Effects Of Electric Pulses On The Microstructures Of Deformed Pure Aluminum And Its Mechanism

As a promising method in materials processing,electric pulse treatment can significantly alter the microstructures and improve the mechanical properties of materials due to its unique thermal,electrical,magnetic and mechanical effects.However,the underlying mechanisms for the microstructures modification by electric pulsing have not yet been fully understood,which limits the development of this technology.In this paper,the microstructure evolution in pure aluminum without solid-state phase transformation under the action of electric pulses will be directly investigated by ex-situ TEM/EBSD techniques and slip trace analysis to reveal the mechanisms.Specifically,in both view of kinetics and thermodynamics,the effects of electric pulses on the microstructural evolutions and the dislocation glides of deformed aluminum were studied.In view of kinetics,the analysis was focused on the characteristics and dynamics of grain boundary and dislocation evolutions upon electric pulsing.In view of thermodynamics,the analysis was focused on whether the electric pulses can provide extra driving force for the movement of grain boundaries and dislocations.Based on the above experimental results,the dominating factor of electric pulses and its mechanism that influences the microstructures were analyzed.Results of this study showed that:?1?The electric pulses could obviously promote the microstructural evolutions and change the microstructural evolution path of deformed aluminum in the period of recovery and recrystallization,although the electric pulses induced thermodynamic driving force for the movement of grain boundaries and dislocations was negligible.The microstructural evolution path changes mainly occurred at the recovery stage.For instance,electric pulses with proper intensity could significantly decrease the interior dislocation density of heavily rolled aluminum in a short period of time,while remaining the average grain size almost unchanged.At a higher intensity,electric pulses could transform the recovery mechanism of heavily rolled aluminum from the migration of triple-junctions to the dissolution of subgrain boundaries and lead to a sharp increase of interior dislocation density at the same time.?2?At cryogenic temperatures?e.g.,T<135 K?electric pulses could induce the non-octahedral dislocation glides?generally forming at high temperatures?e.g.,453 K for aluminum??in heavily rolled aluminum.Occurrence of the non-octahedral dislocation glides could account for the rapid recovery and recrystallization behavior of heavily rolled aluminum,since the degrees of freedom for dislocation glides were substantially increased.Moreover,the activation of non-octahedral dislocation glides upon electric-pulsing may also offer a new/complementary explanation for the electroplasticity.?3?Among a variety of physical effects of electrical pulses,the selective heating effect was proposed to be closely related with the electric pulses induced microstructural evolutions and the non-octahedral dislocation glides in deformed aluminum.The local high temperature caused by the selective heating in the grain boundaries and dislocations?due to their higher electric resistivity?may enhance the movement of grain boundaries and dislocations,and thereby significantly accelerate the microstructural evolutions in recovery and/or recrystallization process.Moreover,the electric pulses induced local high temperature may also activate the non-octahedral dislocation glides in deformed aluminum.The selective heating could not cause the overall temperature rise of bulk materials,since the total volume fraction of crystal defects?including grain boundaries and dislocations?was extremely low(¹0^-3–10^-2).Hence,the selective heating belongs to the global athermal effect of electric pulses.The studies based on the direct observation of microstructural evolutions upon electric-pulsing can deepen understanding of the mechanism for microstructures modification by electric pulses,which may be beneficial to developing electric-pulsing techniques.