There is an increasing demand for high quality Nitinol medical devices and implants to accommodate to the rapid growth in the biomedical market. Extensive studies have confirmed the significant impact of micro-cleanliness of Nitinol on fatigue life. Ultra-clean Nitinol material is required for most critical medical applications such as cardiovascular and neurovascular devices for which integrity and durability are critical. This poses challenges for upstream manufacturers to consistently produce ultra-low inclusion Nitinol mill products. This work is a comprehensive to evaluate hot rolled bars and coils of a new commercial scale ultra-clean Nitinol alloy. The robustness of the alloy production process and stability of product properties was confirmed by examining a large number of mill products manufactured in different campaigns. Extensive characterization and multiple approaches of inclusion analysis demonstrated the consistent ultra-high cleanliness of the products.

Alloys based on CuAl have been a promising option for high-temperature SMAs (HTSMA) because of their procedural and cost advantages over NiTi-based high-temperature SMAs. Despite their excellent shape memory as well as phase stability at temperatures up to 250°C, their brittle behavior and the degradation of the shape memory effect under cyclic stress have provided obstacles to widespread application. While multiple remelting processes are often applied to avoid inhomogeneities in the production of these alloys, single step inductive melting is preferable in terms of productivity, especially for small alloy batches. The goal of this and consecutive work is to characterize, and reduce, the variation of material properties and microstructure in materials prepared by vacuum-induction melting of pure elements and tilt-casting. To this end, castings from pure metals with 5 target chemical compositions from Cu12.5wt.%Al4wt.%Ni to Cu13.2wt.%Al4wt.%Ni were prepared and characterized in terms of transformation temperatures and occurrence of martensitic phases. Samples taken from different positions in the casting were compared. Changes in microstructure with increased aluminum content of the alloy could be assessed in both metallographic and calorimetric analysis. Considerable consistency of transformation temperatures and phase composition in each individual casting, as well as between castings with identical parameters, could be achieved. This points to the high degree of homogenization that can be achieved, even with a single melting cycling and subsequent casting, using suitable induction melting parameters. The absence of oxide and carbide inclusions, despite potential reactions between nickel and the graphite of the crucible, is promising for future casting processes.

Nitinol's thermomechanical properties are well studied and understood below the so-called martensite death (MD) temperature, above which martensite cannot be induced by mechanical stress: Even at high stresses Nitinol stays in the austenite phase. This paper presents tensile tests performed well above MD (>150 °C) with Nitinol specimens laser cut from tube. The investigations show that Nitinol drastically changes its mechanical properties in this temperature range: The superelastic plateau shortens and finally vanishes. Furthermore, Nitinol starts becoming more ductile.

Typical processing techniques involve thermo-mechanically treating the material such as cold working and subsequent annealing to control grain and precipitate size, shape, orientation, and morphology. Shape memory alloy (SMA) mechanical properties rely heavily on microstructural features such as precipitates and grain size to extend fatigue life. Novel approaches to control microstructural features have used laser anneal on amorphous NiTi thin films to recrystallize grains and short-time annealing on NiTi after angular extrusion and cold-drawn fine wires. A recent study examined rapid thermal annealing (RTA) on Ni-lean NiTi- 10 at.% Hf wires as an effective method for controlling grain size and extending actuation fatigue; however, flash annealing or RTA on Ni-rich NiTiHf high-temperature SMA (HTMSA) wires has not been investigated. Based on a larger study, Ni-rich NiTi-20 at.% Hf HTSMA was down-selected for further processing. This study investigates the effect of flash annealing on the thermo-mechanical properties of a Ni-rich Ni50.3Ti29.7Hf20 HTSMA. Microstructural changes were examined using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and transmission electron microscopy (TEM). Actuation fatigue properties were also evaluated at 300 MPa. The results indicate that flash annealing HTSMA wires is an effective method for controlling grain size and extending fatigue life. The heating rate and time held are crucial parameters to control microstructural features such as grain size and coherency.

Achieving stringent chemistry standards is necessary for additive manufacturing of Nitinol shape memory alloys. This work describes an elevated temperature gas-solid reaction processing technique that can be used to precisely regulate the chemistry and phase transformation behaviors of Nitinol components. The technique, Shape memory alloys via Halide-Activated Pack Equilibration (SHAPE), employs established principles of chemical vapor transport to equilibrate a substrate against reactive pack mixtures designed to regulate the chemical potentials of nickel and titanium in accordance with Gibbs' phase rule as means to precisely control substrate phase and elemental composition. The results suggest that SHAPE may find crosscutting potential especially when paired with additive manufacturing or fusion welding of Nitinol to improve product quality. Notwithstanding future applications, SHAPE is limited by solid-state diffusion that, in turn, limits the practical thickness of suitable components to about 1 mm. Opportunities for continued development have been identified for application to other compositions, and to further refine microstructure control.