Cast NiTi Shape‐Memory Alloys

Abstract

Shape-memory alloys, such as nickel-titanium (NiTi), have the unique ability to recover an applied inelastic strain either through the removal of the applied stress (pseudoelasticity) or the application of heat (shape-memory). This capacity has made NiTi a desirable material for many applications including MEMS, [1] biomedical devices, [2,3] and civil infrastructure. [4±6] One of the first mainstream uses of NiTi was in orthodontics, in which drawn NiTi wires were used as arc wires to apply a constant force to a patient's teeth. [2,3] To date, nearly all applications of NiTi employ a deformation processed and machined material. However, due to NiTi's poor workability, especially at higher Ni content, [7] and machinability, [8] fabrication of NiTi is expensive. High cost can make the selection of NiTi an unreasonable choice for many promising applications. Although direct casting could eliminate these costly steps, little work has been done to examine the structure and properties of NiTi in its cast state, limiting the possibility of its use in potential applications.The desire for a method of near net-shape production of NiTi parts has motivated the research of powder metallurgy as a fabrication technique. [9,10] Several methods, including self propagating high temperature synthesis (SHS), [11,12] Hot Isostatic Pressing (HIP), [9,10,13±16] and Metal Injection Molding (MIM) [10,16] are being considered for the production of shapememory materials. Recent attention has been given to HIP due to the achievement of near theoretical densities [10,13] and to MIM for its promising attribute of near net-shape production. [10,13,16] Although the work done by Johansen et al. [9] showed promising results, demonstrating tensile strains larger than 10 % for HIPed NiTi, the majority of the remaining studies demonstrate lower fracture strains under tensile loading (0.6 %±5.1 % [10,13,16] ). The study by McNeese et al. [15] revealed HIPed NiTi with recoverable tensile strains of up to 2 % and recoverable compressive strains of 4.5 %. However, despite progress being made in powder metallurgy, the search for an ideal method for net-shape processing of NiTi that would maintain its unique shape-memory properties is still underway. This was the driving motivation of this study, which aims to correlate processing, structure, and properties of cast NiTi. The link between structure and properties of cast NiTi builds a foundation for the design of net-shape NiTi castings.In general, net-shape casting is a good method for producing parts with complex geometries and is one of the simplest and most direct ways to fabricate a metal component. [17] Typical problems that can arise with a cast material include compositional segregation, porosity, inclusions (oxides, sulfides and nitrides), and shrinkage. [18±21] The solidification of a metal is a thermodynamically driven process that is dependent on such things as initial compositions, temperature, and pressure. [22] For cast alloys, thermodynamics drives the solidification path and co...

Extended Cyclic Deformation Recovery of the Indentation‐Induced Two‐Way Shape‐Memory Effect in Nickel–Titanium

“…The NiTi material investigated post heat treatment does not display any secondary transitions into a third phase such as the R phase. This is expected, based on the heat treatment process and the atomic composition of the alloy investigated here …”

The Enhanced Shape Memory Effect and Mechanical Properties in Thermomechanically Processed Semi‐Equiatomic NiTi Shape Memory Alloy

“…In accord with previous reported results, the observed stress plateau region of the flow curves is attributed to the (i) partial detwinning of martensite which would be true in the case of as‐hot rolled material with fully martensitic structure at room temperature, or to the (ii) partial stress‐induced martensite transformation which would be correct in the case of thermomechanically processed material possessing dual phase structure of austenite and martensite at room temperature. It should be noted that the relatively higher stress level in recoverable region of the processed specimens appears to be reasonable considering the fact that the stress level required for martensitic transformation is higher than that of detwinning . The required stress for martensite reorientation and/or for stress‐induced martensite transformation, i.e., Yield 1, and the typical yield point of the flow curve, i.e., Yield 2 are depicted in Figure b.…”

Beneficial Interface Geometry for Laser Joining of NiTi to Stainless Steel Wires

“…The heat affected zone of the base materials will also undergo some changes. Precipitates in the base NiTi will increase, which also results in lowering the M, temperature [24]. Once the NiTi has recrystallized, the thermodynamic parameters will remain nearly constant though [25].…”