Collaborative Research: Compositionally and Structurally Modulated Ferroelastic Films for Unprecedented Superelastic Properties

大多数金属和金属合金即使在拉伸或压缩少量（通常小于1%）时也会永久变形。相比之下，被称为形状记忆合金（SMA）的特殊类别的材料可以恢复其原始形状，即使在经历大变形（高达10%）后，一旦负载被移除。SMA的这种独特性质导致了广泛的应用，从医疗植入物和机器人到灵活的飞机机翼和太空探索车辆轮胎。然而，SMA的机械行为是高度非线性的，即，即使力发生微小变化，它们的变形也会急剧增加，这会使它们在机械上不稳定。此外，当SMA在变形后恢复其形状时，大量的能量作为热量被浪费。这个综合实验和计算研究项目的主要目标是消除不期望的机械不稳定性并创建更节能的SMA。这是通过在纳米级系统地调节SMA的化学组成和结构来实现的。这种新型合金设计方法，称为纳米级成分和/或结构调制（NCSM），不仅可用于创建机械稳定和节能的SMA，还可用于创建具有高度可调机械性能的其他类型的材料，例如用于骨植入物的钛基合金，其模拟天然骨的强度和刚度。NSCM合金设计策略，独特的实验处理和表征技术，以及该项目中开发的最先进的计算机模拟方法，正在通过会议演讲，在线教程和学术期刊上的文章广泛传播。该项目还通过向高中生进行实际示范、招募代表性不足的少数民族学生进行研究以及与区域社区学院和行业建立劳动力培训伙伴关系，推进教育推广和劳动力发展。 除了晶须或独立的纳米线之外，大多数金属和合金的弹性应变极限小于0.5%。相比之下，铁弹性材料如形状记忆合金（SMA）可以实现高达~ 10%的巨大可恢复应变。然而，SMA中固有的伪弹性非线性导致机械不稳定性，其特征在于应变雪崩驱动的应力平台和大量的应力-应变滞后。这个综合的计算和实验研究项目正在解决这个关键问题，通过引入一种创新的方法，称为纳米级成分和/或结构调制（NCSM），设计和合成下一代SMA。NCSM概念利用NiTi SMA中应力诱导马氏体相变（MT）的临界应力对成分和晶粒尺寸的强烈依赖性，以消除MT期间的应变雪崩，从而实现受控的应变释放。中心假设是，纳米级调制的化学成分和微观结构将引入限制MT过程，有效地抑制自催化和从根本上改变MT特性，导致NiTi SMA是强大的，线性超弹性，无弹性，并具有超低模量。这一假设正在测试合成NCSM镍钛薄膜精确定义的纳米级成分和晶粒尺寸调制使用物理气相沉积，并表征其机械性能使用MEMS的拉伸测试。这些NCSM NiTi薄膜的设计是由分子动力学和相场模拟的计算建模指导的。预计这类新的NCSM SMA可以被设计为表现出各种各样的高度可调的应力-应变行为，这些行为对于各种先进的生物医学、功能和结构应用是期望的。虽然该项目的重点是NiTi形状记忆合金，但NCSM合金设计概念适用于广泛的材料类别，这些材料利用结构相变来定制性能。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

NON-TECHNICAL SUMMARYMost metals and metallic alloys become permanently deformed even when they are stretched or compressed by a small amount (typically less than 1%). In contrast, a special category of materials known as Shape Memory Alloys (SMAs) can regain their original shape even after undergoing large deformations (up to 10%) once the loads are removed. This unique property of SMAs has led to a broad array of applications ranging from medical implants and robotics to flexible airplane wings and space exploration vehicle tires. However, the mechanical behavior of SMAs is highly non-linear, i.e., their deformation can increase drastically even for small changes in force, which can make them mechanically unstable. In addition, significant amounts of energy are wasted as heat when the SMAs recover their shape after being deformed. The primary goal of this integrated experimental and computational research project is to eliminate the undesirable mechanical instability and create SMAs that are more energy efficient. This is being accomplished by systematically modulating the chemical composition and structure of the SMAs at the nanoscale. This novel alloy design approach, termed as Nanoscale Compositional and/or Structural Modulation (NCSM), can be used to create not only mechanically stable and energy efficient SMAs but also other types of materials with highly tunable mechanical properties such as titanium-based alloys for bone implants that mimic the strength and stiffness of natural bones. The NSCM alloy design strategy, unique experimental processing and characterization techniques, and state-of-the-art computer simulation methodologies developed in this project are being broadly disseminated via conference talks, online tutorials, and articles in academic journals. The project is also advancing educational outreach and workforce development through hands-on demonstrations to high school students, recruitment of underrepresented minority students for conducting research and workforce training partnerships with regional community colleges and industries. TECHNICAL SUMMARYThe elastic strain limit of most metals and alloys is less than 0.5%, except for whiskers or freestanding nanowires. Ferroelastic materials such as shape memory alloys (SMAs), in contrast, can achieve giant recoverable strains of up to ~10%. However, the inherent nonlinearity of pseudo-elasticity in SMAs results in mechanical instability, characterized by strain avalanche driven stress plateaus and substantial stress-strain hysteresis. This integrated computational and experimental research project is addressing this pivotal issue by introducing an innovative approach, termed as Nanoscale Compositional and/or Structural Modulation (NCSM), to the design and synthesis of the next generation of SMAs. The NCSM concept capitalizes on the strong dependency of the critical stress for stress-induced martensitic transformation (MT) in NiTi SMAs on composition and grain size to eliminate strain avalanches during MT, and thus enable controlled strain release. The central hypothesis is that nanoscale modulations in chemical composition and microstructure will introduce confinements to the MT process, effectively suppress autocatalysis and fundamentally change the MT characteristics, leading to NiTi SMAs that are strong, linear superelastic, hysteresis-free, and have ultralow modulus. This hypothesis is being tested by synthesizing NCSM NiTi films with precisely defined nanoscale compositional and grain size modulations using physical vapor deposition, and characterizing their mechanical behavior using MEMS based tensile testing. The design of these NCSM NiTi films is being guided by computational modeling using molecular dynamics and phase field simulations. It is anticipated that this new class of NCSM SMAs can be designed to exhibit a wide array of highly tunable stress-strain behaviors that are desirable for a variety of advanced biomedical, functional, and structural applications. Although the focus of the project is on NiTi SMA, the NCSM alloy design concept is applicable to a broad class of materials for which structural phase transformations are utilized to tailor the properties.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.