Shape Memory Alloys
1. SMA recover to their original shape when heated. Will they return to deformed shape when cooled again?
SMA can be deformed when they are cool. They recover to the original shape when heated, but do not return to deformed shape when cooled again.
The force needed to deform the alloy when they are cool is approx. one third of the force that will be generated by the same alloy when they are heated. It is possible to memorize the alloy with two different shapes for Heated and Cooled conditions, however, it is not realized for the industrial use yet due to several demerits such as “volume of deformation cannot be controlled”, “low reliability”, and “high cost”.
To have the application move the opposite direction for the Heated / Cooled time, it is popular to use alternative power source from outside such as bias spring to move when cooled.
2. Up to how much deformation is applicable?
If the number of the thermal cycles is small for the application, distortion up to 7 % strain will be completely recovered. If over 10,000 cycles are required, the maximum shear strain should be controlled within 1%. (it may slightly vary by the alloy type and memory treatment conditions.)
3. How strong is the Force generated when heated? The value of Elasticity?
The recovery force when heated is approx. one third of force that of equivalent Stainless Steel Alloy would generate. The force at the low temperature is even one third force at heating. The following table shows mechanical performances at high and low temperatures.
4. How accurate can the shape recovery temperature be set?
Recovery temperature usually designated by specifying requested Af temperature (the temperature alloy will fully recover its original shape when heated). We generally ask for approx. +/- 5 °C allowance at the sample stage.
Following conditions will seriously damage the shape memory property and its fatigue life. It is good idea to avoid them.
– Bind and leave in the temperature more than 60°C higher than Af temperature.
– Leave without binding in the temperature more than 200°C higher than Af point.
– Actuate in the temperature from 60°C lower than Af point to higher than Af point in repetition.
5. How is Fatigue Life? How many times can they retrieve the original shape?
Assuming the maximum shear strain within 1 %, and following the conditions of Q4, several hundred thousand cycles can be achieved.
If either one of above condition can not be set, recovery force will be dramatically reduced before 1,000 cycles and will not be able to return to its original shape.
6. How is Thermal Conductivity? Response speed of actuation by heat?
Thermal conductivity is not so good, it is about same as stainless steel. It settles itself to be low temperature phase when reaching 15°C below the Af point. Response speed is dependent on how fast can it be heated to set temperature, and how quickly can it be cooled.
7. How is Workability? Welding, soldering or stamping is possible?
Welding same SMA each other is possible, but it may result welded portion physically weaker or thermal property to be changed. Welding with other alloy is generally impossible.
Soldering is also possible, however, the conditions are severe. Mechanical jointing such as clamping is preferred.
Its plate can be pressed up to about 0.5 mm thickness, but durability of pressing die will be the issue. Plates thicker than 0.5 mm will be processed with electric extrude wire or laser processor.
8. Can it be heated by flowing electric current?
Heating the SMA by flowing electric current can be considered when applying SMA in the application as an actuator with even temperature situation.
Fatigue property by over heating, and recovery speed on cooling should be considered.
9. Does Nickel or the elution of Nickel affect the human body such as Nickel allergies?
Elution of Nickel ions is found in the human body. However, depending on the environment, it shows the same or less elution performance of the Stainless Steel 304 alloy. It should be evaluated with the environmental condition of each medical device in advance.
10. What shapes or types are available of Ni-Ti alloys?
We have much record of round wire, square and rectangular wires, tubes and coil springs, but not plates or tapes.
Certainly depending on the condition as generating force, or deformation ratio and other, but generally speaking, coil spring has many advantages such as fatigue property, and easiness of applying bias spring together in the application.