Shape memory alloys (SMA) and superelastic alloys (NITINOL) Flashcards
What is the basic phenomena that results in shape memory and superelastic effects?
Thermoelastic martensitic transformation of nickel-titanium (Ni-Ti) alloys.
How to explain this phenomena in simple terms for shape memory alloys?
SMAs have 2 sets of properties which occur above and below the transformation temperature
What happens at low temps for shape memory alloys?
A uniform martensite structure occurs which has low yield strength and is easily deformed
What happens at high temps for shape memory alloys?
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.
How can the transition temp zone be moved?
By choosing different alloying ratios and it can be modified further by cold work and final heat treatment.
What is superelastic alloy?
A special form of Ni-Ti where the transition temp is set below normal ambient (room temp).
What happens when a superelastic alloy is loaded?
It transforms directly to the deformed martensite phase.
What does this stress induced martensite transformation allow?
Strain values of up to 6% – only 0.5% for SS!
Can you draw the diagrams of stress strain behaviours for SMA/Superelastic alloys?
What percentage Ni and Ti are Ni-Ti alloys?
Approx 50% Ni and 50% Ti
What happens if the Ni is increased slightly, say by 1%?
This strongly depresses the phase transformation temp and increases the yield strength of the austenite.
What can be added to lower the transformation temp?
Iron and chromium
By varying the elements what range can the transformation temp vary between?
-10 degC to 100 degC with repeatable accuracy of +/- 4 degC
Where can the heat for transformation be from?
Hot air gas or liquid, radiant heat or by electrical resistance heating
Can you draw a schematic showing the mechanisms of the shape memory and superelastic effects?