed in pure aluminium and such alloys as Al-Ag, Al-Li and Al-Zn in which the atomic size difference is relatively small. (2) This yielding occurred above a lower temperature with increase of the solute concentration and decrease of the strain rate. (3) Work hardening after the yield drop was practically zero. (4) The stress drop in this high temperature yielding was not so rapid as those in usual yielding phenomena, but it proceeded relatively slowly. The preyield strain in this yielding was fairly large. (5) Under the conditions where this type of yielding was observed, the flow stress was severely affected by the test temperature and the strain rate. This temperature and strain rate dependence of the flow stress can be expressed by the state equation of \dotε=A σ^n exp(-U_0/kT). In Al-5.5 at%Mg, n=2.8∼3.3 and U0=1.4 eV. In Al-2.2 at%Cu, n=4.5 and U0=1.5 eV. (7) Any heterogeneous deformation related to this yielding phenomenon was never observed, and that the flow stress was independent of the grain size.From these experimental results it was concluded that the plastic deformation under the conditions where the high temperature yielding occurs is controlled by the viscous motion of dislocations which drag the soulte atmosphere around them, and the remarkable yield drop observed is due to the increase of mobile dislocation density during plastic deformation, as proposed originally by Johnston.