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

非技术摘要：热形状记忆合金(SMA)是一种有趣的材料，它可以变形并在加热后恢复其初始形状。特别是，最广为人知的SMA之一是镍钛，这是一种镍和钛的比例接近1：1的合金(即NiTi)。由于这种形状恢复行为，镍钛通常用于执行器或开关等应用。此外，NiTi是一种生物相容性材料，因此可用于从支架到骨植入的各种生物医学应用。虽然到目前为止，传统的块状镍钛已被证明非常有用，但可以通过引入开放孔隙率来以各种方式增强性能来进一步改进它。例如，通过包括开口孔洞，表面积与体积比增加，从而提高加热和冷却效率，从而导致热形状记忆效应的更快响应时间。从生物医学的角度来看，另一个例子是开放的孔洞允许骨骼向内生长，有助于镍钛种植体的整合。然而，由于NiTi的高熔点和反应性，制造这种具有开放孔洞的NiTi结构是具有挑战性的。解决这一问题的一种方法是从纯镍制备多孔结构，然后在其上沉积钛，以便一旦均质，就会形成具有形状记忆行为的镍钛结构。本研究将致力于将纯镍丝转化为三维NiTi丝编织结构。该项目将提供包括在当地小学在内的各种推广机会，展示NiTi向学生介绍材料科学学科的形状记忆行为。我们还将制作一系列YouTube视频，讨论NiTi结构的加工和形状恢复特性。技术摘要：多孔NiTi结构具有形状记忆性能、低刚度和高表面积的组合，适用于执行器、骨植入物和阻尼器等应用。最常见的多孔NiTi的合成方法是自蔓延高温合成，其中混合的Ni和Ti粉末反应失控-导致不希望的金属间化合物相和失控的Kirkendall气孔。我们试图探索一种新的方法，在镀钛镍丝中形成具有形状记忆或超弹性行为的近等原子NiTi合金的受控互扩散。本项目旨在了解金属丝尺寸对Ni-Ti互扩散行为和Kirkendall孔演化的影响。具体地说，随着线材直径(即其体积与表面积的比)的减小，我们预计会从(I)大线材的横截面上有大量的Kirkendall孔(体积行为)，到(Ii)较细线材的横截面上只有一个孔，到(Iii)非常细的线材没有孔，因为空位扩散到表面而不是形成孔。为此，我们将进行一项系统的研究，探索镍-钛互扩散和Kirkendall孔随线材直径的变化的机制和动力学。纯Ni丝将通过填充粘结在钛表面涂覆钛，然后进行均化，同时捕捉各种中间相和最终相(NiTi2、NiTi、Ni3Ti)和Kirkendall孔的演化(尺寸、分数、位置)。将使用非原位金相技术和原位X射线层析技术对金属丝进行表征。从这些实验中获得的知识将被应用于NiTi(1D)丝、(2D)弹簧和(3D)编织支架的制造。为了验证实验结果和最终预测优化的互扩散参数，将建立相场模型来预测涂层和均匀化过程中的组织演变(相和气孔)。此外，基于断层扫描数据的有限元模型将被开发来研究Kirkendall孔对力学(形状记忆和超弹性)行为的影响。各种研究人员已经证明Kirkendall孔可以被利用来产生中空结构。然而，目前还没有关于样品大小和空间限制对Kirkendall孔洞形成和稳定性的影响的系统研究。对样品尺寸效应(与空位扩散距离与导线直径之比有关)对Ni-Ti互扩散行为和Kirkendall孔隙率的基本了解将为选择合适的制备多孔NiTi结构的工艺条件提供指导。它还将提供对利用柯肯德尔效应在其他金属系统中创建这种中空结构的机制和优化的更全面的见解。此外，该项目中进行的实验和开发的模型将能够制造具有独特形状恢复性能的线基或弹簧基微结构NiTi结构，这是目前的拉丝和织造方法无法在如此小的长度规模上生产的。这种编织结构将适合于驱动、减震和生物医学应用，利用其改善的渗透性、更大的表面积、更低的硬度以及高形状记忆和超弹性。

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.