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

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 equi-