Nickel-Titaniumalloy is often formed into wires; however, the manufacture of these wires isquite difficult as the Ni-Ti alloy displays some odd characteristics and actunlike other metals when being machined.
Ni-Ti alloy exhibits desirablecharacteristics in stretching, compressing, scaling, and ductility, but itstolerance falls when drawn at high temperatures (above 450°C) (Salonitis et al., 2009).However, the alloy displays the opposite characteristics when drawn at lowtemperatures (below 100°C). As a result, the process of manufacture for Nitinolwires tends to be: hot-roll the alloy into wires as thin as 2.5-3.
0 millimetres,at this point the alloy only experiences semi-super-elasticity due to the low yieldstrength of their austenite. Austenite is the crystal structure formed by thealloy when heated to high temperatures, austenite is often used to describe the”metallic, non-magnetic solid solution of carbon and iron that exists in steelabove the critical temperature of 723°C” (Berglund, 2006), however it can also be used to describethe Face-centred cubic structure of other alloys as well. The wires are then furtherdrawn. There are reports that claim that it is possible to tensile-deformed ina ductile manner at up to 50% strain prior to fracture but the wire experiencesextreme strain hardening which hampers the drawing process (Liu et al., 1997cited in Lei et al.
, 2010). Wu et al. suggest using a multi-pass drawingtechnique around the martensite start temperature (Wu, Lin and Yen, 1996).Martensite again is mostly used to refer to a form of steel crystallinestructure but is also used to describe a crystalline structure formed by diffusionlesstransformation. Diffusionless transformation meaning a phase change that occurswithout the long-range diffusion of atoms but with a collective movement ofmany atoms, such as, an entire layer moving together instead of just a few atoms.The ordered form of movement displayed in such a reaction lead some to refer tothem as ‘military transformations’ (Porterand Easterling, 1997).