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A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 1 9 4 7 NiTi ALLOY HISTORY In the early 1950s, W.J. Buehler and his colleagues at the Naval Ordnance Laboratory began researching the binary nick- el-titanium alloy system seeking a replacement for nickel-copper alloys, which could be used in structural applications ex- posed to elevated temperatures [1] . Compositions studied ranged from 45-65 wt% Ni, and a wide array of thermophysical and mechanical properties were evaluated. Buehler designated these alloys with the name “NITINOL” followed by the nickel con- tent in weight percent. Thus, NITINOL 50 contains equal amounts (by wt%) of nickel and titanium while NITINOL 60 contains 20 wt%more nickel than titanium. During experiments with bent wires made of equiatomic NiTi (known by its weight percent name NitinoI 55), the Bue- hler team observed that heating slightly above room temperature sometimes resulted in the wires returning to their original shape prior to bending. Buehler recognized this as a “shape memory effect,” and it was the driving force behind its use first as fluid couplings and dental wires, and later in a wide variety of potential applications. Alloys with nickel content above about 57 wt%are not commonly used for their shapememory effect, and when specially heat treated and processed are dimension- ally stable structural alloys. Much additional work was done on nickel-rich alloy compositions like Nitinol 60, but problems with sporadic brittle frac- ture and difficulties in machining and fabrication severely limited any major developments. Despite limited success making fasteners and rudimentary hand tools, the Ni-rich NiTi alloys were left in a largely undeveloped state [2] . from Superelastic Ni-Ti Alloys for Aerospace Mechanism Ap- plications,” NASA/TM—2011-217105, September 2012. 4. NASA Specification for 60Ni-40Ti Billets, NASA MSFC- SPEC-3706, April 2016. 5. C. DellaCorte and W.A.Wozniak, “Design and Manufactur- ing Considerations for Shockproof and Corrosion-Immune Superelastic Nickel-Titanium Bearings for a Space Station Application,” NASA TM, 2012-216015. 6. “High Hardness, High Elasticity Intermetallic Compounds For Mechanical Components,” U.S. Patent 10,364,483, July 2019. 9 FEATURE