NiTi Alloys for Structural and Tribological Applications: The Other Side of Superelastics

Nickel-Titanium based “NITINOL” alloys are widely known for their unusual shape memory and superelastic properties, as discovered in the 1950’s by W.J. Buehler and his colleagues at the Naval Ordnance Laboratory [1]. Few practitioners today are aware that NiTi was initially explored as a simple high temperature structural alloy for missile nosecones and other defense applications for which cupronickel alloys like Monel were commonly used.
Monel excelled in corrosive applications but rapidly looses strength at elevated temperatures. Buehler’s investigation of NiTi alloys, therefore, was not a radical departure from previous research on Ni-Cu alloys. Lower density, improved strength, and higher temperature capability while retaining similar electrical and non-magnetic characteristics were among the motivations to consider NiTi.

Buehler’s early efforts to explore the NiTi compositional landscape is well documented in the literature, as were the challenges he and his team faced trying to fabricate NiTi into test specimens and engineered products [1]. When near equiatomic Ni/Ti ratios were investigated, the specimens had variable damping properties and were relatively soft. When the nickel content was increased, the alloys became much stronger and harder but were prone to erratic and even explosive fracture during cooling or cutting. Wet chemical analyses, X-ray diffraction and extensive thermo and mechanical testing of a range of “NITINOL” alloys from “NITINOL 50” to “NITINOL 63” were researched. Despite difficulties faced with working the alloys into various forms and shapes, these early pioneers were able to fabricate wires, fasteners and even hand tools from NITINOL alloys.