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Nitinol Glossary

A brief glossary of common terms related to nitinol.

Af-Temperature

Temperature, above which the phase transformation from martensite to austenite is fully completed during heating of the alloy.

Apeak-or Ap-Temperature

Temperature, at which the phase transformation from martensite to austenite shows the maximum of heat flow during heating. The alloy needs a certain amount of energy (heat flow) to initiate the phase transformation and to move the phase boundary through the alloy. The maximum is a value which can be accurately measured in a DSC equipment, whereas the As- and Af- temperature measurements are sometimes lacking reproducibility.

As-Temperature

Temperature, at which the phase transformation from martensite to austenite is initiated during heating of the alloy.

Actuator

A device made of a shape memory alloy that is able to perform mechanical work as it undergoes a phase transformation upon heating. Actuation force and displacement are functions of the device’s geometry and design.

Annealing

Annealing is a heat treatment that alters a material, changing properties like hardness and strength. Annealing is used to bring about ductility, soften material, relieve internal stresses, refine the structure of a material by making it homogeneous and improve cold working properties. Nitinol is annealed to attain its superelasticty, and to shape set it.

Austenite

Austenite is the parent phase of nitinol. The crystal structure of austenite is defined as a simple cubic and exists in an ordered crystal lattice that is very stable at high temperatures. See martensite.

Bend and Free Recovery (BFR)

Tests which trace the shape recovery as a function of temperature.

Biocompatibility

General expression for the suitability of a material for use in the human body and in the endogenous fluids. For nitinol, a multitude of clinical studies and long clinical history have shown that its biocompatibility is excellent, especially in terms of cytocompatibility, haemocompatibility, genocompatibility, and corrosion performance. The most current studies report that the overall biocompatibility of nitinol alloys is comparable to stainless steel and Titanium alloys. In some applications, such as stents, recent studies suggest that the biocompatibility of nitinol alloys may be even superior to stainless steel.

DSC (Differential Scanning Calorimetry)

Calorimetric method for the characterization of a shape memory alloy with respect to its transformation temperatures. The DSC measures the specific heat flow, which changes constantly during the phase transformation. The overall accuracy and the ease of usage of the DSC equipment together with the high reproducibility makes it the most suitable method for alloy characterization. The main disadvantage of the DSC is the fact, that material deformation and external load cannot be simulated with this measurement.

Elastic Modulus

The elastic modulus refers to the ratio of stress to strain. The value of this proportion is a constant and depends on the material being deformed as well as the nature of the deformation. It is typically summarized through Hooke’s Law:

 

Stress-Strain Curve for Typical Nonferrous Alloy:

1: True elastic limit

2: Proportionality limit

3: Elastic limit

4: Offset yield strength

Hysteresis

a) thermal hysteresis:
The thermal hysteresis means generally the difference between the Apeak- and the Mpeak- temperatures. The hysteresis appears during passing of the transformation temperatures and is affected by a number of parameters (alloy composition, thermo-mechanical treatment, external load, etc.).

b) mechanical hysteresis:
The mechanical hysteresis appears during loading and unloading of a shape memory component in its high temperature phase above Af. The nitinol component shows a large amount of strain during loading, which recovers during unloading. The necessary force to initiate the transformation during loading is higher as the released force during unloading.

Martensite

Martensite is the daughter phase of nitinol. The crystal structure is more complex that austenite, and is known as monoclinic. It is stable at low temperatures. Once deformed in martensite, nitinol will remain deformed until heated to austenite where it will return to its pre-deformed shape (the shape memory effect).

Martensite

Crystallographic description of the low temperature phase of a shape memory alloy, which starts to form during cooling of the high temperature phase austenite when the Ms-temperature is passed. The formation of martensite is completed below the Mf-temperature. But the martensite can also be induced during loading of the austenite above Mf. This is related to a large amount of recoverable strain and is called _Superelasticity>.

One Way Effect (or 'thermal shape memory')

The one way effect occurs in a shape memory alloy, which has been deformed below its lower transformation temperature Ms. The obvious deformation is called pseudoplastic because the alloy recovers shape during subsequent heating into its austenitic high temperature phase. During this shape change the shape memory alloy is capable of providing a significant amount of work output and can be used as an actuator. The subsequent cooling in to the austenitic phase is normally not subject to a reverse shape change, as long as there is no external stress during austenite - martensite transformation.

Nitinol

Common trade name for the commercially most important family of shape memory alloys (nitinol alloys).

Permanent Set

Unrecovered strain experienced by a material upon unloading to a particular strain value.

Phase transformation

Physical mechanism, which is the metallurgical basis of the shape memory effect. The term martensitic phase transformation describes the formation of martensite during cooling OR during loading with an external stress of the austenitic high temperature phase.

R-Phase

The R-Phase of nitinol is an intermediate martensitic phase that competes with the martensite phase. The R-Phase has a rhombohedral crystal structure.

Superelasticity

Sometimes compared to a rubber-like deformability of shape memory alloys, which occurs during application of an external stress on the austenitic high temperature phase above Af. Thus, the superelastic effect is related to about 8% of recoverable elastic stress, for which temperature changes are not necessary. The superelasticity occurs basically in the same alloys as the thermal shape memory (one way effect).

Strain

Strain is a measure of deformation of a material.

Stress

Stress is force per unit area. Standard units are pounds per square inch or megapascals.

Superelasticity

Sometimes compared to a rubber-like deformability of shape memory alloys, which occurs during application of an external stress on the austenitic high temperature phase above Af. Thus, the superelastic effect is related to about 8% of recoverable elastic stress, for which temperature changes are not necessary. The superelasticity occurs basically in the same alloys as the thermal shape memory (one way effect).

Transformation Temperatures

Temperature values, at which start and finish of the phase transformation can be measured (Ms-, Mf-, As-, Af-temperature).

Thermomechanical treatment

Treatment of a shape memory alloy consisting out of a combination between cold working steps and annealing processes. The purpose of the thermomechanical treatment is the adjusting of a number of functional properties, like e.g. the phase transformation temperatures. Usually the annealing of the component or the semi-finished shape is the last step of the fabrication process.

Two-way Effect

Special form of the thermal shape memory effect, in which not just the heating but also the cooling of the shape memory element are subject to shape changes. This effect is induced either by a special thermomechanical treatment (_Training>) or through external stresses during cooling. For design and industrial application of actuators this latter option is highly preferred for as multitude of reasons.

Ultimate tensile strength (UTS):

The maximum resistance to fracture

Vacuum Arc Remolding

VAR is performed by striking an electrical arc between the raw material and a water-cooled copper strike plate. Melting is done in a high vacuum, and the mold itself is water cooled copper, so no carbon is introduced during melting. High purity alloys result from this process.

Vacuum Induction Melting

VIM is done by using alternating magnetic fields to heat the raw materials in a crucible (generally carbon). This is also done in a high vacuum, but carbon is introduced during the process.