Statement of objectives and methods - Porous NiTi structures offer several properties that make them attractive for a wide range of applications such as actuators, bone implants, and dampers. However, the formation of stable intermetallic phases and uncontrolled Kirkendall pores undermine the fabrication of porous NiTi structures via methods based on the solid-state diffusion of Ni and Ti, e.g., self-propagating high-temperature synthesis. This project aims at understanding the wire size effect on the Ni-Ti interdiffusion behavior and Kirkendall pore evolution in Ti-coated Ni wires, which, upon homogenization, form near equiatomic NiTi alloys with shape memory or superelastic behavior. To this end, the investigator proposes a systematic study that will explore the mechanisms and kinetics of Ni-Ti interdiffusion and Kirkendall pore evolution as a function of wire diameter ranging from 10 to 200 µm. Pure Ni wires will be coated with Ti via a gas phase deposition technique known as pack cementation and subsequently homogenized at various temperatures and times to determine how to control the Kirkendall porosity. Knowledge gained from these experiments will be applied to the fabrication of NiTi wires, 2D springs, and 3D-woven scaffolds exhibiting shape memory or superelastic properties, with or without Kirkendall pores. Samples will be characterized using both ex situ metallographic techniques and also in situ X-ray tomography. For verification of the experimental results, a phase-field model will be developed to predict the microstructural evolution during coating and homogenization. Additionally, finite-element modeling based on tomographic data will be used to investigate the effect of Kirkendall porosity on mechanical and shape recovery behavior. As a final step, samples will be coated with Hf and subsequently homogenized to modify the phase transformation temperatures. Intellectual merit - A significant amount of research has been conducted regarding harnessing the Kirkendall effect to produce hollow structures. However, there has not yet been a systematic investigation on the effect of size and spatial confinement on the Kirkendall porosity formation and stability. A better understanding of the size effect on Ni-Ti interdiffusion behavior and Kirkendall porosity will not only serve as a guideline for choosing appropriate processing conditions for production of porous NiTi structures, but will also provide more general insight into the mechanisms and limitations of using the Kirkendall effect for creating such hollow structures in other systems. Moreover, the techniques used in this project would enable manufacturing of wire-based micro-architectured materials with shape recovery properties, which are otherwise difficult to produce at such length scales using traditional wire drawing and weaving methods. Broader impact - Solid-state fabrication techniques provide an inexpensive route for the production of porous NiTi structures for actuation and biomedical applications. This study will provide the necessary information and guidelines for tailoring the microstructure and controlling the Kirkendall porosity to achieve the desired properties. The pack cementation technique applied to wire-based structures in this study can also be used for the processing of other material geometries such as foils and ribbons, and for the deposition of other elements such as Al, Si, Fe, Co, Mn, and Mo. Therefore, the methods developed here could translate to other metallic systems with applications in the automotive, aerospace, and biomedical indus
|Effective start/end date||9/1/16 → 2/28/21|
- National Science Foundation (DMR-1611308)
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