Shape memory metals pdf writer
A shape-memory alloy is an alloy that can be deformed when cold but returns to its pre-deformed "remembered" shape when heated. It may also be called memory metal , memory alloy , smart metal , smart alloy , or muscle wire.
Parts made of shape-memory alloys can be lightweight, solid-state alternatives to conventional actuators such as hydraulic , pneumatic , and motor -based systems.
They can also be used to make hermetic joints in metal tubing. The two most prevalent shape-memory alloys are copper - aluminium - nickel and nickel - titanium NiTi , but SMAs can also be created by alloying zinc , copper , gold and iron.
Shape Memory Alloy
NiTi alloys change from austenite to martensite upon cooling; M f is the temperature at which the transition to martensite completes upon cooling. Accordingly, during heating A s and A f are the temperatures at which the transformation from martensite to austenite starts and finishes. Repeated use of the shape-memory effect may lead to a shift of the characteristic transformation temperatures this effect is known as functional fatigue, as it is closely related with a change of microstructural and functional properties of the material.
The transition from the martensite phase to the austenite phase is only dependent on temperature and stress, not time, as most phase changes are, as there is no diffusion involved. Similarly, the austenite structure receives its name from steel alloys of a similar structure. It is the reversible diffusionless transition between these two phases that results in special properties.
While martensite can be formed from austenite by rapidly cooling carbon - steel , this process is not reversible, so steel does not have shape-memory properties. The difference between the heating transition and the cooling transition gives rise to hysteresis where some of the mechanical energy is lost in the process.
The shape of the curve depends on the material properties of the shape-memory alloy, such as the alloy's composition  and work hardening. The shape memory effect SME occurs because a temperature-induced phase transformation reverses deformation, as shown in the previous hysteresis curve.
Typically the martensitic phase is monoclinic or orthorhombic B19' or B Since these crystal structures do not have enough slip systems for easy dislocation motion, they deform by twinning —or rather, detwinning. Martensite is thermodynamically favored at lower temperatures, while austenite B2 cubic is thermodynamically favored at higher temperatures. Since these structures have different lattice sizes and symmetry, cooling austenite into martensite introduces internal strain energy in the martensitic phase.
To reduce this energy, the martensitic phase forms many twins—this is called "self-accommodating twinning" and is the twinning version of geometrically necessary dislocations. Since the shape memory alloy will be manufactured from a higher temperature and is usually engineered so that the martensitic phase is dominant at operating temperature to take advantage of the shape memory effect, SMAs "start" highly twinned.
When the martensite is loaded, these self-accommodating twins provide an easy path for deformation.
Applied stresses will detwin the martensite, but all of the atoms stay in the same position relative to the nearby atoms—no atomic bonds are broken or reformed as they would be by dislocation motion. Thus, when the temperature is raised and austenite becomes thermodynamically favored, all of the atoms rearrange to the B2 structure which happens to be the same macroscopic shape as the B19' pre-deformation shape.
Shape-memory alloys have different shape-memory effects. Two common effects are one-way and two-way shape memory. A schematic of the effects is shown below.
The procedures are very similar: starting from martensite a , adding a reversible deformation for the one-way effect or severe deformation with an irreversible amount for the two-way b , heating the sample c and cooling it again d.
When a shape-memory alloy is in its cold state below A s , the metal can be bent or stretched and will hold those shapes until heated above the transition temperature. Upon heating, the shape changes to its original. When the metal cools again it will remain in the hot shape, until deformed again.
With the one-way effect, cooling from high temperatures does not cause a macroscopic shape change. A deformation is necessary to create the low-temperature shape. The two-way shape-memory effect is the effect that the material remembers two different shapes: one at low temperatures, and one at the high-temperature shape. A material that shows a shape-memory effect during both heating and cooling is said to have two-way shape memory. This can also be obtained without the application of an external force intrinsic two-way effect.
The reason the material behaves so differently in these situations lies in training. Training implies that a shape memory can "learn" to behave in a certain way. Under normal circumstances, a shape-memory alloy "remembers" its low-temperature shape, but upon heating to recover the high-temperature shape, immediately "forgets" the low-temperature shape. However, it can be "trained" to "remember" to leave some reminders of the deformed low-temperature condition in the high-temperature phases.
There are several ways of doing this. SMAs display a phenomenon sometimes called superelasticity, but is more accurately described as pseudoelasticity. Pseudoelasticty still achieves large, recoverable strains with little to no permanent deformation, but it relies on more complex mechanisms. SMAs exhibit at least 3 kinds of pseudoelasticty.
The two less-studied kinds of pseudoelasticity are pseudo-twin formation and rubber-like behavior due to short range order. The main pseudoelastic effect is comes from a stress-induced phase transformation. The figure on the right exhibits how this process occurs. Here a load is isothermally applied to a SMA above the austenite finish temperature, A f , but below the martensite deformation temperature, M d.
The figure above illustrates how this is possible, by relating the pseudoelastic stress-induced phase transformation to the shape memory effect temperature induced phase transformation.
The material initially exhibits typical elastic-plastic behavior for metals. However, once the material reaches the martensitic stress, the austenite will transform to martensite and detwin.
As previously discussed, this detwinning is reversible when transforming back from martensite to austenite.
Nitinol, Memory Metal, Formgedächtnislegierung, Büroklammer
If large stresses are applied, plastic behavior such as detwinning and slip of the martensite will initiate at sites such as grain boundaries or inclusions. The material will recover nearly all strain that was induced from the structural change, and for some SMAs this can be strains greater than 10 percent.
In a plot of strain versus temperature, the austenite and martensite start and finish lines run parallel. The SME and pseudoelasticity are actually different parts of the same phenomenon, as shown on the left.
The key to the large strain deformations is the difference in crystal structure between the two phases. Austenite generally has a cubic structure while martensite can be monoclinic or another structure different from the parent phase, typically with lower symmetry.
For a monoclinic martensitic material such as Nitinol, the monoclinic phase has lower symmetry which is important as certain crystallographic orientations will accommodate higher strains compared to other orientations when under an applied stress. Thus it follows that the material will tend to form orientations that maximize the overall strain prior to any increase in applied stress.
In crystallography, a twin boundary is a two-dimensional defect in which the stacking of atomic planes of the lattice are mirrored across the plane of the boundary. Depending on stress and temperature, these deformation processes will compete with permanent deformation such as slip.
Interfaces and inclusions will provide general sites for the transformation to begin, and if these are great in number, it will increase the driving force for nucleation. The first reported steps towards the discovery of the shape-memory effect were taken in the s.
Greninger and Mooradian observed the formation and disappearance of a martensitic phase by decreasing and increasing the temperature of a Cu-Zn alloy.
The basic phenomenon of the memory effect governed by the thermoelastic behavior of the martensite phase was widely reported a decade later by Kurdjumov and Khandros and also by Chang and Read The nickel-titanium alloys were first developed in — by the United States Naval Ordnance Laboratory and commercialized under the trade name Nitinol an acronym for Nickel Titanium Naval Ordnance Laboratories. Their remarkable properties were discovered by accident.
A sample that was bent out of shape many times was presented at a laboratory management meeting.
One of the associate technical directors, Dr. David S.
Muzzey, decided to see what would happen if the sample was subjected to heat and held his pipe lighter underneath it. To everyone's amazement the sample stretched back to its original shape. These materials are of particular interest as the magnetic response tends to be faster and more efficient than temperature-induced responses.
Metal alloys are not the only thermally-responsive materials; shape-memory polymers have also been developed, and became commercially available in the late s. Many metals have several different crystal structures at the same composition, but most metals do not show this shape-memory effect.
The special property that allows shape-memory alloys to revert to their original shape after heating is that their crystal transformation is fully reversible. In most crystal transformations, the atoms in the structure will travel through the metal by diffusion, changing the composition locally, even though the metal as a whole is made of the same atoms.
A reversible transformation does not involve this diffusion of atoms, instead all the atoms shift at the same time to form a new structure, much in the way a parallelogram can be made out of a square by pushing on two opposing sides.
At different temperatures, different structures are preferred and when the structure is cooled through the transition temperature, the martensitic structure forms from the austenitic phase. Shape-memory alloys are typically made by casting, using vacuum arc melting or induction melting.
Magnetic shape-memory alloy
These are specialist techniques used to keep impurities in the alloy to a minimum and ensure the metals are well mixed. The ingot is then hot rolled into longer sections and then drawn to turn it into wire.
The way in which the alloys are "trained" depends on the properties wanted. The "training" dictates the shape that the alloy will remember when it is heated. This occurs by heating the alloy so that the dislocations re-order into stable positions, but not so hot that the material recrystallizes. The copper-based and NiTi-based shape-memory alloys are considered to be engineering materials.
This Breakthrough Shape-Memory Metal Practically Never Wears Out
These compositions can be manufactured to almost any shape and size. The yield strength of shape-memory alloys is lower than that of conventional steel, but some compositions have a higher yield strength than plastic or aluminum.
The high cost of the metal itself and the processing requirements make it difficult and expensive to implement SMAs into a design.
As a result, these materials are used in applications where the super elastic properties or the shape-memory effect can be exploited. The most common application is in actuation. One of the advantages to using shape-memory alloys is the high level of recoverable plastic strain that can be induced. This compares with a maximum strain 0. SMA have many advantages over traditional actuators, but do suffer from a series of limitations that may impede practical application.
In numerous studies, it was emphasised that only a few of patented shape memory alloy applications are commercially successful due to material limitations combined with a lack of material and design knowledge and associated tools, such as improper design approaches and techniques used. SMA actuators are typically actuated electrically, where an electric current results in Joule heating. Deactivation typically occurs by free convective heat transfer to the ambient environment.
Consequently, SMA actuation is typically asymmetric, with a relatively fast actuation time and a slow deactuation time.