Shape memory alloys (SMA) and superelastic alloys (NITINOL)

Question 1

What is the basic phenomena that results in shape memory and superelastic effects?

Answer 1

Thermoelastic martensitic transformation of nickel-titanium (Ni-Ti) alloys.

Question 2

How to explain this phenomena in simple terms for shape memory alloys?

Answer 2

SMAs have 2 sets of properties which occur above and below the transformation temperature

Question 3

What happens at low temps for shape memory alloys?

Answer 3

A uniform martensite structure occurs which has low yield strength and is easily deformed

Question 4

What happens at high temps for shape memory alloys?

Answer 4

There is a threefold increase in stiffness, the metal takes on properties of a high tensile spring steel and returns to the shape previously imprinted on it by heat treatment.

Question 5

How can the transition temp zone be moved?

Answer 5

By choosing different alloying ratios and it can be modified further by cold work and final heat treatment.

Question 6

What is superelastic alloy?

Answer 6

A special form of Ni-Ti where the transition temp is set below normal ambient (room temp).

Question 7

What happens when a superelastic alloy is loaded?

Answer 7

It transforms directly to the deformed martensite phase.

Question 8

What does this stress induced martensite transformation allow?

Answer 8

Strain values of up to 6% – only 0.5% for SS!

Question 9

Can you draw the diagrams of stress strain behaviours for SMA/Superelastic alloys?

Answer 9

Yes/no

Question 10

What percentage Ni and Ti are Ni-Ti alloys?

Answer 10

Approx 50% Ni and 50% Ti

Question 11

What happens if the Ni is increased slightly, say by 1%?

Answer 11

This strongly depresses the phase transformation temp and increases the yield strength of the austenite.

Question 12

What can be added to lower the transformation temp?

Answer 12

Iron and chromium

Question 13

By varying the elements what range can the transformation temp vary between?

Answer 13

-10 degC to 100 degC with repeatable accuracy of +/- 4 degC

Question 14

Where can the heat for transformation be from?

Answer 14

Hot air gas or liquid, radiant heat or by electrical resistance heating

Question 15

Can you draw a schematic showing the mechanisms of the shape memory and superelastic effects?

Answer 15

yes/no

Question 16

Can you explain/describe the stress strain curve for an ordinary metal?

Answer 16

In an ordinary metal, strain caused by an external stress disappears if it is below the limit of elasticity of the metal. If a deforming strain is given to the metal beyond its elastic limit, a permanent deformation remains, the strain being reduced only by as much as the elastic deformation even when the stress is removed.

Question 17

Can you explain/describe the stress strain curve for a superelastic alloy?

Answer 17

The superelastic alloy given a deforming strain beyond the range of elasticity and therefore an apparent plastic deformation recovers to its original shape when it is relieved of the external stress.

Question 18

Can you explain/describe the stress strain curve for a shape memory alloy?

Answer 18

The shape memory alloy deforms plastically under a stress but returns to its original shape when heated.

Question 19

What can the shape memory alloy be used for?

Answer 19

As a thermo-sensor and actuator where properties make it a superior alternative to existing mechanisms. SMA are also used in louver applications.

Question 20

What can superelastic alloys be used for?

Answer 20

Medical applications such as orthodontic teeth correction as a replacement to conventional stainless steel.

Question 21

What are the main features of SMA/superelastic alloys?

Answer 21

1. High recovery force, stress up to 600MPa can be generated upon heating
2. Totally silent in operation – often without lubrication
3. Large recoverable deformation strain, up to 6% for a few cycles
4. Good resistance to strain controlled fatigue when used in long life applications
5. Excellent corrosion resistance
6. Biocompatibility for medical applications – comparible to series 300 SS.

Question 22

What is the most common configuration for SMA actuators?

Answer 22

Wire wound into a tension or compression spring shape, giving large stroke

Question 23

How is a spring made?

Answer 23

Using a hard drawn wire, forming of a helix on a shaft and clamping the shape while heating for 1 hour at 450-500 degC.

Question 24

What is the spring equation?

Answer 24

(G.d.delta)/(pi.n.D^2) = (8.k.P.D)/(Pi*d^3)
Draw spring to label D, d and P
delta = spring deflection
n = number of turns
k = constant = (D/d)/((D/d) - 1)
G = modulus of rigidity or shear modulus of elasticity: G=E/2(1+nu)

Question 25

Can you draw the diagram showing the concept of differential actuators?

Answer 25

Yes or no

Question 26

Why is manufacturing Ni-Ti SMA and shaping it for a specific purpose not an easy task?

Answer 26

Ti is very reactive so melting must be done in an inert atmosphere. Machining Ni-Ti through cutting methods is difficult, as is welding, brazing and soldering.

Question 27

What are better methods of creating specific shapes?

Answer 27

Grinding, shearing and punching.