Electro-work hardening of metals induced by the athermal electromigration effect

Abstract

Direct current stressing treatment has been proposed as an effective potential method to modify the microstructures and optimize the mechanical properties of metals. Two contributions that could give rise to variations in the microstructure and mechanical properties of metals are involved in the direct current stressing treatment: thermal effects from Joule heat generation and athermal effects from electron-lattice interaction. The present study, for the first time, provides direct evidence of the work hardening behavior of pure Sn metals induced by the athermal electromigration effect, which is referred to the “electro-work hardening.” The micro-hardness of the Sn strip increased for all of the current densities (4–7 × 103 A/cm2) and current stressing times (1 h and 6 h) that were investigated. The maximum micro-hardness increment achieved could be as high as 27.3% at 5 × 103 A/cm2 for 1 h as compared with the as-annealed Sn strip benchmark. The thermal benchmark experiment further evidences that the thermal Joule heating effect did not contribute to any variations in the micro-hardness, suggesting the predominant athermal effect of electromigration. The high-resolution transmission electron microscope lattice images further revealed a high volume of dislocations produced in the Sn matrix, which was responsible for the electro-work hardening behavior. The electro-work hardening mechanism incorporated in a direct current stressing treatment can be applied in metals without the need for macroscopic mechanical deformation pre-treatment, which is regarded as a non-deformation material treatment. This innovative method for inducing work hardening is considered to have great potential for applications in which products have already been shaped.