Nickel-Titanium 50% strain prior to fracture but the

Nickel-Titanium alloy is often formed into
wires; however, the manufacture of these wires is quite difficult as the Ni-Ti
alloy displays some odd characteristics and behave unlike other metals when
being machined. Ni-Ti alloy exhibits desirable characteristics in stretching,
compressing, scaling, and ductility, but its tolerance falls when drawn at high
temperatures (above 450°C)
(Salonitis et al., 2009). However, the
alloy displays the opposite characteristics when drawn at low temperatures
(below 100°C). As a result, the process of manufacture for Nitinol wires 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 yield strength
of their austenite. Austenite is the crystal structure formed by the alloy when
heated to high temperatures, austenite is often used to describe the “metallic,
non-magnetic solid solution of carbon and iron that exists in steel above the
critical temperature of 723°C” (Berglund, 2006),
however it can also be used to describe the Face-centred cubic structure of other
alloys as well. The wires are then further drawn. There are reports that claim
that it is possible to tensile-deformed in a ductile manner at up to 50% strain
prior to fracture but the wire experiences extreme strain hardening which hampers
the drawing process (Liu et al., 1997 cited in Lei et al., 2010). Wu et
al. suggest using a multi-pass drawing technique around the martensite start
temperature (Wu, Lin and Yen, 1996). Martensite again is mostly used to
refer to a form of steel crystalline structure but is also used to describe a
crystalline structure formed by diffusionless transformation. Diffusionless
transformation meaning a phase change that occurs without the long-range diffusion
of atoms but with a more homogenous movement of many 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 to them as ‘military transformations’.