Effect of Temperature on the Kinetics of Localized Plasticity Autowaves in Lüders Deformation

Abstract

The paper analyzes the elastoplastic transition in Fe–0.025 wt. % C at a temperature of 296–503 K and strain rate of 6.67·10−6–3.33·10−3 s−1. The analysis shows that the lower yield stress increases by a power law with increasing the strain rate, and that its rate sensitivity decreases linearly with increasing the test temperature. At temperatures lower than 393 K, the rate sensitivity of the lower yield stress is normal, and at 393–503 K, it is zero. In the range 393–503 K, the kinetics of the Lüders bands is changed from steady to discrete, and the higher the strain rate, the higher the temperature of this transition. Using the available data on the dynamics of dislocations and diffusion of interstitial impurities in the test alloy, it is demonstrated that the kinetics of Lüders bands are controlled by the effect of dynamic strain aging. If the arrest time of mobile dislocations tw at barriers which are overcome via thermal activation is comparable with the precipitation time of interstitial atoms ta at these dislocations, the motion of a Lüders band is discrete, and the band represents an excitation wave of localized plasticity; its refractory period is determined by the time of dynamic strain aging. If ta >> tw, the band moves monotonically and represents a switching autowave. The results of the analysis suggest that the effect of serrated yielding at the lower temperature boundary of blue brittleness can be suppressed by increasing the strain rate. When the arrest time of dislocations tw decreases, the comparability of tw and ta is broken, and no excitation autowave is formed. The data reported in the paper can be used to develop warm rolling technologies for materials with a sharp elastoplastic transition.

Keywords

• elastoplastic transition
• lower yield stress
• temperature
• blue brittleness
• serrated deformation
• dynamic strain aging