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A novel arc heat treatment technique was applied to design a uniquely graded super duplex stainless steel (SDSS), by subjecting a single sample to a steady state temperature gradient for 10 h. A ne ...

Ti-Ni shape memory alloy thin films were deposited using RF magnetron sputtering. The lattice constants, the transformation temperatures and shape memory behavior were measured by the X-ray diffractometry, DSC and mechanical tests, respectively. These films showed a perfect shape memory effect which is the same as that of bulk alloys. The shape recovery stress and strain measured in the thin films were large enough for fabricating microactuators. In an age-treated Ni-rich Ti-Ni thin films, the shape recovery stress was more than 600MPa, while the shape memory strain amounted up to 2.6%. The thin films showed both the martensitic and R-phase transformations. Accordingly, they showed a two-stage deformation upon cooling ; the strain induced in each stage recovered perfectly upon heating. The transformation temperatures and shape memory characteristics were strongly affected by age-treatment in Ni-rich thin films, while they were not in Ni-poor thin films.

This thesis explores the possibilities of design optimization techniques for designing shape memory alloy structures. Shape memory alloys are materials which, after deformation, can recover their initial shape when heated. This effect can be used for actuation. Emerging applications for shape memory alloys are e.g. miniaturized medical instruments with embedded actuation, as well as microsystem components. However, designing effective shape memory alloy structures is a challenging task, due to the complex material behavior and the close relationship between geometry, electrical, thermal and mechanical properties of the structure. In this thesis, various approaches are developed to combine optimization algorithms with computational modeling of shape memory alloy structures. The focus is on the shape memory behavior of NiTi alloys that exhibit the R-phase/austenite transformation. Dedicated computationally efficient constitutive models are formulated to capture this behavior and predict the performance of designs. The considered optimization approaches include deterministic shape optimization, shape optimization under bounded-but-unknown uncertainty, gradient-based shape optimization and topology optimization. Together they provide a collection of efficient and systematic techniques to generate well-performing designs. Their applicability and effectiveness is evaluated by application to design studies of realistic complexity, involving the design of miniature grippers and steerable catheters. The developed design optimization techniques are expected to be of great use for the design of future instruments and devices that utilize shape memory alloy actuation.

The contributions of electronic and crystallographic components to the semiconductor → metal transition in VO2 have been estimated from resistivity, E.P.R., and calorimetric measurements of the electronic and thermodynamic properties of GaxV−xO2, where 0 < x < 0.0130. E.P.R. and resistivity measurements indicate a decrease in the metallic character of the high-temperature R phase with increasing x, and calorimetric measurements of the energetics of the transition show a decrease in the enthalpy and entropy of the transition with increasing levels of doping. This concomitant decrease in enthalpy and metallic character with increasing x implies a strong contribution of the electronic entropy to the transition. An extrapolation of the combined electronic and calorimetric data for GaxV−xO2 to pure VO2 suggest that the electronic entropy comprises about 60% of the total entropy of transition in VO2.

MnAl(r) phase is investigated by both X-ray powder method and transmission electron microscopy. The unit cell is concluded to have “body-centred rhombohedral” symmetry, with lattice constants of a=8.96 Å and α=89° 1' when the alloy has composition of 43 at. per cent of manganese. Single crystalline electron diffraction patterns also assure this unit cell. Surface etching patterns of the alloy, which often look like arrow feathered patterns, are also observed on transmission electron micrographs. The micro-structure can be interpreted on the basis of above cell and a model of configuration of the subgrains is proposed, boundaries of which, shafts and barbs, are indexed as {100} and {110} respectively.

Investigations on the interconnection between the polarization rotation and crack propagation are performed for [110]-oriented 74Pb(Mg1/3Nb2/3)O3-26PbTiO3 relaxor ferroelectric single crystal under electric loadings along [001] direction. The crystal is of predominantly monoclinic MA phase with scatter distributed rhombohedral (R) phase under a moderate poling field of 900 V/mm in [001] direction. With magnitude of 800 V/mm, a through thickness crack is initiated near the electrode by electric cycling. Static electric loadings is then imposed to the single crystal. As the applied static electric field increases, domain switching in the monoclinic MA phase and phase transition from MA to R phase occur near the crack. The results indicate that the crack features a conducting one. Whether domain switching or phase transition occurs depends on the intensity of the electric field component that is perpendicular to the applied electric field.