| This dissertation work consists of five topics: processing, microstructure and thermal stability, mechanical properties, tribological behavior, and fatigue crack growth behavior of Ni/Sn multilayer composites. These composites, with different layer thicknesses and volume fractions, were prepared using electrodeposition technique. The adhesion between Ni and Sn layers was improved by depositing a very thin Cu film on each Sn layer surface before the subsequent Ni deposition. Deposit textures of Ni(200) and Sn(101) were detected in the multilayer structure. Composition profile at interfaces of the Ni/Sn as-deposited sample was fairly sharp. However, extended annealing of the layered composites at 210{dollar}spcirc{dollar}C led to diffusion of Ni atoms into Sn layers, and formation of Ni-Sn intermetallic compounds {dollar}rm (Nisb3Sn, Nisb3Snsb2, and Nisb3Snsb4){dollar}.; Mechanical properties of these composites were measured by tensile and microhardness tests. The multilayers displayed enhanced ultimate tensile strength (UTS), yield strength (YS), and microhardness. The YS and microhardness increased significantly with decreasing layer thickness, and this dependence followed the Hall-Petch relation. The observed strengthening was attributed to dislocation pinning at interfaces of the multilayers.; Tribological behavior of these composites was evaluated from two types of wear tests, pin-on-disc and block-on-ring. Both results suggested that the soft Sn acted as a solid-lubricant between the contacting surfaces. Discontinuous Sn films were detected on the surfaces of both the specimens and the steel rings, and it was found that the wear resistance greatly depended on the thickness, area coverage, composition, and stability of this interfacial film. Specimens with lower Sn content and/or thinner layer spacing exhibited enhanced wear resistance than those with higher Sn and/or thicker layers. The enhancement in wear resistance was attributed to the Sn solid-lubrication effect and the superior strength of the multilayer microstructure.; Fatigue crack growth behavior of the multilayer composites was investigated using three-point bend fatigue tests. Two cracking processes were observed. One was the primary fatigue crack propagating from notch tip normal to the multilayers. Another was the extensive intergranular cracking within the Sn layers. The primary crack growth was effectively arrested by the intergranular cracking in the first Sn layer because it reduced the stress intensity factor at the crack tip. Thus, Sn layers acted as a barrier to the primary fatigue crack growth. The Ni layers were totally intact and supported the structure, so the integrity of the composites was maintained. Consequently, the Ni/Sn multilayered composites exhibited super fatigue damage tolerance. |
