Shape memory alloys (SMAs) are metals that "remember" their original shapes.
Shape memory alloys (SMA’s) are metals which exhibit two very unique properties, pseudo-elasticity and the shape memory effect. SMA’s are useful for such things as actuators which are materials that “change shape, stiffness, position, and other mechanical characteristics in response to temperature or electromagnetic fields”. The alloys that exhibit so-called “shape memory” can undergo surprisingly large amounts of strain and then, upon temperature increase or unloading, revert to their original shape. Ni-Ti based shape memory alloys have to date provided the best combination of materials properties for most commercial applications.
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Af – Austenite Finish
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The temperature at which the Martensite to Austenite transformation is completed in a single stage transformation.
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As – Austenite Start
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The temperature at which the Martensite to Austenite transformation begins on heating in a single stage transformation.
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Activation Range
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The distance a spring can be stretched, or the angle a wire can be bent, without permanent deformation; the useful, working range of a spring or wire.
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Anneal
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A heat treat process designed to remove the effects of cold work and/or aging heat treatments.
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Austenite
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The high temperature phase of Nickel Titanium. In its austenitic phase, NiTi is stiff and elastic and will immediately return to shape after bending. (See Superelasticity.) Austenitic NiTi wire has the B2 body-centered cubic crystal structure.
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BFR Test
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The Bend and Free Recovery test provides an analysis of bend angle to temperature, and it can be used in determining As and Af temperatures of a NiTi wire.
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Compression Spring
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Springs which are manufactured with open spaces between helixes. Compression springs are typically used in applications were they are compressed to provide a “pushing” force.
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DSC Analysis
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Differential Scanning Calorimetry – a laboratory procedure used to identify the thermally induced transformations of a particular alloy.
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Elasticity
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The apparent ease with which a wire can be deflected without incurring permanent deformation.
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Extension Spring
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Springs which are manufactured with closed coils, such that the helixes are wound tightly together. Extension springs are typically used in tensile to provide pulling forces.
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Hysteresis
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The difference in bending loads during loading and unloading in a test such as 3 Point Bending.
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Martensite
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The low temperature phase of Nickel Titanium. In its martensitic phase, Niti is soft and bends easily. NiTi will stay in this state until exposed to heat above its TTR. NiTi in its martensitic state is responsive to chilling, as it becomes even softer and more easily bent. Martensitic NiTi has the B19 monoclinic crystal structure.
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Md – Martensite Desist
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The temperature above Af at which stress induced martensite will no longer form. Typically =80°C above functional Af.
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Mf - Martensitic Finish
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The temperature at which the transformation from Austenite to Martensite is completed on cooling a single stage transformation.
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Ms – Martensite Start
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The temperature at which the transformation from Austenite to Martensite begins on cooling in a single stage transformation.
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Permanent Deformation
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Usually in reference to the angle or percentage that a wire or component does not return to its original shape after being strained.
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Resiliency
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A material’s ability to resist permanent deformation when bent.
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Shape Memory
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An apparent plastic deformation in the martensitic phase, which undergoes a thermo-elastic change in the crystal structure when heated through its transformation temperature range, resulting in a recovery of the deformation. The ability for a preformed wire, below its TTR, to be deformed to some degree and then, upon heating, to return to its original shape. Further details are available on the Material Characteristics page.
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Stress Induced Martensite
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The spontaneous phase change of the crystal lattice from Austenite to Martensite as stress is applied to the material at a temperature above Af. Stress Induced Martensite spontaneously reverts to Austenite as stress is removed, unless permanent deformation has been introduced into the material.
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Superelasticity
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A nonlinear recoverable deformation behavior of Nickel Titanium shape memory alloys at temperatures above Af. The nonlinear deformation arises from the stress-induced formation of martensite on loading and the spontaneous reversion of this crystal structure to austenite upon unloading. Further details are available on the Material Characteristics page.
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Tensile Strength
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When stretched, the maximum force a wire can withstand without breaking.
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Thermal
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Term used for martensitic wire, as it is responsive to heat-activation. See Martensite.
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Transformation Temperature Range (TTR)
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For shape memory alloys, such as Nickel Titanium, a change of phase occurs, such as from martensite to austenite, in a specific temperature range. The TTR is adjustable by several means to produce desired characteristics.
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Shape Memory Alloys (SMA's) are novel materials which have the ability to return to a predetermined shape when heated. When an SMA is cold, or below its transformation temperature, it has a very low yield strength and can be deformed quite easily into any new shape--which it will retain. However, when the material is heated above its transformation temperature it undergoes a change in crystal structure which causes it to return to its original shape. If the SMA encounters any resistance during this transformation, it can generate extremely large forces. This phenomenon provides a unique mechanism for remote actuation.
The most common shape memory material is an alloy of nickel and titanium called Nitinol. This particular alloy has very good electrical and mechanical properties, long fatigue life, and high corrosion resistance.
Shape memory alloys, however, are not for all applications. One must take into account the forces, displacements, temperature conditions, and cycle rates required of a particular actuator.
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