Size Effect on the Evolution of Kirkendall Pores in Ti-Coated Ni Wires

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Non-technical Abstract: Thermal shape memory alloys (SMAs) are an interesting type of material that can be deformed and recover their initial shape upon heating. In particular, one of the most commonly known SMAs is Nitinol, which is an alloy with a near 1:1 ratio of nickel and titanium (i.e. NiTi). Because of this shape recovery behavior, Nitinol is commonly used for applications such as actuators or switches. Additionally, NiTi is a biocompatible material and, hence, can be used for a variety of biomedical applications from stents to bone implants. While traditional bulk Nitinol has proven very useful thus far, it can be further improved by introducing open porosity to enhance the properties in a variety of ways. For example, by including open porosity, the surface area to volume ratio increases, thereby increasing the heating and cooling efficiency leading to faster response times of the thermal shape memory effect. Another example from the biomedical perspective is that the open porosity allows for ingrowth of bone aiding in the integration of the NiTi implant. However, manufacturing such NiTi structures with open porosity is challenging due to the high melting point and reactive nature of NiTi. One way to overcome this issue is to fabricate porous structures from pure Ni and then deposit Ti on them such that, once homogenized, NiTi structures with shape memory behavior will be formed. This research will focus on transforming pure Ni wires into three-dimensional NiTi wire-woven structures. This project will provide various outreach opportunities including at local elementary schools, demonstrating the shape memory behavior of NiTi to introduce students to the discipline of materials science. We will also be creating a series of YouTube videos that will discuss the processing and shape recovery properties of NiTi structures.Technical Abstract: Porous NiTi structures offer a combination of shape-memory behavior, low stiffness, and high surface area useful for applications such as actuators, bone implants, and dampers. The most common synthesis method for porous NiTi - self-propagating high-temperature synthesis where mixed Ni and Ti powders are reacted uncontrollably - resulting in undesirable intermetallic phases and uncontrolled Kirkendall pores. We seek to investigate a novel approach where controlled interdiffusion in Ti-coated Ni wires forms near equiatomic NiTi alloys with shape-memory or superelastic behavior. This project aims at understanding the wire size effect on the Ni-Ti interdiffusion behavior and Kirkendall pore evolution. Specifically, as the wire diameter (i.e. its volume-to-surface ratio) decreases, we expect a transition from (i) numerous Kirkendall pores in the cross-section (bulk behavior) for large wires, to (ii) a single pore in the cross-section for thinner wires, to (iii) no pores for very thin wires, as vacancies diffuse to the surface instead of forming pores. To this end, we will be conducting a systematic study that explores 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 pack cementation and subsequently homogenized while capturing the evolution (size, fraction, location) of the various intermediate and final phases (NiTi2, NiTi, Ni3Ti) and the Kirkendall pores. Wires will be characterized using both ex situ metallographic techniques and in situ X-ray tomography. Knowledge gained from these experiments will be applied to the fabrication of NiTi (1D) wires, (2D) springs, and (3D) woven scaffolds. For verification of the experimental results and eventual prediction of optimized interdiffusion parameters, a phase-field model will be developed to predict the microstructural evolution (phases and pores) during coating and homogenization. Also, finite-element models based on tomographic data will be developed to investigate the effect of Kirkendall pores on mechanical (shape-memory and superelastic) behavior.Various researchers have demonstrated that Kirkendall pores can be harnessed to produce hollow structures. However, there has not yet been a systematic investigation on the effect of sample size and spatial confinement on the Kirkendall porosity formation and stability. A fundamental understanding of the sample size effect (linked to the ratio of vacancy diffusion distance to wire diameter) on Ni-Ti interdiffusion behavior and Kirkendall porosity will serve as a guideline for choosing appropriate processing conditions for production of porous NiTi structures. It will also provide more general insight into the mechanisms and optimization of using the Kirkendall effect for creating such hollow structures in other metallic systems. Moreover, the experiments performed and models developed in this project will enable manufacturing of wire- or spring-based micro-architectured NiTi structures with unique shape-recovery properties, which are impossible to produce at such small length scales via current wire drawing and weaving methods. Such woven structures will be suitable for actuation, damping, and biomedical applications, taking advantage of improved permeability, increased surface area, lower stiffness and high shape memory and superelasticity.