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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 new experimental approach was used to map precipitation in microstructure, covering aging temperatures of up to 1430 °C. The microstructure was characterized and functionality was evaluated via hardness mapping. Nitrogen depletion adjacent to the fusion boundary depressed the upper temperature limit for austenite formation and influenced the phase balance above 980 °C. Austenite/ferrite boundaries deviating from Kurdjumov–Sachs orientation relationship (OR) were preferred locations for precipitation of σ at 630–1000 °C, χ at 560–1000 °C, Cr2N at 600–900 °C and R between 550 °C and 700 °C. Precipitate morphology changed with decreasing temperature; from blocky to coral-shaped for σ, from discrete blocky to elongated particles for χ, and from polygonal to disc-shaped for R. Thermodynamic calculations of phase equilibria largely agreed with observations above 750 °C when considering nitrogen loss. Formation of intermetallic phases and 475 °C-embrittlement resulted in increased hardness. A schematic diagram, correlating information about phase contents, morphologies and hardness, as a function of exposure temperature, is introduced for evaluation of functionality of microstructures., QC 20180521

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 new experimental approach was used to map precipitation in microstructure, covering aging temperatures of up to 1430 °C. The microstructure was characterized and functionality was evaluated via hardness mapping. Nitrogen depletion adjacent to the fusion boundary depressed the upper temperature limit for austenite formation and influenced the phase balance above 980 °C. Austenite/ferrite boundaries deviating from Kurdjumov–Sachs orientation relationship (OR) were preferred locations for precipitation of σ at 630–1000 °C, χ at 560–1000 °C, Cr2N at 600–900 °C and R between 550 °C and 700 °C. Precipitate morphology changed with decreasing temperature; from blocky to coral-shaped for σ, from discrete blocky to elongated particles for χ, and from polygonal to disc-shaped for R. Thermodynamic calculations of phase equilibria largely agreed with observations above 750 °C when considering nitrogen loss. Formation of intermetallic phases and 475 °C-embrittlement resulted in increased hardness. A schematic diagram, correlating information about phase contents, morphologies and hardness, as a function of exposure temperature, is introduced for evaluation of functionality of microstructures., Funding details: VINNOVA; Funding details: 2016-02834; Funding details: 20140130; Funding text: The financial support from the KK-foundation for the research school SiCoMaP ( 20140130 ) and the DUWELTOOL project ( 2016-02834 ) funded by Vinnova is acknowledged.

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 new experimental approach was used to map precipitation in microstructure, covering aging temperatures of up to 1430 °C. The microstructure was characterized and functionality was evaluated via hardness mapping. Nitrogen depletion adjacent to the fusion boundary depressed the upper temperature limit for austenite formation and influenced the phase balance above 980 °C. Austenite/ferrite boundaries deviating from Kurdjumov–Sachs orientation relationship (OR) were preferred locations for precipitation of σ at 630–1000 °C, χ at 560–1000 °C, Cr2N at 600–900 °C and R between 550 °C and 700 °C. Precipitate morphology changed with decreasing temperature; from blocky to coral-shaped for σ, from discrete blocky to elongated particles for χ, and from polygonal to disc-shaped for R. Thermodynamic calculations of phase equilibria largely agreed with observations above 750 °C when considering nitrogen loss. Formation of intermetallic phases and 475 °C-embrittlement resulted in increased hardness. A schematic diagram, correlating information about phase contents, morphologies and hardness, as a function of exposure temperature, is introduced for evaluation of functionality of microstructures., Funding details: VINNOVA; Funding details: 2016-02834; Funding details: 20140130; Funding text: The financial support from the KK-foundation for the research school SiCoMaP ( 20140130 ) and the DUWELTOOL project ( 2016-02834 ) funded by Vinnova is acknowledged.

Transformation behaviors of the technologically important polycrystalline Ti50.75Ni47.75Fe1.50 shape memory alloy were investigated using differential scanning calorimeter (DSC) and powder diffraction techniques. DSC revealed that there are two-stage (i.e., cubic -> trigonal -> monoclinic) martensitic phase transformations on cooling and a one-step transformation (monoclinic -> cubic) on heating. In situ structural refinement of cubic -> trigonal -> monoclinic on cooling was carried out using the D1A high-resolution neutron powder diffractometer at the Institut Laue-Langevin Neutrons for Science in Grenoble, France. Results showed that the phases involved during the phase transition are consistent with the differential scanning calorimeter cooling curve, and the refined crystal structure parameters obtained from Rietveld refinements with the generalized spherical harmonic description agreed reasonably well with X-ray single-crystal data. Subsequently, a combined neutron and synchrotron structural refinement for each phase was conducted because the trial refinements initially using only the synchrotron data of trigonal phase yielded a false minimum with a somewhat high goodness-of-fit chi(2). Results obtained from the combined neutron and synchrotron data of the cubic, trigonal, and monoclinic phases show that the same minimum goodness-of-fit indices were always obtained. (C) 2007 International Centre for Diffraction Data.

This article illustrates the opportunities that combining computational modeling and systematic design optimization techniques offer to facilitate the design process of shape memory alloy (SMA) structures. Focus is on shape memory behavior due to the R-phase transformation in Ni-Ti, for which a dedicated constitutive model is formulated. In this paper, efficient topology and shape optimization procedures for the design of SMA devices are described. In order to achieve fast convergence to optimized designs, sensitivity information is computed to allow the use of gradient-based optimization algorithms. The effectiveness of the various optimization procedures is illustrated by numerical examples, including the design of a miniature SMA gripper and a steerable SMA active catheter. It is shown that design optimization enables designers of SMA structures to systematically enhance the performance of SMA devices for a variety of applications., Department of Precision and Microsystems Engineering, Mechanical, Maritime and Materials Engineering

Transformation behaviors of the technologically important polycrystalline Ti50.75Ni47.75Fe1.50 shape memory alloy were investigated using differential scanning calorimeter (DSC) and powder diffraction techniques. DSC revealed that there are two-stage (i.e., cubic -> trigonal -> monoclinic) martensitic phase transformations on cooling and a one-step transformation (monoclinic -> cubic) on heating. In situ structural refinement of cubic -> trigonal -> monoclinic on cooling was carried out using the D1A high-resolution neutron powder diffractometer at the Institut Laue-Langevin Neutrons for Science in Grenoble, France. Results showed that the phases involved during the phase transition are consistent with the differential scanning calorimeter cooling curve, and the refined crystal structure parameters obtained from Rietveld refinements with the generalized spherical harmonic description agreed reasonably well with X-ray single-crystal data. Subsequently, a combined neutron and synchrotron structural refinement for each phase was conducted because the trial refinements initially using only the synchrotron data of trigonal phase yielded a false minimum with a somewhat high goodness-of-fit chi(2). Results obtained from the combined neutron and synchrotron data of the cubic, trigonal, and monoclinic phases show that the same minimum goodness-of-fit indices were always obtained. (C) 2007 International Centre for Diffraction Data.

This article illustrates the opportunities that combining computational modeling and systematic design optimization techniques offer to facilitate the design process of shape memory alloy (SMA) structures. Focus is on shape memory behavior due to the R-phase transformation in Ni-Ti, for which a dedicated constitutive model is formulated. In this paper, efficient topology and shape optimization procedures for the design of SMA devices are described. In order to achieve fast convergence to optimized designs, sensitivity information is computed to allow the use of gradient-based optimization algorithms. The effectiveness of the various optimization procedures is illustrated by numerical examples, including the design of a miniature SMA gripper and a steerable SMA active catheter. It is shown that design optimization enables designers of SMA structures to systematically enhance the performance of SMA devices for a variety of applications., Department of Precision and Microsystems Engineering, Mechanical, Maritime and Materials Engineering

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., Mechanical Maritime and Materials Engineering

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., Mechanical Maritime and Materials Engineering