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

镀钛镍丝柯肯德尔孔演化的尺寸效应

• 批准号: 1611308
• 负责人: David Dunand
• 金额: $ 39.68万
• 依托单位: Northwestern University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-09-01 至 2021-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1611308&HistoricalAwards=false
• 关键词: Size; Effect; Evolution; Kirkendall; Pores

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.

非技术摘要：热形状记忆合金（SMA）是一种有趣的材料类型，可以在加热时变形并恢复其初始形状。特别地，最常见的SMA之一是镍钛诺，其是具有接近1：1的镍和钛（即NiTi）比率的合金。由于这种形状恢复行为，镍钛诺通常用于诸如致动器或开关的应用。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物的各种生物医学应用。虽然传统的块体镍钛诺迄今已被证明非常有用，但它可以通过引入开放孔隙率来进一步改进，以各种方式增强性能。例如，通过包括开孔孔隙，表面积与体积比增加，从而增加加热和冷却效率，导致热形状记忆效应的更快响应时间。从生物医学角度来看的另一个例子是，开放孔隙允许骨向内生长，有助于NiTi植入物的整合。然而，由于NiTi的高熔点和反应性质，制造这种具有开放孔隙率的NiTi结构是具有挑战性的。克服该问题的一种方法是由纯Ni制造多孔结构，然后在其上存款Ti，使得一旦均匀化，将形成具有形状记忆行为的NiTi结构。本研究将专注于将纯镍丝转化为三维NiTi丝编织结构。该项目将提供各种推广机会，包括在当地小学，展示NiTi的形状记忆行为，向学生介绍材料科学学科。我们还将创建一系列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结构提供了形状记忆行为、低刚度和高表面积的组合，适用于致动器、骨植入物和阻尼器等应用。多孔NiTi最常见的合成方法-自蔓延高温合成，其中混合的Ni和Ti粉末反应不受控制-导致不期望的金属间相和不受控制的Kirkendall孔。我们试图研究一种新的方法，其中控制互扩散在钛涂层镍丝形成近等原子NiTi合金的形状记忆或超弹性行为。本计画旨在了解丝材尺寸对镍钛互扩散行为及柯肯达尔孔隙演化之影响。具体地，随着线直径（即其体积与表面比）减小，我们预期从（i）大线的横截面中的许多Kirkendall孔（本体行为）到（ii）较细线的横截面中的单个孔到（iii）非常细的线的无孔的转变，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孔演变的机制和动力学，作为金属丝直径的函数，范围从10到200 µm。纯镍导丝将通过包埋渗碳涂覆钛，随后均质化，同时捕获各种中间相和最终相（NiTi 2、NiTi、Ni 3 Ti）以及Kirkendall孔的演变（尺寸、分数、位置）。将使用非原位金相技术和原位X射线断层扫描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到NiTi（1D）线，（2D）弹簧和（3D）编织支架的制造中。为了验证实验结果并最终预测优化的互扩散参数，将开发相场模型来预测涂层和均匀化过程中的微观结构演变（相和孔）。此外，将开发基于断层扫描数据的有限元模型，以研究Kirkendall孔对机械（形状记忆和超弹性）行为的影响。许多研究人员已经证明，Kirkendall孔可以用来制造中空结构。然而，目前还没有一个系统的调查的影响，样品的大小和空间限制的Kirkendall孔隙形成和稳定性。一个基本的理解的样品尺寸效应（连接到空位扩散距离线直径的比率）对镍钛互扩散行为和Kirkendall孔隙率将作为一个指导方针，选择适当的加工条件生产多孔NiTi结构。它还将提供更一般的洞察机制和优化使用柯肯德尔效应在其他金属系统中创建这样的中空结构。此外，在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特性的基于线或弹簧的微结构NiTi结构，这是不可能通过当前的拉丝和编织方法在如此小的长度尺度上生产的。这种编织结构将适合于致动、阻尼和生物医学应用，利用改进的渗透性、增加的表面积、较低的刚度和高形状记忆和超弹性。