Browsing by Author "Wan, Li"
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Item Influence of Fe Underlayers on Stress Evolution of Ti in Ti/Fe Multilayers(2016-09-07) Thompson, Gregory B.; Wan, Li; University of Alabama TuscaloosaA series of 40–2 nm bilayer spacing Ti/Fe multilayers were sputter-deposited. As the length scale of individual Ti layers equaled to 2 nm, Ti phase transforms from a hexagonal close packed (hcp)- to-body centered cubic (bcc) crystal structures for equal layer thicknesses in Ti/Fe multilayers. Further equal reductions in bilayer spacing to less than 1 nm resulted in an additional transformation from a crystalline to amorphous structure. Atom probe tomography reveals significant intermixing between layers which contributes to the observed phase transformations. Real-time, intrinsic growth stress measurements were also performed to relate the adatom mobility to these phase transformations. For the hcp Ti/bcc Fe multilayers of equivalent volume fractions, the multilayers undergo an overall tensile stress state to a compressive stress state with decreasing bilayer thickness for the multilayers. When the above phase transformations occurred, a modest reduction in the overall compressive stress of the multilayer was noted. Depending on the Fe thickness, the Ti growth was observed to be a tensile to compressive growth change to a purely compressive growth for thinner bilayer spacing. Fe retained a tensile growth stress regardless of the bilayer spacing studied.Item Influence of the Nb Growth Surface on the Allotropic Ti Phase Transformation in Nb/Ti/Nb Nanolaminates(2016-09-14) Thompson, Gregory B.; Wan, Li; Yu, Xiao-Xiang; University of Alabama TuscaloosaAs the thickness of a thin film is decreased, the interfacial structure becomes paramount and crystals can undergo phase transformations. Molecular dynamic simulations have been performed to capture how such transformation could occur under the growth surface of a film. An hcp to bcc transition in Ti for Ti/Nb multilayers was used as the case studies. The simulations had good agreement with experiments. The simulations further predicted a mixed phase state for Ti for particular equal layer thicknesses.Item Interrelationship of in Situ Growth Stress Evolution and Phase Transformations in Ti/W Multilayered Thin Films(2016-06-24) Thompson, Gregory B.; Wan, Li; Yu, Xiao-Xiang; Zhou, Xuyang; University of Alabama TuscaloosaThis paper addresses the in situ growth stress evolution and phase transformation of bcc to hcp Ti in Ti/W multilayered thin films. A series of equal layer thicknesses from 20 nm to 1 nm were deposited. As the bilayer thickness reduced, the overall film stress became less compressive until the Ti transformed from hcp (at the larger layer thicknesses) to bcc in the 1 nm/1 nm multilayer. The pseudomorphic bcc stabilization resulted in a recovery of the compressive stress to values near that for the bulk phase stabilized for the 5 nm/5 nm multilayer. A discernable change in stress slope was noted for the bcc to hcp Ti transition as a function of Ti layer thickness. The stress states for each film, during film growth, are rationalized by the lattice matching of the phase with the growth surface. These results are coupled to a molecular dynamics deposition simulation which revealed good agreement with the experimentally observed transformation thickness.Item Phase stability in ti/bcc multilayered thin films(University of Alabama Libraries, 2016) Wan, Li; Thompson, Gregory B.; University of Alabama TuscaloosaMaterials structures with large surface area-to-volume ratios can exhibit size dependent physical and chemical properties that are different than their bulk form. These changes are often related to the material adopting a different crystallographic phase. Often these phase transformations are serendipitously observed with the criteria for their stability difficult to ascertain. This work elucidates the underpinnings of phase stability behavior in the nanoscale regime by providing a systematic study using Ti/bcc multilayered thin film architectures. The influences of lattice misfit, layer thickness, composition and chemical intermixing on the phase stability are determined. In situ thin film growth stresses of these materials are measured and correlated to the interfacial stress evolution to help rationalize the stability behavior. X-ray and electron diffraction have been employed to determine the phase with atom probe tomography used to characterize the chemical compositions within the materials and across the interfaces. This work will delineate how intrinsic film stress drives compositional intermixing across such interfaces which can thermodynamically promote phase transformations.