ADVANCED MATERIALS & PROCESSES | NOVEMBER 2026 26 ENHANCED TRANSFORMATION STRESS AND CRYOGENIC SUPERELASTICITY IN A NiTiNbY SHAPE MEMORY ALLOY Thermomechanical processing of a shape memory alloy provides stable superelastic performance over extremes of temperature and stress, making it suitable for demanding wide-temperature and strength-critical environments. S. Cai* and J.E. Schaffer* Fort Wayne Metals Research Products Corp., Indiana Superelastic alloys are widely used in medical devices and are gaining increasing interest in aerospace, energy, robotics, and defense applications. While conventional NiTi alloys exhibit excellent superelasticity, their moderate transformation (plateau) stress and relatively low stiffness can limit performance in applications requiring higher force output, reduced component thicknesses, structural rigidity, and energy absorption. In addition, growing demand for materials capable of reliable operation at cryogenic temperatures has created a need for alloys that combine high plateau stress, high stiffness, and robust low-temperature superelasticity. Previous work from the authors demonstrated that exceptionally high transformation plateau stress and stiffness can be achieved in a Ni46.7Ti42.8Nb10.5 (at.%) alloy[1]. However, this alloy contained approximately 10 vol% Nb-rich phase, which does not contribute to superelasticity and can reduce strain recovery after large deformation. In situ synchrotron x-ray diffraction studies revealed that the high plateau stress was primarily associated with the NiTiNb matrix and dissolved Nb, rather than the Nb-rich phase itself[1]. This finding suggests that similar mechanical performance may be attainable with substantially lower Nb content. Based on this concept, a NiTiNbY alloy with reduced Nb content was developed through alloy design and thermo- mechanical processing. A small amount of Y was added to reduce the volume fraction of large Ti4Ni2O inclusions by forming relatively smaller Y2O3 [2]. The effects of heat treatment on microstructure, phase transformation behavior, and mechanical properties were systematically investigated, with particular emphasis on cryogenic superelasticity. The results demonstrate that the developed alloy exhibits high transformation stress, elevated stiffness, and stable superelastic performance at extremely low temperatures, making it a promising candidate for next-generation medical devices, high-force actuators, aerospace systems, and cryogenic engineering applications. FEATURE MATERIAL AND EXPERIMENTS Using raw elements, about 450 kg of 53.4Ni-39.5Ti6.8Nb-0.1Y (wt%) was melted in a vacuum induction melting furnace and further refined in a vacuum arc remelting furnace and cast into a ø300 mm ingot. Chemistry was analyzed by inductively coupled plasma atomic emission spectroscopy. Oxygen is ~200 ppm tested by inert gas fusion method (ASTM E1409). Microstructure and composition were studied by scanning electron microscopy with energy dispersive x-ray spectroscopy. The as-cast ingot was homogenized at 1000°C for 72 h before hot forging and hot rolling to ~6 mm in diameter followed by cold drawing to wires of ~0.3 mm in diameter by traditional wire drawing and annealing process. The amount of cold work after the last process anneal was ~60% in area reduction. The process annealing was conducted at 850°C for varied dwell time depending on the alloy type and wire diameter. Cold drawn wires were heat treated at temperatures between 350 and 800°C for a dwell time of 5 minutes in an argon-filled tube furnace and rapidly cooled to room temperature in less than 1 minute. This wide heat treating temperature range allows us to study the effect of heat treating temperature or residual cold-work and mechanical capabilities of these alloys. Phase transfor- mation temperatures were measured by differential scanning calorimetry (DSC) testing in a temperature range from 150 to -150°C at a rate of 10°C/min. Loading-unloading tensile testing was carried out on an Instron tensile tester at room temperature. During the test, samples were loaded to a strain of 6% and unloaded to zero stress at a strain rate of 0.8×10-3 s-1; then reloaded to fracture. Sample gauge length was 127 mm. Deformation strains were determined based upon crosshead displacement. Tensile tests were performed between -130 and 150°C to evaluate the temperature range within which the material can retain superelasticity. *Member of ASM International
RkJQdWJsaXNoZXIy MTYyMzk3NQ==