A Quantitative In Situ SEM Bending Method for Stress Relaxation of Microscale Materials at Room Temperature

Although the time-dependent deformation behaviors of microscale materials have been investigated through experiments with uniaxial loading conditions, the influence of the strain gradient has not been clearly clarified due to the lack of appropriate testing methods. In the current study, to investigate the stress relaxation behavior of microscale single-crystal copper (Cu) at room temperature, a quantitative in situ SEM bending experiment is presented using microcantilever specimens of single-crystal Cu. The microcantilever specimens were fabricated using a focused ion beam, and a tungsten (W) layer was deposited onto the front surface to eliminate the error induced by the penetration of the stiff indenter into the metallic specimen. The yield stress of microscale single-crystal Cu is determined to be 445 MPa by a monotonic loading test, showing an apparent size effect, and no strain hardening is observed due to single-slip deformation. On the other hand, the stress relaxation behavior of the microscale single-crystal Cu consists of both a continuous stress relaxation and an abrupt stress decrease due to a strain burst. The activated volume in each dwell stage is obtained by thermodynamics theory and is found to be mainly related to the abrupt stress decrease. The value of the activated volume indicates that the continuous stress drops in the 1st and 2nd dwell stages are attributed to the evolution of dislocation structures by the single slip on system B4, while the dislocation pile-up near the neutral plane leads to the dominance of cross slip on the stress relaxation behavior in the bending plateau. The proposed microcantilever bending experiment is applicable to explore the time-dependent deformation behavior of small-scale materials.