Young Stress Analyst Competition

Objectives: Nanocrystalline (nc) metals present excellent mechanical properties such as strength and fatigue resistance but often show low ductility. Furthermore, nc systems show moderate to high rate sensitivity at room temperature which might help restoring the ductility but can have disadvantage of creep/relaxation effects in applications. Thin metallic films constitute ideal candidates for looking at the mechanics of nc systems as they can be easily produced with nano-grained structures frequently involving only one grain over the thickness (for high resolution transmission electron microscopy (HRTEM) observation) and sharp textures. Time dependent relaxation/creep mechanisms are amplified in nc materials compared to traditional microcrystalline systems which can be due to a change in deformation mechanisms. Different methods have been used to characterize the rate sensitive creep or relaxation behaviour at the nanoscale, involving nanoindentation, bending and direct tensile testing. The main shortcomings involve the difficulty to impose very small strain rates typical of real applications and to perform in-situ relaxation tests in order to characterize the deformation mechanisms. Also, the sensitivity to drift is increased at the nanoscale, which hampers imposing a constant load during long periods of time. In the present project a novel technique for stress and strain evolution measurement called "on-chip testing" and allowing in-situ HRTEM observations has been used for the first time for creep/relaxation experiments of nc Pd free standing beams. The concept of the on-chip method is to use the internal stress present in a long beam, 'the actuator' (30 nm-thick Si 3 N 4), to deform another material attached to it, 'the specimen' (90 nm-thick ~2 µm wide Pd ribbon) by selectively back etching the Si substrate and SiO 2 sacrificial layer yielding the actuator to contract and deform the Pd beams and enable direct in-plane TEM observation (Figure 1a). The stress and strain in the deformed Pd specimens are provided by the measurement of the specimen elongation. A complete stress-strain curve is generated by varying the actuator versus specimen length ratio to impose different 1 deformations. The relaxation experiment is performed by measuring the deformation at different time intervals. Note that both stress and strain vary with time. The stress and strain evolution during relaxation has been measured under pure uniaxial tension (Figure 1b). The fracture strain is around 3% and the yield stress ~500±60 MPa close to the internal stress measured after deposition. The mean activation volume was also measured for different plastic strains …