The effect of interfaces on the stability and mechanical properties of polycrystalline multilayers

In this work, the effects of the structure and free energies of solid-solid interfaces on the microstructural stability and mechanical properties of polycrystalline multilayered materials were examined.; In polycrystalline multilayers, the ratio of grain boundary energy to interfacial free energy determines the microstructural stability of the system. In immiscible elemental systems (“A/B” multilayers), the layer having the higher grain boundary energy (typically the layer with the higher melting temperature) tends to be the less stable layer, and is more likely to pinch off or break down at high temperatures. In metal/intermetallic systems (“A/ABx” multilayers), the elemental (“A”) layer tends to be less stable than the intermetallic layer. The effects of relative crystallographic orientation on stability of individual grain triple junctions are described.; Understanding interfacial free energy is critical to the design of stable polycrystalline multilayers. This work describes the first equilibrium biaxial zero-creep measurements of interfacial free energy, which were performed on Ag/Ni multilayers.; Solid-solid interfaces control the plastic deformation behavior of polycrystalline multilayers as well. Plastic deformation in Cu/Nb multilayers was studied in detail. Dislocation-controlled plasticity and grain boundary strengthening was observed both at room temperature and at elevated temperatures. A pronounced reduction in this effect was observed as temperature increased and as strain rates decreased.; Creep rates and time-dependent plastic deformation mechanisms were also studied in the Cu/Nb system. Over a range of grain sizes from 0.5 microns to 5.0 microns, two distinct creep regimes were observed at 600°C. Power Law creep dominates at high stresses. At low stresses, the dependence of creep rate on grain size indicates that an interface-controlled creep mechanism is operating. The rate of generation and annihilation of vacancies at the grain boundaries and interfaces is the rate-controlling phenomenon in this deformation mechanism. Activation energy for creep at low stresses is consistent with the activation energy for the generation of vacancies at Nb grain boundaries.