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Taken from Book Materials Science and Engineering by William D. Callister, Jr (Very Good Book)
A relatively new group of metals that exhibit an interesting (and practical) phenomenon
are the shape-memory alloys (or SMAs). One of these materials, after having been deformed, has the ability to return to its pre-deformed size and shape upon being subjected to an appropriate heat treatment—that is, the material “remembers” its previous size/shape. Deformation normally is carried out at a relatively low temperature, whereas, shape memory occurs upon heating1.
Materials that have been found to be capable of recovering significant amounts of deformation (i.e., strain) are nickel–titanium alloys (Nitinol2, is their tradename), and some copper-base alloys (viz.Cu–Zn–Al and Cu–Al–Ni alloys).
Figure 1: Time-lapse photograph that demonstrates the shapememory effect. A wire of a shape-memory alloy (Nitinol) has been bent and treated such that its memory shape spells the word “Nitinol”. The wire is then deformed and, upon heating (by passage of an electric current), springs back to its pre-deformed shape; this shape recovery process is recorded on the photograph. [Photograph courtesy the Naval Surface Warfare Center (previously the Naval Ordnance Laboratory)].
A shape memory alloy is polymorphic that is, it may have two crystal structures (or phases), and the shape-memory effect involves phase transformations between them.
One phase (termed an austenite phase) has a body-centered cubic structure that exists at elevated temperatures; its structure is represented schematically by the inset shown at stage 1 of Figure 2. Upon cooling, the austenite transforms spontaneously to a martensite phase, which is similar to the martensitic transformation for the iron–carbon system that is, it is diffusionless, involves an orderly shift of large groups of atoms, occurs very rapidly, and the degree of transformation is dependent on temperature; temperatures at which the transformation begins and ends are indicated by “Ms ” and “Mf” labels on the left vertical axis of Figure 2.In addition, this martensite is heavily twinned3, asrepresented schematically by the stage 2 inset, Figure 2. Under the influence of an applied stress, deformation of martensite (i.e., the passage from stage 2 to stage 3, Figure 2) occurs by the migration
of twin boundaries—some twinned regions grow while others shrink; this deformed martensitic structure is represented by the stage 3 inset. Furthermore, when the stress is removed, the deformed shape is retained at this temperature. And, finally, upon subsequent heating to the initial temperature, the material reverts back to (i.e., “remembers”) its original size and shape (stage 4).
Figure :2 :Diagram illustrating the shape memory effect. The insets are schematic representations of the crystal structure
at the four stages. and denote temperatures at which the martensitic transformation begins and ends. Likewise for the
austenite transformation, and represent beginning and end transformation temperatures. (Adapted from BALL, Philip MADE TO MEASURE.)
This stage 3–stage 4 process is accompanied by a phase transformation from the deformed
martensite to the original high-temperature austenite phase. For these shape memory alloys, the martensite-to-austenite transformation occurs over a temperature range, between temperatures denoted by “ As” (austenite start) and “Af” (austenite finish) labels on the right vertical axis of Figure 2. Of course, this deformation–transformation cycle may be repeated for the shape memory material.
The original shape (to be remembered) is created by heating to well above the "Af"temperature (such that the transformation to austenite is complete), and then restraining the material to the desired memory shape for a sufficient time period.
For example, for Nitinol alloys, a one-hour treatment at 500 C is necessary.
Although the deformation experienced by shape-memory alloys is semipermanent, it is not
truly “plastic” deformation, neither is it strictly “elastic”.
Rather, it is termed “thermoelastic,” since deformation is nonpermanent when the deformed
material is subsequently heat treated. The stress versus strain behavior of a thermoelastic material is presented in Figure 3. Maximum recoverable deformation strains for these materials are on the order of 8%.
Figure 3: Typical stress–strain behavior of a shape-memory alloy, demonstrating its thermoelastic
behavior. The solid curve was generated at a temperature below that at which the martensitic transformation is complete (i.e.,Mf of Figure 2). Release of the applied stress corresponds to passing from point P to point Q. Subsequent heating to above the austenite–completion transformation temperature ( Af of Figure 2), causes the deformed piece to resume its original shape (along the dashed curve from point Q to point R).[Adapted from ASM Handbook, Vol. 2, Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, J. R.
Davis (Manager of Handbook Development)]
For this Nitinol family of alloys, transformation temperatures can be made to vary over a wide temperature range (between about -200 C and 110 C ), by altering the Ni–Ti ratio, and also by the addition of other elements.
One important SMA application is in weldless, shrink-to-fit pipe couplers used for hydraulic lines on aircraft, for joints on undersea pipelines, and for plumbing on ships and submarines. Each coupler (in the form of a cylindrical sleeve) is fabricated so as to have an inside diameter slightly smaller than the outside diameter of the pipes to be joined. It
is then stretched (circumferentially) at some temperature well below the ambient. Next the coupler is fitted over the pipe junction, and then heated to room temperature; heating causes the coupler to shrink back to its original diameter, thus creating a tight seal between the two pipe sections.
There is a host of other applications for alloys displaying this effect—for example, eyeglass
frames, tooth-straightening braces, collapsible antennas, greenhouse window openers, antiscald control valves on showers, women’s foundations, fire sprinkler valves, and in biomedical applications (as blood-clot filters, self-extending coronary stents, and bone anchors). Shape-memory alloys also fall into the classification of “smart materials”
since they sense and respond to environmental (i.e., temperature) changes.
1 Alloys that demonstrate this phenomenon only upon heating are said to have a one-way shape memory. Some
of these materials experience size/shape changes on both heating and cooling; these are termed two-way shape
memory alloys. In this discussion, we discuss the mechanism for only the one-way shape-memory.
2 “Nitinol” is really an acronym for nickel-titanium Naval Ordnance Laboratory, where this alloy was discovered.
Taken from Book Materials Science and Engineering by William D. Callister, Jr (Very Good Book)
A relatively new group of metals that exhibit an interesting (and practical) phenomenon
are the shape-memory alloys (or SMAs). One of these materials, after having been deformed, has the ability to return to its pre-deformed size and shape upon being subjected to an appropriate heat treatment—that is, the material “remembers” its previous size/shape. Deformation normally is carried out at a relatively low temperature, whereas, shape memory occurs upon heating1.
Materials that have been found to be capable of recovering significant amounts of deformation (i.e., strain) are nickel–titanium alloys (Nitinol2, is their tradename), and some copper-base alloys (viz.Cu–Zn–Al and Cu–Al–Ni alloys).
A shape memory alloy is polymorphic that is, it may have two crystal structures (or phases), and the shape-memory effect involves phase transformations between them.
One phase (termed an austenite phase) has a body-centered cubic structure that exists at elevated temperatures; its structure is represented schematically by the inset shown at stage 1 of Figure 2. Upon cooling, the austenite transforms spontaneously to a martensite phase, which is similar to the martensitic transformation for the iron–carbon system that is, it is diffusionless, involves an orderly shift of large groups of atoms, occurs very rapidly, and the degree of transformation is dependent on temperature; temperatures at which the transformation begins and ends are indicated by “Ms ” and “Mf” labels on the left vertical axis of Figure 2.In addition, this martensite is heavily twinned3, asrepresented schematically by the stage 2 inset, Figure 2. Under the influence of an applied stress, deformation of martensite (i.e., the passage from stage 2 to stage 3, Figure 2) occurs by the migration
of twin boundaries—some twinned regions grow while others shrink; this deformed martensitic structure is represented by the stage 3 inset. Furthermore, when the stress is removed, the deformed shape is retained at this temperature. And, finally, upon subsequent heating to the initial temperature, the material reverts back to (i.e., “remembers”) its original size and shape (stage 4).
Figure :2 :Diagram illustrating the shape memory effect. The insets are schematic representations of the crystal structure
at the four stages. and denote temperatures at which the martensitic transformation begins and ends. Likewise for the
austenite transformation, and represent beginning and end transformation temperatures. (Adapted from BALL, Philip MADE TO MEASURE.)
This stage 3–stage 4 process is accompanied by a phase transformation from the deformed
martensite to the original high-temperature austenite phase. For these shape memory alloys, the martensite-to-austenite transformation occurs over a temperature range, between temperatures denoted by “ As” (austenite start) and “Af” (austenite finish) labels on the right vertical axis of Figure 2. Of course, this deformation–transformation cycle may be repeated for the shape memory material.
The original shape (to be remembered) is created by heating to well above the "Af"temperature (such that the transformation to austenite is complete), and then restraining the material to the desired memory shape for a sufficient time period.
For example, for Nitinol alloys, a one-hour treatment at 500 C is necessary.
Although the deformation experienced by shape-memory alloys is semipermanent, it is not
truly “plastic” deformation, neither is it strictly “elastic”.
Rather, it is termed “thermoelastic,” since deformation is nonpermanent when the deformed
material is subsequently heat treated. The stress versus strain behavior of a thermoelastic material is presented in Figure 3. Maximum recoverable deformation strains for these materials are on the order of 8%.
behavior. The solid curve was generated at a temperature below that at which the martensitic transformation is complete (i.e.,Mf of Figure 2). Release of the applied stress corresponds to passing from point P to point Q. Subsequent heating to above the austenite–completion transformation temperature ( Af of Figure 2), causes the deformed piece to resume its original shape (along the dashed curve from point Q to point R).[Adapted from ASM Handbook, Vol. 2, Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, J. R.
Davis (Manager of Handbook Development)]
For this Nitinol family of alloys, transformation temperatures can be made to vary over a wide temperature range (between about -200 C and 110 C ), by altering the Ni–Ti ratio, and also by the addition of other elements.
One important SMA application is in weldless, shrink-to-fit pipe couplers used for hydraulic lines on aircraft, for joints on undersea pipelines, and for plumbing on ships and submarines. Each coupler (in the form of a cylindrical sleeve) is fabricated so as to have an inside diameter slightly smaller than the outside diameter of the pipes to be joined. It
is then stretched (circumferentially) at some temperature well below the ambient. Next the coupler is fitted over the pipe junction, and then heated to room temperature; heating causes the coupler to shrink back to its original diameter, thus creating a tight seal between the two pipe sections.
There is a host of other applications for alloys displaying this effect—for example, eyeglass
frames, tooth-straightening braces, collapsible antennas, greenhouse window openers, antiscald control valves on showers, women’s foundations, fire sprinkler valves, and in biomedical applications (as blood-clot filters, self-extending coronary stents, and bone anchors). Shape-memory alloys also fall into the classification of “smart materials”
since they sense and respond to environmental (i.e., temperature) changes.
1 Alloys that demonstrate this phenomenon only upon heating are said to have a one-way shape memory. Some
of these materials experience size/shape changes on both heating and cooling; these are termed two-way shape
memory alloys. In this discussion, we discuss the mechanism for only the one-way shape-memory.
2 “Nitinol” is really an acronym for nickel-titanium Naval Ordnance Laboratory, where this alloy was discovered.
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