In this thesis, I planned to fabricate Cu/Ni metallic multilayers with equal layer thicknesses on different substrates by using magnetic sputtering technique. My objective was to characterize the texture, structure and hardness, in order to study strengthening mechanisms and nanotwins in the Cu/Ni multilayers. Sputtered, highly textured (111) and (100) Cu/Ni multilayers with individual layer thickness, h, vary from 1 to 200 nm. At greater h, X-ray diffraction (XRD) patterns of Cu and Ni (100 or 111) peaks are clearly separated indicating that the interface between Cu and Ni is semi-coherent. When h decreases to 5 nm or less, XRD spectra show significant peak distortions due to coherency stress. High resolution microscopy studies confirm the coexistence of nanotwins and coherent layer interfaces in highly (111) textured Cu/Ni mutilayers. Nanoscale twins can be formed in Cu at all h and in Ni at smaller h. Multilayer hardnesses increase with decreasing h, approach maxima at h of 2.5-5 nm, and show softening thereafter. A detail comparison between (111) and (100) textured Cu/Ni is made in both microstructure and strengthening. In this thesis, the possible mechanisms to form high density growth twins in Ni are discussed. Furthermore, the influences of both coherent layer interfaces and twin interfaces on strengthening mechanisms are discussed.

<p><a>Aluminum (Al) alloys have been widely used in </a>industry as light-weight structural materials. However, the mechanical strength of the strongest Al alloys is still much lower than most high-strength steels. This thesis aims to investigate the fabrication and mechanical behaviors of nanotwinned high-strength Al alloys and multilayers.</p> <p>Twin boundaries are special grain boundaries with mirror symmetry. Twin boundaries can generate slip discontinuity and block the transmission of dislocations, and serve as dislocation sources to accommodate plasticity. However, twinning in Al is rare due to its high stacking fault energy and low unstable stacking fault energy. In this thesis, we used multiple methods to introduce high-density twins into Al and achieve outstanding mechanical properties and thermal stability. </p> <p>Certain type of solutes can greatly increase the twin density in Al by decreasing the stacking fault energy of Al and retarding the detwinning process. Nanotwinned Al-Ni and Al-Ti binary alloys fabricated by magnetron sputtering show high strength, good deformability, and unique deformation mechanisms. Furthermore, deformation and thermal stability of binary nanotwinned Al alloys can be enhanced by adding a third or fourth solute element. </p> <p>Interfaces can facilitate twin formation in Al as well. High-density twins and stacking faults were introduced into Al by using Al/Ti layer interfaces. Nanotwinned Al/Ti multilayers have ultra-high strength, superb deformability and thermal stability. This thesis provides promising pathways to fabricate Al alloys and composites with high strength and good thermal stability.</p>