Infoscience

Thesis

Mechanical properties of tailored nanostructured alloys produced by electrodeposition

The studies presented in this thesis aim to extend the current knowledge and understanding of the mechanical behavior of nanocrystalline materials with respect to temperature- and time dependence. Free standing electrodeposited nanocrystalline nickel specimen with mean grain sizes in the order of 40nm are synthesized and investigated. A miniaturized in situ test rig is developed, capable of performing uniaxial tensile tests on dedicated miniaturized specimen. A novel displacement controlled in situ high temperature indentation system is presented, capable of performing sharp tip indentation and uniaxial micropillar compression experiments up to 600°C. Elevated temperature micropillar strain rate jump tests are performed to extract the strain rate sensitivity factor, apparent activation volume and activation energy. The results suggest grain boundary diffusion to be the rate controlling deformation mechanism. Post deformation imaging also indicates the activity of dislocation based mechansims. To compare the results to established techniques, micro specimen are tested under three different load cases: uniaxial tension, uniaxial compression and hydrostatic pressure. The results compare well to the previous, suggesting the rate controlling mechanism to be the same for all three load cases. A transient load relaxation tests is developed to assess time dependent plasticity on short and intermediate time scales. The results suggest grain boundary mediated processes to be rate controlling. To enhance the identification of specific deformation mechanisms, elevated temperature micropillar strain rate jump- and load relaxation tests are also conducted on an inert gas condensated nanocrystalline palladium-gold model alloy. Distinct features of this material are extremely high chemical purity and small, homogeneous grain sizes in the order of  10nm. Shear transformation mediated plasticity is observed, similar to the deformation of metallic glasses. The demonstrated methods are relevant for the Swiss watch industry. A series of mechanical tests on standard watch materials are compared. An approach for the electrodeposition of a nickel-tungsten alloy for watch applications with improved thermal stability and less pronounced time dependent plastic behavior is presented.

Related material

Contacts

EPFL authors