TechNyou Education: Shape Memory Alloys

Nitinol is an alloy made of nickel and titanium
with a capacity to change shape and then change back again.
This shape changing behavior
sets it in a special class of man-made materials
known and Shape Memory Alloys. If nitinol wire is given a certain shape
it remembers that shape. It can be twisted or deformed into new shapes at
lower temperatures.
But, if it is reheated to it's remembering temperature, it will quickly
recover its remembered shape.
The shape memory behaviors
result from temperature induced changes
to its crystal structures at the atomic level.
Nitinol occupies a niche of its own among shape memory alloys as an
engineering material.
It mimics human tendons and human hair and has many applications in
orthodontics,
optometry,
electronics and medical surgery.
It is also valued by artists
for its sculptural and kinetic qualities.
In this experiment,
we demonstrate how we can manipulate the crystal structures in some man-made
materials to influence their behaviors.
The experiment will be conducted in three parts
to explore the properties of nitinol
and show how it can be programmed
at certain temperatures to remember a shape.
This is a very simple experiment to do,
we need only a few items of equipment,
approximately 10 centimeters of each of the following wires:
nitinol, copper,
and a steel paper clip,
a set of tongs or pliers,
a 1 litre beaker,
a bunsen burner,
lighter or matches,
hot water, a hairdressers dummy head wired with nitinol as shown,
a hair dryer.
This experiment is about shape memory alloys
and this is a shape memory alloy known as nitinol
which is made from nickel and titanium.
In Part One of the experiment, we will use three different metals:
the nitinol wire,
a steel paper clip
and copper wire.
We will bend the three straight wires at room temperature
so each wire is similarly shaped.
Our three metal wires all now have a V-shape.
We want to know which, if any, of these materials
will return to its original shape.
So first we drop in our paper clip.
As you can see the shape we bent into it is still the same in the hot
water.
We'll now drop in the copper wire.
It too has kept its new V-shape.
Now let's try the nitinol wire.
We have to be very careful when it goes into the hot water because it will
change shape quickly.
As you can see
the nitinol wire has recovered its original straight shape.
We've just observed how differently nitinol behaves from the other metals
and identical conditions in controlled temperatures.
The two common metals retained their assigned new shape. But, nitinol appeared
to remember
and recover its original shape.
Let's run that memory test again.
This time we will use a hairdryer.
Meet Dolly, our nitinol model.
She's just had her hair done.
These are nitinol curls of course,
but clearly Dolly doesn't need a perm. Or does she?
Watch what happens when we pass hot air
over these curls.
Well, so much for the nitinol perm.
We've just seen the same behavior.
The nitinol hair remembered it's pre-programmed shape
and with a little encouragement from our hair salon and the right temperature,
it just snapped back straight again.
Part 3 will take our experiment further.
You will see how simple it is to give a shape memory alloy, like nitinol,
and a new shape it will remember,
by setting that shape as a memory at a high temperature.
In the first part of this experiment, we used nitinol wire
with a pre-programmed shape
that is a straight shape that has been built into its memory. We deformed it at
room temperature.
But, at a certain temperature known as the transition temperature,
it sprang back to its programmed shape.
How did we do that?
In this final part of the experiment we show you.
This time we bend the nitinol wire into another shape say,
roughly a loop. Then we place the wire loop in the bunsen burner flame. We can see
the wire heating. What we can't see is the crystal structure changing.
This is what gives nitinol a new memory.
We are reprogramming it structural behavior
at a specific temperature so the nitinol atoms remember the shape.
We'll allow the wire to cool down,
then we'll straighten it out.
It's pretty much straight. Watch what happens now
when we drop it into hot water.
Look, we have our loop back.
That's how easy it is.
Our nitinol wire has a new memory.
From here on, it's a loop.