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

Arizona State University, Scottsdale AZ

Investigators

Abstract

NON-TECHNICAL SUMMARY Most 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 undergraduate and graduate students for conducting research and workforce training partnerships with regional community colleges and industries. TECHNICAL SUMMARY The 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.