Shape Memory Alloys Flashcards
What are SMA used as?
They are used as actuators, due to it’s ability to remember a particular shape.
Describe the phases of SMA.
In a SMA, two different phased can be present inside the material:
- Austenite (parent phase)
- Martensite (product phase)
The austenitic phase is generated during the production of the alloy (at high temperature) and this is why is called parent phase.
The martensitic phase is obtained from the first phase (during cooling) and it’s called product phase. The crystalline structure in the two phases is different:
1) The austenite shows a body centered cubic structure with a very high level of symmetry. It exhibits very good mechanical properties.
2) The martensite shows a monoclinic structure with a lower level of symmetry and exhibits lower stiffness.
The austenitic phase can be in only one state. The martensitic phase can be:
- Twinned. It is obtained when we go from austenite to martensite by cooling with no load applied. It is not oriented and it has 24 possible orientations of couple of cells.
- Detwinned. By appling the load, we orient the martensite and we obtained this phase.
Also the R-Phase exists, which is an intermediate phase during the cooling from Austenite to Martensite.
How is it possible to pass from Austenite to Martensite and vice versa?
1) It is possible to pass from martensite twinned to austenite by heating.
2) It is possible to pass from Austenite to Martensite (twinned and detwinned) by cooling
3) By applying a stress in austenite we can directly move towards martensite detwined. When we remove the stress, if the material is in temperature where it is stable in martensite, it will remain in that state (detwinned).
4) It is possible to pass from Martensite detwinned to Austenite by applying a stress if the temperate is higher than AF.
5) It is not possible to go from Md to Mt.
What are the transformation temperatures?
There are 4 transformation temperatures.
- Austenite Start
- Austenite finish
- Martensite start
- Martensite finish
There is an hysteresis on the Martensitic fraction - Temperature graph. The area inside the graph is the energy dissipated during a complete transformation.
What happens in the microscopic behaviour?
The state which corresponds to the minimum of Gibbs free energy is the stable state. The orientation of the couple of cells and the movement inside the structure are possible due to the presence of mobile planes called habit planes (they are responsible for the reversible transformation).
What is the macroscopic behaviour?
By moving from one phase to another, the material is able to show macroscopic changes:
i) After an elongation, the material remains elongated
ii) When heated, the material recovers its original shape
The two drivers drivers of the transformation are the strain and the temperature change (heating). The martensitic transformation is a thermo-elastic reversible transformation, not diffusive transformation (it is instantaneous).
What do the thermo-mechanical properties of SMA depend on?
They depend on:
- The chemical composition
- The manufacturing process (eg cold working). This has an impact on characteristic temperatures.
- Thermal treatments
- Thermo-mechanical cycles (both during training and operative life)
What are the most known SMA and what are its properties?
The most known shape memory alloys is the NiTiNOL (Nickel-Titanimum-Naval-Ordinance-Laboratory). It is a equi-atomic alloy of Ni and Ti.
1) By increasing the presence of Ti, the transformation temperatures increase.
2) The activatation temperatures vary from -30 to 170 deg celcius.
3) The maximum recoverable strain is in the range of 5-8%.
4) The material is corrosion resistand and is biocompatible.
What do the characteristic temperatures depend on?
The characteristic temperatures depend on the stress applied (it increases with the stress). So if a stress is applied, we need to increase the temperature in the material in order to start the transformation.
Ms = MS0 + σ/CM and Mf = MF0 + σ/CM.
As = AS0 + σ/CA and Af = AF0 + σ/CA.
CA, CM constants.
What is the critical temperature Md?
Md is a critical temperature where if T>Md, we can’t promote the transformation towards martensite detwinned only by applying stress. That is because if the force required to induce the transformation is too high we obtain yielding of the material before the transformation occurs.
How can one obtain the transformation temperatures without stress applied?
The Differential Scanning Calorimeter Analysis can be used. When the transformation occurs, the system reads the release of energy.
How can one obtain the transformation temperatues at different stress levels?
They can be obtained with iso-stress mechanical tests. A constant stress is applied inside the material and the transformation is promoted by heating - cooling. When the material transforms, it is able to recover the strain (intial shape). By observing the change of the deformation inside the material, it is possible to associate the starting point and the ending point of the cycle to characteristic temperatures.
What is the One Way memory effect?
The material is able to remember one shape when moving towards the austenitic state: we start from a point where the material is in twinned martensite and there is no strain inside the material. By increasing the stress applied, we go from twinned to detwinned and we obtain a permanent deformation. After removing the load, the deformation remains in the material. The deformation can be recovered by increasing the temperature up to Af. After cooling, the material recovers the shape and is in martensite twinned phase, with no strain inside the material.
What is the Two Ways memory effect?
Two way memoery effect means that the material is able to remember a second shape associated to a state where twinned and detwinned martensite coecist. It is possible to associate different shapes to this partially detwinned state. There are some thermomechanical treatments (called training) needed to train the material to remember this particular shape.
At the beggining the material is in a partially detwinned state. By increasing the load, all the material becomes detwinned. After removing the load, we have the recovery of the elastic strain in the detwinned state. At this point, by increasing temperature, the austenitic transformation starts after reaching As. When Af is reached, the strain generated during the transformation from partially detwinned to 100% detwinned martensite is recovered. Now, by cooling the material, the martensitic transformation begins after Ms. Now the material presents a deformation when moving from austenite to martensite. After reaching Mf the material returns to the initial point without changing shape.
What is superelasticity?
Superelasticity (or pseudoelasticity) is the ability of the material to recover large deformation during load-unload cycle. The effect happens if the material is loaded at T>Af.
With the incresae of the stress, after an initial elastic part of the curve stress vs strain in austenite, we can observe a plateau. (the material can be elongated until the martensite is completely oriented with a more or less constant stress). Then there is the elastic part of the curve in detwinned martensite. If we don’t reach the yielding point, the material is able to recover its original shape after removing the load. The recovery is possible thank to the transformation towards austenite, since the material is stable in austenite (T>Af). The generation of martensite starting from austenite by applying a stress is known as SIM (Stress Induced Martensite)
