Applications of shape memory alloys in space engineering past and future
A shape-memory alloy SMA , smart metal , memory metal , memory alloy , muscle wire , smart alloy is an alloy that "remembers" its original shape and that when deformed returns to its pre-deformed shape when heated. This material is a lightweight, solid-state alternative to conventional actuators such as hydraulic , pneumatic , and motor -based systems. Shape-memory alloys have applications in robotics and automotive , aerospace and biomedical industries. The two most prevalent shape-memory alloys are copper - aluminium - nickel , and nickel - titanium NiTi alloys but SMAs can also be created by alloying zinc , copper , gold and iron.
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.
Shape Memory Alloy Engineering
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 alloying. 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: 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: 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 also display superelasticity , which is characterized by the recovery of relatively large strains with some dissipation. In addition to temperature-induced phase transformations, martensite and austenite phases can be induced in response to mechanical stress.
When SMAs are loaded in the austenite phase i. Upon continued loading and assuming isothermal conditions, the twinned martensite will begin to detwin , allowing the material to undergo plastic deformation. If the unloading happens before plasticity , the martensite transforms back to austenite, and the material recovers its original shape by developing a hysteresis. For example, these materials can reversibly deform to very high strains — up to 7 percent.
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.
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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.
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- Future of shape memory alloy and its utilization.
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. 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.
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. 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.
Future of shape memory alloy and its utilization
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.
A number of methods have been proposed to reduce SMA deactivation time, including forced convection,  and lagging the SMA with a conductive material in order to manipulate the heat transfer rate. Novel methods to enhance the feasibility of SMA actuators include the use of a conductive " lagging ". This heat is then more readily transferred to the environment by convection as the outer radii and heat transfer area are significantly greater than for the bare wire.
This method results in a significant reduction in deactivation time and a symmetric activation profile. As a consequence of the increased heat transfer rate, the required current to achieve a given actuation force is increased. SMA is subject to structural fatigue — a failure mode by which cyclic loading results in the initiation and propagation of a crack that eventually results in catastrophic loss of function by fracture. The physics behind this fatigue mode is accumulation of microstructural damage during cyclic loading. This failure mode is observed in most engineering materials, not just SMAs.
As a result of cyclic loading both mechanical and thermal , the material loses its ability to undergo a reversible phase transformation.
For example, the working displacement in an actuator decreases with increasing cycle numbers. The physics behind this is gradual change in microstructure—more specifically, the buildup of accommodation slip dislocations. Eric Randy Reyes Politud.
Shape-memory alloy - Wikipedia
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