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Proceedings Papers
Hyperelastic Behavior of Knitted TiNi Mesh under Uniaxial Tension
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SMST 2022, SMST 2022: Extended Abstracts from the International Conference on Shape Memory and Superelastic Technologies, 73-74, May 16–20, 2022,
Abstract
View Papertitled, Hyperelastic Behavior of Knitted TiNi Mesh under Uniaxial Tension
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for content titled, Hyperelastic Behavior of Knitted TiNi Mesh under Uniaxial Tension
TiNi-based alloys belong to the class of materials with shape memory effects and superelasticity, which are currently being actively studied and successfully used in engineering and medicine. In these alloys, their natural ability to undergo large inelastic deformations and return to their original shape by increasing temperature or relieving stress takes place. The key characteristic of these phenomena is thermoelastic martensitic transformations (MT). The problem of biocompatibility of implants is relevant, as the number of operations using implants in various fields of medicine is growing rapidly. Currently, several studies are underway on the deformation behavior of biological tissues and various implant materials. Wires made of TiNi are one of the most important metal biomedical materials used in endovascular surgery, orthodontics, soft tissue plastics in the form of stents, catheters, orthodontic archwires, metal-knitted materials. Textile implants should be singled out from a wide range of structures made of thin TiNi wire, with the help of which complex surgical problems are solved. A variety of mesh structures made of titanium nickelide are characterized by a particular complexity of deformation characteristics, the manifestation of which in the implant-bio-tissue interface is difficult to predict. To create the appropriate mechanical behavior of an implant in the form of mesh structures, it is necessary to study their deformation behavior. Therefore, to describe the functioning of a superelastic implant in the interface with a biological tissue, the aim of this work is to study the deformation behavior of wire samples 40, 60, and 90 µm thick from the TiNi alloy and metal knit made from them by the method of uniaxial tension. TiNi wires exhibit the effect of superelasticity at a relative strain of 4-6%. Under uniaxial tension of knitted mesh made of these wires, the effect of superelasticity was not detected. It has been found that the cyclic tension diagrams of knitted mesh show behavior inherent in hyperelastic materials. The total tensile load is unevenly distributed in the knitwear, in contrast to the uniformly distributed load when testing the wire.
Proceedings Papers
Epitaxial NiTi Thin Films: A 3D Puzzle
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SMST 2022, SMST 2022: Extended Abstracts from the International Conference on Shape Memory and Superelastic Technologies, 75-76, May 16–20, 2022,
Abstract
View Papertitled, Epitaxial NiTi Thin Films: A 3D Puzzle
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for content titled, Epitaxial NiTi Thin Films: A 3D Puzzle
NiTi films are widely used in micro applications due to their shape memory and superelasticity properties. When customizing the material for a specific miniature device, it is vital to understand the underlying martensitic microstructure, how it forms, and how it affects the shape memory effect. Up to now, most research on the martensitic microstructure in NiTi concentrates on NiTi bulk, but results derived from bulk materials are not always applicable for films as well. Even though polycrystalline NiTi films are widely available, these films contain grain boundaries, which hamper or even inhibit a scale bridging analysis of the martensitic microstructure. Therefore, the martensitic microstructure in NiTi films and its formation remains mostly unexplored. To improve NiTi for applications in miniature devices, it is thus helpful to study films without grain boundaries as model systems. In this study, the authors analyze single crystalline NiTi films grown by DC magnetron sputter deposition. These epitaxial films grow without large angle grain boundaries and make it possible to analyze the martensitic microstructure over several length scales. The work analyzed the martensitic microstructure and its nucleation with microscopy and X-ray methods and compared these measurements with orientation relationships calculated with the phenomenological theory of martensite. The results are the starting point to understand the formation of a hierarchical martensitic microstructure of NiTi in three dimensions.
Proceedings Papers
Tensile Deformation of B19′ Monoclinic Martensite in Nanocrystalline NiTi Wires
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SMST 2022, SMST 2022: Extended Abstracts from the International Conference on Shape Memory and Superelastic Technologies, 77-78, May 16–20, 2022,
Abstract
View Papertitled, Tensile Deformation of B19′ Monoclinic Martensite in Nanocrystalline NiTi Wires
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for content titled, Tensile Deformation of B19′ Monoclinic Martensite in Nanocrystalline NiTi Wires
When deformed in martensite state, NiTi undergoes pseudoplastic deformation taking place via motion of intervariant interfaces (called martensite reorientation or detwinning), followed by plastic deformation of the B19′ monoclinic martensite. The state of the art view is that: (i) the martensite reorientation proceeds via detwinning of <011> type-II twin laminates created by the martensitic transformation upon cooling and (ii) the reoriented martensite deforms plastically via dislocation slip. Although this view might be correct for single crystals and large grain size polycrystals, doubts existed whether it applies also for nanocrystalline NiTi which displays (001) compound twinned microstructures after stress free cooling from the austenite. The authors performed systematic experimental investigations of martensitic microstructures (postmortem TEM) and textures (in-situ HEXRD) evolving during tensile tests on nanocrystalline NiTi wires until fracture. The results indicate that the widespread view of the martensite reorientation as "detwinning" is incorrect. Plastic deformation of martensite proceeds via peculiar deformation mechanism involving (20-1) and (100) deformation twinning assisted by [100]/(011) dislocation slip. It enables the nanocrystalline NiTi wire to deform plastically at ~1 GPa engineering stress up to very large plastic strains ~50% and refines the austenitic microstructure down to nanoscale. Upon unloading and heating, reverse martensitic transformation takes place leaving large recoverable as well as unrecovered strains and high density of {114} austenite twins in the microstructure.