| A type of plastic instability, the Portevin-Le Chatelier (PLC) effect accompanies repetitive yielding in stress-strain curves and a localization of deformation (known as a PLC band) is exhibited during deformation in many alloys under appropriate conditions. This phenomenon is generally explained as being a consequence of dynamic strain aging (DSA), i.e. dynamic pinning and unpinning interactions between mobile dislocations and solute atoms. However, the microscopic processes of DSA have not been confirmed by experiment. The PLC effect will make a smooth surface routh and reduce the mechanical properties, such as ductility and fatigue cycle. Therefore, the research on the PLC effect is of important theoretical significance and application value.The critical strain of the PLC effect marks the boundary between stable and unstable flow. Two series of tension tests were conducted at different temperatures at a certain strain rate and at different strain rates at room temperature. At low temperatures or at high strain rates, the normal behavior of the critical strain is observed, i.e. the critical strain decreases with increasing temperature or increases with increasing strain rate. At high temperatures or at low strain rates, the inverse behavior of the critical strain is observed, i.e. the critical strain increases with increasing temperature or decreases with increasing strain rate. By comparing the stress-strain curves at different temperatures, the lower and upper envelope curves are identified respectively. Before the critical strain, the stress follows the lower envelope curve at low temperatures, whereas it follows the upper envelope curve at high temperatures. The subsequent serrations are upward in the region of normal behavior but downward the region of inverse behavior. Two critical mechanisms are proposed for the normal and inverse behaviors according to the serration directions. In the normal behavior case, few mobile dislocations are pinned by solute atoms before the critical strain, and the critical strain is dependent on the first pinning process. In the inverse behavior case, mobile dislocations are pinned by the solute prior to escape, and the critical strain depends on the first unpinning process. The calculated results based on the two critical mechanisms are highly consistent with the experiment.Secondly, the influence of nature aging time on the PLC effect in an Al-Mg-Si alloy is studied. The serration amplitude decreases with increasing nature aging time of0-2h and then keep constant. In nature aging time of0-l h, the strength, serration period and the critical strain keep constant, the statistical aspect of serration ampltide presents half-peak distribution, and the PLC band changes from type A to type B with increasing strain, In following nature aging time, the strength, serration period and the critical strain increase with increasing nature aging time, the statistical aspect of serration ampltide presents power-law distribution, and the PLC band is always type A. The analysis in relation to the precipitation sequence and DSA shows that the serration amplitude is denpendent on the solute concentration and the other characters are influenced by the precipitation with strengthening effect.The PLC bands in a Ni-Co-based superalloy are observed with digital image correlation. The width of the PLC band is about4-4.5mm and the angle between the PLC band and tension axis is about60°. The strain is much larger within the PLC band than the average strain. The elastic shrinkage, i.e. the area with nagetive strain, is observed outside the PLC band. By using digital speckle shearography, the behavior of out-plane deformation of the PLC bands is studied. The shape of the PLC band, the continuous propagation and the intermittent fashion of the PLC bands are presented. The processes of position changing and transition of the inclined angle are also observed. |
