Mechanism of the High Temperature Yield Point Phenomenon in Some Aluminium Alloys

A new type yield point phenomenon observed in some binary aluminium alloys such as Al-Mg and Al-Cu which are deformed at high temperatures above about 350°C has been reported in a previous papar. The present paper is devoted to a detailed explanation of the mechanism which controls the high temperature yield point phenomenon. The proposed theory in this work concerns the viscous motion of dislocations dragging the Cottrell atmosphere around them and the state equaton of deformation derived theoretically explains quite well the phenomenon observed at high temperatures. In the viscous motion of a dislocation, a fairly larger stress is needed to increase the dislocation velocity. This means that a remarkable yield drop must occur due to the dislocation multiplication during the deformation. This is the fundamental idea of the proposed theory to explain the high temperature yield point phenomenon. Comparing the state equations obtained experimentally and theoretically, it is deduced that the apparent relation between strain rate and stress, in which the strain rate increases in proportion to about the third power of the stress, occurs as a result of the proportional relationship of dislocation density to the second power of the stress and the proportional dependence of the dislocation velocity to the stress. The process of the increasing dislocation density calculated from the stress-strain curves shows that the density increases very rapidly in the initial deformation stage up to about 0.1% plastic strain and then gradually increases to an equilibrium density determined by the tensile conditon. For example, the equilibrium density is 2×10⁹ cm⁻² and 7×10⁸ cm⁻², respectively, when strained at 400°C at strain rates of 3×10⁻³ sec⁻¹ and 4×10⁻⁴ sec⁻¹, and 2×10⁷ cm⁻² at 500°C when the strain rate is 4×10⁻⁴ sec⁻¹. The density saturation to an equilibrium value is attained more rapidly at a higher temperature.<BR>The stress-strain curves obtained by rapidly changing the tensile speed agree with the theoretical predictions from the strain rate dependence of the dislocation density. Further the theoretical prediction of the critical temperature, above which the viscous motion of dislocations dragging solute atmospheres controls the deformation, agrees well with the critical temperature for the yield point phenomenon determined experimentally. This shows that of the theory serves fairly well to explain the observation.