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 act
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 collective 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’ (Porter
and Easterling, 1997).