Severe room-temperature grain coarsening induced by cyclic strain in UFG gold and copper films as an alternative plasticity mechanismThursday (29.09.2016) 15:15 - 15:30 Part of:
One of the well-known ways to tailor the mechanical properties of pure metals is to design the microstructure. For instance, ultra-fine grained (UFG) or nanocrystalline materials exhibit significantly higher yield stresses in comparison to coarse-grained samples. However, it is also known that grain growth can occur in fine-grained materials even at room temperature if mechanical stress is applied. Despite the large number of experimental observations, little is known about the mechanisms of room-temperature grain coarsening and its relationship to the damage formation.
In this contribution the room-temperature grain coarsening induced by cyclic tensile strain in UFG Au and Cu films is investigated by means of a detailed electron backscatter diffraction analysis. It will be shown that very strong and homogeneous grain coarsening occurs in 500 nm thick gold films where the average grain size grows from 200 nm to approximately 2 µm during cyclic loading. The development of grain coarsening with the cycle number can be described by two stages. During the first stage the grain coarsening occurs without any topological changes of the surface of the film. Within the second stage further grain coarsening is accompanied by grain rotation as well as by formation of slip bands and extrusions. By means of interrupted testing-characterization technique a basic model which explains the grain coarsening in the frameworks of a thermodynamic driving force concept is presented. For copper films it will be demonstrated that there is a strong correlation between the formation and propagation of localized extrusion/crack couples and dynamic grain coarsening. These observations suggest that non-conservative motion and elimination of grain boundaries act as an alternative energy dissipation mechanism replacing the conventional slip-based plasticity at the early stages of cyclic deformation.