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

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1
Q

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

A

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

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2
Q

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

A

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

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3
Q

What happens at low temps for shape memory alloys?

A

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

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4
Q

What happens at high temps for shape memory alloys?

A

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.

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5
Q

How can the transition temp zone be moved?

A

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

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6
Q

What is superelastic alloy?

A

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

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7
Q

What happens when a superelastic alloy is loaded?

A

It transforms directly to the deformed martensite phase.

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8
Q

What does this stress induced martensite transformation allow?

A

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

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9
Q

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

A

Yes/no

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10
Q

What percentage Ni and Ti are Ni-Ti alloys?

A

Approx 50% Ni and 50% Ti

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11
Q

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

A

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

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12
Q

What can be added to lower the transformation temp?

A

Iron and chromium

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13
Q

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

A

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

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14
Q

Where can the heat for transformation be from?

A

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

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15
Q

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

A

yes/no

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16
Q

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

A

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.

17
Q

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

A

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.

18
Q

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

A

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

19
Q

What can the shape memory alloy be used for?

A

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

20
Q

What can superelastic alloys be used for?

A

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

21
Q

What are the main features of SMA/superelastic alloys?

A
  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.
22
Q

What is the most common configuration for SMA actuators?

A

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

23
Q

How is a spring made?

A

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.

24
Q

What is the spring equation?

A

(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)

25
Q

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

A

Yes or no

26
Q

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

A

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.

27
Q

What are better methods of creating specific shapes?

A

Grinding, shearing and punching.