The property, above the reverse transformation finish temperature, of recovering large strains of several percent on unloading through stress-induced martensitic transformation and its reverse. Ni-Ti alloys are the typical example.
Japanese name: 超弾性
Superelasticity appears in 1 practice question across 1 topic, including Biomaterials.
Field: Biomaterials
When a superelastic alloy is loaded in tension above Af, the parent phase first deforms elastically in the ordinary way, and at a certain stress a flat region (plateau) appears in which large strain develops at almost constant stress. This is the range in which the parent phase transforms to stress-induced martensite. On unloading, the martensite reverts to the parent phase along a plateau at a lower stress, and the strain returns almost to zero. The difference in stress between the loading and unloading plateaus is called hysteresis, and in each cycle an energy corresponding to the area enclosed by the curve is lost as heat.
The plateau stress depends strongly on temperature: the higher the temperature, the more stable the parent phase, so a larger stress is needed for transformation. This relation is expressed by the Clausius–Clapeyron equation, and for Ni-Ti the plateau stress is reported to rise by a few MPa per K (roughly 5–8 MPa/K). At or above the temperature Md, where the stress needed for transformation exceeds the stress for slip, slip occurs instead of transformation and the strain no longer recovers. Superelasticity can therefore be used between Af and Md. The transformation temperatures of Ni-Ti change greatly with slight differences in Ni content, so they are tuned precisely through composition and heat treatment.
If the same alloy is deformed in the martensitic state below Mf, the strain remains after unloading, and the shape recovers when it is heated above Af. This is the shape memory effect; the difference from superelasticity is whether the reverse transformation is caused by removing the stress or by heating. Also, while the elastic strain limit of most metallic materials is less than 1%, superelasticity recovers strains of about 6–8%. The essential difference is that the strain arises not from stretching interatomic distances, as in ordinary elasticity, but from a change of crystal structure. A nearly constant force partway through deformation is also a feature that ordinary elastic bodies lack.
In orthodontic wires, the stress on the unloading plateau is nearly constant, so the force on the teeth changes little even as the teeth move and the strain decreases. Self-expanding stents are folded thin, carried into a blood vessel by a catheter and, when pushed out, expand to their original diameter and hold the vessel open with a constant force. For this, Af is set below body temperature. Guidewires and the files used in root canal treatment exploit the fact that they recover even after large bending. On the other hand, evaluation of long-term reliability, such as fatigue under repeated deformation and surface treatments that suppress Ni release, is essential.
Answer: C. Under load the parent phase transforms to martensite (stress-induced martensitic transformation), and on unloading it reverts and the strain recovers
Above Af, the parent phase with the B2 structure is stable. When stress is applied, the parent phase transforms to martensite at a certain stress (stress-induced martensitic transformation), and a large strain develops at a nearly constant stress. On unloading the martensite becomes unstable and returns to the parent phase, so strains of up to about 6–8% recover. This property of deforming greatly and returning at a nearly constant force is used in orthodontic wires that move teeth with a constant force and in stents that are folded thin and inserted into blood vessels. Plastic deformation by dislocation slip is a permanent strain that does not recover on unloading. Strengthening with precipitates only raises the yield stress; the Young's modulus hardly changes, and it does not explain superelasticity.
Both come from martensitic transformation and its reverse. Superelasticity is used above Af, and the shape recovers simply on unloading; the shape memory effect involves deforming below Mf and recovering the shape by heating. Even in the same Ni-Ti alloy, which one appears depends on the relation between the service temperature and the transformation temperatures.
For Ni-Ti the limit of recoverable strain is about 6–8%. Beyond this, dislocation slip occurs even in the martensite and permanent strain remains. Above Md superelasticity itself does not occur, so the range of service temperature also needs attention.
The usual method is differential scanning calorimetry (DSC), which gives Ms, Mf, As and Af from the exothermic and endothermic peaks accompanying transformation on heating and cooling. For products, Af is also found by bending a specimen at low temperature, heating it and finding the temperature at which its shape has fully recovered.
Strengthening with precipitates only raises the yield stress; the Young's modulus hardly changes, and it does not explain superelasticity.
Which correctly describes the mechanism of superelasticity …Martensite Dislocation Young's modulus Biocompatibility Biodegradable polymer Bone cement Hydroxyapatite Osseointegration Stress shielding β titanium alloy