Laser additive manufacturing of bulk and porous shape-memory NiTi alloys: From processes to potential biomedical applica
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NiTi shape-memory alloys Shape memory is a fascinating material property, enabled by complex deformation mechanisms that create the “shape memorizing” ability in specific metallic and polymeric materials. Shape-memory materials can recover their primary shape after deformation (applied under specific temperature/ stress conditions) as a response to a thermal or mechanical command. This capacity of shape-memory materials allows for a wide range of applications, from biomedical implants and devices to sensors, actuators, aerospace components, and even fashion items.1–6 This article focuses on NiTi (nickel titanium or nitinol) intermetallics, since they are the most utilized shape-memory alloy. NiTi intermetallics are ductile in contrast to commonly known brittle intermetallics, hence they are commonly referred to as alloys. NiTi alloys regain their original shape through a reversible martensitic transformation (i.e., low-temperature martensite ↔ high-temperature austenite) (see Figure 1).7–9 The low-temperature martensite deforms through reorientation and detwinning of martensite lattice structure. Subsequent heating transforms the martensite (monoclinic with low symmetry)
to austenite (body-centered cubic with high symmetry) and recovers the original shape. This type of shape-memory effect is also known as a thermal-memory effect. Conversely, when the austenite is stressed within a specific temperature range, it transforms to martensite. Since martensite is unstable without stress at those temperatures, it transforms back to austenite upon unloading, reversing the deformation. This results in a large elastic response, called superelasticity.10–13 Accordingly, the martensite ↔ austenite transformation temperature is the most important factor in NiTi alloys; it defines the application at an intended working condition. Besides the shape-memory ability of NiTi alloys, they exhibit other valuable characteristics (e.g., good biocompatibility [comparable to conventional stainless steel and titanium, despite some existing concerns]),14–17 low stiffness (important for biomedical applications where bone stress shielding or bone healing is an issue),18,19 good corrosion resistance (similar to 300 series stainless steel or titanium alloys),20,21 superb wear resistance,22–24 high strength, and excellent ductility.22,25 These characteristics broaden the applications and performance of NiTi devices.
Sasan Dadbakhsh, Department of Mechanical Engineering, KU Leuven, Belgium; [email protected] Mathew Speirs, Department of Mechanical Engineering, KU Leuven, Belgium; [email protected] Jan Van Humbeeck, Department of Mechanical Engineering, KU Leuven, Belgium; [email protected] Jean-Pierre Kruth, Department of Mechanical Engineering, KU Leuven, Belgium; [email protected] doi:10.1557/mrs.2016.209
• VOLUME • www.mrs.org/bulletin 2016 Materials Research Society MRS BULLETIN 41 • OCTOBER Downloaded© from http:/www.cambridge.org/core. Temple University Libraries, on 13 Dec 2016 at 21:04:17, subject
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