Superelastic NiTi honeycombs: fabrication and experiments

In this paper we demonstrate a new class of superelastic NiTi honeycomb structures. We have developed a novel brazing technique that has allowed us to fabricate Nitinol-based cellular structures with relative densities near 5%. Commercially available nickel-rich Nitinol strips were shape-set into corrugated form, stacked, and bonded at high temperature by exploiting a contact eutectic melting reaction involving pure niobium. After heat treatment to restore transformational superelastic response, prototype honeycomb structures were subjected to severe in-plane compression loading at room temperature. The specimens exhibited good specific strength, high specific stiffness, and enhanced shape recovery compared to monolithic shape memory alloys (SMAs). Compressive strains of over 50% could be recovered upon unloading. The demonstrated architectures are simple examples of a wide variety of possible built-up topologies, enabled by the bonding method, that can be engineered for customizable net section properties, arbitrary shape, and kinematically enhanced thermomechanical shape-memory and superelastic response.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/58146/2/sms7_1_S17.pd

Low-density open-cell foams in the NiTi system

It is shown that open-cell metallic foams having very low density, and that display martensite transformations required for shape memory and superelastic behavior, can be fabricated using a powder-metallurgy technique. Results are presented on experiments in which a polymeric precursor foam was coated with an equiatomic NiTi powder slurry and subsequently sintered to yield foams with relative densities as low as 0.039. Although contaminated with interstitial impurities, they displayed unambiguous calorimetric signature of the B2→B19′B2→B19′ transformation. The results are of considerable significance to potential applications requiring ultralightweight structures with the unusual dissipative and strain-recovery properties of NiTi shape-memory materials. © 2003 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71336/2/APPLAB-82-16-2727-1.pd

The influence of ion beam mixed Ni---Al surface layers on fatigue in polycrystalline nickel

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26250/1/0000331.pd

Shape memory alloy honeycombs: experiments & simulation

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76238/1/AIAA-2007-1739-156.pd

DRAFT IMECE2003-41216 CRYSTALLIZATION OF SPUTTERED NiTi FILMS

ABSTRACT Sputtered, crystalline thin films of nickel-titanium (NiTi) can display both superelastic properties and the shape memory effect, either of which may be used in films for MEMS sensors and actuators. However, direct deposition of crystalline NiTi films requires high deposition temperature and cooldown can lead to catastrophic delamination from extrinsic residual stress. To avoid delamination, especially for thick films (>5 micrometers) the amorphous form of NiTi can be sputter deposited at a low temperature, patterned and etched, released, and then crystallized to develop the proper microstructure

Surface form Memory in NiTi Shape Memory Alloys by Laser Shock Indentation

An indentation-planarization method for NiTi shape memory alloys has been developed that produces a robust surface topographical memory effect that we call surface form memory , orSFM. Surface form memory entails reversible transitions between one surface form (flat) and another (say, wavy) that occur on changing temperature. These transitions are cyclically stable and exhibit very high mechanical energy density. Our previous study has demonstrated SFM transitions in NiTi alloys derived from quasistatic (i.e., low strain rate) spherical indents, as well as other geometries. Here, we report on experiments using confined laser ablation to indent a similar martensitic NiTi substrate, but in the dynamical regime (very high strain rate). As in the quasistatic case, subsurface plastic strain gradients are created via martensite twinning reactions, and later by dislocation-mediated slip. The resulting defects and stress fields support the two-way shape memory effect underlying SFM. In the dynamical case however, relative cyclic two-way displacements are found to be significantly larger, when normalized to the initial indent depth, than is the case with quasistatic indentation. This confers certain processing and boundary condition advantages. Analysis of the shock dynamics is found to be consistent with the observed surface displacements