UC Santa Barbara

Metallic multilayers are composites comprised of alternating layers of two or more metal components. Laminates with micro and nanoscale layer thicknesses have extraordinary properties when compared to the single phase metals; these advantages are due to the presence of and structural constraints imposed by bi-metal interface boundaries. High densities of bi-metal interfaces provide enhanced resistance to structural damage under irradiative and high impact shock conditions. While single phase nanocrystalline materials easily recrystallize and coarsen, metallic multilayers can maintain nanoscale microstructures at elevated temperatures. Constraints of the laminate structure and bi-metal interfaces cause this system to outperform its metallic components. Thermal instabilities can degrade this structure over time at temperature to different degrees depending on layer thicknesses.

This dissertation presents theory, experiments and modeling of high temperature behavior of copper-niobium multilayer composites made by accumulative roll bonding (ARB). Composites with different nominal layer thicknesses were assessed to characterize the influence of bi-metal interface densities and microstructure on mechanisms of deformation and structural evolution.

The creep behavior of bulk, ARB Cu-Nb multilayers was assessed at 400 C as a function of layer thickness, ranging from 2 microns to 65 nanometers. Similar to single phase metallic systems, three regimes were observed during creep: transient, steady-state and tertiary. The mechanism controlling minimum creep rate for all conditions tested has a strong dependence on stress, consistent with dislocation-dominated creep. Unlike the conventional effect of grain size on creep resistance, the unique result of this study reveals that decreasing length scale increases creep resistance.

The deformation response of ARB copper-niobium multilayers was modeled as a composite system, accounting for the behavior of Nb and Cu phases individually. The model assumes a continuous laminate structure of 50% Cu and 50% Nb in which deformation is controlled by stage II creep for the copper and plasticity in Nb. The results identify regimes in which composite steady-state creep at constant stress can be achieved, and illustrates that strain-hardening in the niobium plays a critical role in the transient response of the multilayer, which can dominate the creep lifetime. The combination of experiments and models strongly suggest that dislocation climb mechanisms in the copper control the time-response at 400 C for all layer thicknesses tested.

Heat treatments were performed at high temperatures for copper (0.50-0.57 Tm), but at which niobium diffusivity is limited (0.25-0.28 Tm). Annealing times range from 30 minutes to 30 days at temperature. Structural evolution and hardness measurements were assessed for laminates with layer thicknesses ranging from 8 microns to 30 nanometers. After 30 days, 65 nanometers laminates remain nanostructured with no significant coarsening. Voids at bi-metal interfaces grow larger and more frequent with time; interfacial void formation is a phenomenon that has not been previously reported in these systems. Analysis of structural evolution mechanisms and system stability will be discussed in relation to microstructure, thermal property mismatch, and the driving force for copper recrystallization.