Browsing by Author "Yu, Xiao-Xiang"
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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 Plasticity mechanisms in HfN at elevated and room temperature(Nature Portfolio, 2016-10-06) Vinson, Katherine; Yu, Xiao-Xiang; De Leon, Nicholas; Weinberger, Christopher R.; Thompson, Gregory B.; University of Alabama Tuscaloosa; Drexel UniversityHfN specimens deformed via four-point bend tests at room temperature and at 2300 degrees C (similar to 0.7 T-m) showed increased plasticity response with temperature. Dynamic diffraction via transmission electron microscopy (TEM) revealed < 110 > {111} as the primary slip system in both temperature regimes and < 110 > {110} to be a secondary slip system activated at elevated temperature. Dislocation line lengths changed from a primarily linear to a curved morphology with increasing temperature suggestive of increased dislocation mobility being responsible for the brittle to ductile temperature transition. First principle generalized stacking fault energy calculations revealed an intrinsic stacking fault (ISF) along < 112 > {111}, which is the partial dislocation direction for slip on these close packed planes. Though B1 structures, such as NaCl and HfC predominately slip on < 110 > {110}, the ISF here is believed to facilitate slip on the {111} planes for this B1 HfN phase.