Fort Wayne Metals - Technical Blog

Super-elastic alloys with gigapascal plateau strengths: Advancing medical and space applications

Thu, 04 Mar 2021 12:00:00 Z

Nitinol alloys have long been a core material in medical and industrial applications due to their unique super-elastic properties. Fort Wayne Metals developed a new generation of Nitinol-based super-elastic (SE) alloys that push the previous boundaries of strength and performance. With upper plateau strengths exceeding 1.0 GPa and lower plateau strengths above 600 MPa, these advanced materials unlock new possibilities across a range of demanding applications.

Enhanced strength for medical applications

The ability to achieve high plateau stresses allows medical device engineers to develop components with thinner profiles while maintaining or even increasing mechanical performance. For instance, vascular guidewires and orthodontic arch wires made from these advanced Nitinol alloys can deliver greater force in smaller wire diameters.

Tensile stress-strain testing of one such alloy, NiTiNbY, revealed remarkable improvements over traditional binary Nitinol alloys. NiTiNbY demonstrated a forward loading plateau stress of 1100 MPa—an increase of more than 65% compared to conventional Nitinol’s 650 MPa. Additionally, its unloading plateau stress is double that of standard Nitinol, while its axial elastic modulus is 40-50% higher. These improvements mean that medical devices, such as stents and guidewires, can be designed with thinner structures without compromising outward force or performance.

While higher stresses can enhance push-ability and torque in guidewires, engineers must consider durability, particularly for permanent implants. However, for temporary devices, the benefits are substantial. Orthodontic arch wires, for example, can provide double the bending force compared to traditional Nitinol wires, enhancing treatment efficiency.

Superelasticity at cryogenic temperatures for space applications

One of the most remarkable attributes of these alloys is their ability to retain super-elastic properties even at extremely low temperatures. Traditional Nitinol alloys transform to martensite upon cooling, losing their super-elastic behavior. However, NiTiNbY and similar compositions maintain stable super-elastic recovery even at -130°C, as shown in stress-strain testing. This property makes them ideal for applications in space exploration and other extreme environments.

Potential space applications include deployable structures, springs, and even advanced tire designs for extraterrestrial exploration. A spring or textile-based tire made from NiTiNbY alloy could be compacted for transport and then deployed to operate over a broad temperature range while maintaining high load-carrying capabilities.

If you are interested in the future of Nitinol alloy spring tires and our collaboration with NASA, please explore our most recent research.

The challenge of finding a Nitinol alternative

Mon, 23 Nov 2020 15:09:00 Z

Nitinol has long been the material of choice for medical devices that require super-elasticity. Its ability to recover from deformation without permanent damage makes it essential in applications like stents, guidewires, and orthopedic implants. However, concerns over Nitinol’s nickel content with potential allergic reactions from patients have driven research into alternative materials with comparable properties.

The rise of Ni-free β-Ti alloys

One promising alternative is a Ni-free β-Ti alloy composed of Ti-40Hf-13Nb-4.5Sn (wt.%). This alloy demonstrates large and stable super-elastic behavior at room temperature, a key requirement for medical applications. Unlike other β-Ti alloys that struggle to achieve the recoverable strain of Nitinol, this material exhibits up to 5.3% total recoverable strain at 6% deformation—putting it much closer to Nitinol’s performance.

Key advantages of Ti-Hf-Nb-Sn (THNS) alloy

This Ni-free β-Ti alloy offers several advantages beyond eliminating nickel:

Comparable mechanical properties: Its mechanical strength and fatigue performance are in the same range as superelastic Nitinol.
Customizable plateau strengths: Similar to Ni-rich Nitinol, the alloy's performance can be adjusted through low-temperature aging.
Manufacturability at scale: The alloy has been successfully produced in 100 kg quantities using commercial production lines, making it feasible for industrial-scale applications.
High X-ray visibility: The presence of hafnium enhances radiopacity, which is beneficial for medical imaging during procedures.
Biocompatibility and corrosion resistance: These characteristics make it a strong candidate for implants, especially for nickel-sensitive patients.

Overcoming manufacturing challenges

Many β-Ti alloys have struggled with manufacturability due to high heat treatment requirements and oxidation concerns. However, this alloy achieves optimal mechanical properties at lower temperatures (~600°C), reducing the risk of oxidation and making shape-setting more practical. Unlike other high-zirconium β-Ti alloys, which pose ignition risks during processing, this alloy has demonstrated safer large-scale production capabilities.

A substitute for Nitinol?

Despite years of research, no Ni-free β-Ti alloy has been successfully substituted for Nitinol in the medical device industry—until now. With its stable super-elasticity, comparable mechanical properties, and improved manufacturability, this Ti-Hf-Nb-Sn alloy presents a viable alternative for applications such as orthopedic implants, dental devices, and neurovascular components.

As the medical industry continues to prioritize patient safety and material innovation, this breakthrough may pave the way for the next generation of superelastic biomaterials.