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Design and development of innovative material compositions and mechanisms based on shape memory alloys (SMAs) were accomplished as part of the Consortium for the Advancement of Shape Memory Alloy Research and Technology (CASMART) 2nd student design challenge. The challenge consisted of developing an expandable space habitat where the retention and deployment of such structure employs the use of SMAs. Student groups were provided with an initial set of requirements and given an option to design the material or design with the material, within six months. This paper collectively documents the evaluation, alloy selection, fabrication, testing, and implementation processes of three teams, including the tools and information used to successfully design, develop, and implement SMA material systems and habitat technologies. For the mechanism design, students used a combination of superelastic rods and shape memory springs/wires to design collapsible rings to fold and deploy the habitat. Publicly available design tools were used to size the SMA components based on the provided loading scenarios. For the material system development, compositions based on CuAlMn, NiTiFe, NiTi and NiTiSn alloys were explored and designed to satisfy the given set of requirements. Details pertinent to these designs are described in this work, along with lessons learned.

Nickel-rich (54–56 at.% Ni) NiTi-based alloys have gained increased attention for their high hardness, corrosion resistance, strength, and wear resistance, leading to their development for high-performance bearings and other wear applications. An investigation of a broader compositional range of NiTiHf alloys, in terms of Ni and Hf content, is presented in this study. Their Vickers micro-hardness, 3 ball-on-rod rolling contact fatigue, and compression performances are benchmarked against early compositions identified for bearing applications, e.g., Ni55Ti45 and Ni54Ti45Hf1. The results show that by varying heat treatments and alloy composition, certain NiTiHf alloys can exhibit long life (~ 107 cycles) at 10–20% greater contact stress levels (up to 2.2–2.4 GPa), together with up to 20% greater hardness (up to 770 HV) and 30% larger compressive yield strengths (up to 3.4 GPa) than the original bearing compositions. These improvements are attributed to the ability to develop alloys with high-volume fractions of fine precipitate phases. Interestingly, a variety of combinations of different precipitate strengthening phases, depending on alloy composition and heat treatment, were able to achieve improvements relative to Ni55Ti45 and Ni54Ti45Hf1, demonstrating the versatility of the NiTiHf system.

This study investigates the thermal and mechanical properties that arise from aging Ni-rich Ni–Ti (Nitinol) at temperatures below 250 °C, well below those commonly used to fabricate medical devices. We demonstrate that the Ni50.8–Ti49.2 composition decomposes at temperatures as low as room temperature and discuss the unusual changes in thermal and mechanical behaviors compared to common aging treatments, such as separation of Martensite and R-phase transformations. Using such aging treatments, superelasticity can be achieved without the presence of austenite at body temperature (Af > 45 °C, well above the body temperature). Furthermore, the influence of R-phase on mechanical response and the disparity between thermal and mechanical behavior is discussed in detail.

Porous NiTi shape memory alloys have applications in the biomedical and aerospace fields. Recent developments in metal additive manufacturing have made fabrication of near-net-shape porous products with complicated geometries feasible. There have also been developments in tailoring site-specific microstructures in metals using additive manufacturing. Inspired by these developments, we explore two related mechanistic phenomena in a simplified representation of porous shape memory alloys. First, we computationally elucidate the connection between pore geometry, stress concentration around pores, grain orientation, and strain-band formation during tensile loading of NiTi. Using this, we present a method to engineer local crystal orientations to mitigate the stress concentrations around the pores. Second, we experimentally document the growth of cracks around pores in a cyclically loaded superelastic NiTi specimen. In the areas of stress concentration around holes, cracks are seen to grow in large grains with [1 1 0] oriented along the tensile axis. This combined work shows the potential of local microstructural engineering in reducing stress concentration and increasing resistance to propagation of cracks in porous SMAs, potentially increasing the fatigue life of porous SMA components.

Two prevalent myths of Nitinol mechanics are examined: (1) Martensite is more compliant than austenite; (2) Texture-free Nitinol polycrystals do not exhibit tension–compression asymmetry. By reviewing existing literature, the following truths are revealed: (1) Martensite crystals may be more compliant, equally stiff, or stiffer than austenite crystals, depending on the orientation of the applied load. The Young’s Modulus of polycrystalline Nitinol is not a fixed number—it changes with both processing and in operando deformations. Nitinol martensite prefers to behave stiffer under compressive loads and more compliant under tensile loads. (2) Inelastic Nitinol martensite deformation in and of itself is asymmetric, even for texture-free polycrystals. Texture-free Nitinol polycrystals also exhibit tension–compression transformation asymmetry.

A general procedure to optimize shape memory alloys (SMAs) for specific engineering performance metrics is outlined and demonstrated through a study of ternary, NiTiX high-temperature SMAs, where X = Pd, Hf, Zr. Transformation strains are calculated using the crystallographic theory of martensite and compared to the cofactor conditions, both requiring only lattice parameters as inputs. Measurements of transformation temperatures and hysteresis provide additional comparisons between microstructural-based and transformation properties. The relationships between microstructural-based properties and engineering performance metrics are then thoroughly explored. Use of this procedure demonstrates that SMAs can be tuned for specific applications using relatively simple, fast, and inexpensive measurements and theoretical calculations. The results also indicate an overall trade-off between compatibility and strains, suggesting that alloys may be optimized for either minimal hysteresis or large transformation strains and work output. However, further analysis of the effects of aging shows that better combinations of uncompromised properties are possible through solid solution strengthening.

A micromechanical study of the effect of varying amounts of tensile strains on the microstructures and subsequent mechanical behaviors of superelastic Nitinol rods is presented. It is found that strains up to ~8–9 % develop microstructures that assist both forward and reverse transformation relative to un-strained material. This superelastic phenomenon is explained to be analogous to two-way shape memory effect in Nitinol actuation materials. These results provide understanding as to why such “pre-strains” may lead to improvements in subsequent superelastic fatigue life. Beyond 9 %, a drastic change is observed, as large amounts of martensite (75 % and more) are retained in unloaded samples. Thus, a competition between transformation, plasticity, and reorientation is found to give rise to microstructures that inhibit complete transformation. Furthermore, even though similar inelastic strain magnitudes are observed in loading and unloading plateaus, micromechanical mechanisms differ substantially from samples with less pre-strain. For example, in highly pre-strained samples at least half of the plateau strains are due to martensite reorientation, whereas, in low and moderately pre-strained samples nearly the entirety of the plateau strain is due to transformation. We also find that latent heat of plastic flow is larger than latent heat of transformation.