SMAs are materials that can be deformed at low temperature and recover their original shape upon heating. Shape memory alloys (SMAs) belong to a class of shape memory materials (SMMs), which have the ability to ‘memorise’ or retain their previous form when subjected to certain stimulus such as thermo mechanical or magnetic variations. SMAs has found commercial application in a broad range of industries including automotive, aerospace, robotics and biomedical.
Currently, SMA actuators have been successfully applied in low frequency vibration and actuation applications. This material is a lightweight, solid-state alternative to conventional actuators such as hydraulic, pneumatic, and motor-based systems in robotics and automotive, aerospace and biomedical industries.
Compared to conventional actuators, SMAs provide high force (per volume/weight) allowing lightweight compact actuator designs, eliminates extraneous systems (hydraulic, pneumatic, etc.), responds to temperature change, which eliminates the need for sensors and electronics and enables simple, frictionless designs that result in less maintenance
SMAs can be used in passive, active, or superelastic design applications. Passive design applications result from the material heating during normal operation resulting in an actuation force. Active design applications use the material below its transformation temperature and of supplemental heat to provide an on-demand actuation force. Superelastic design applications use the material above its transformation temperature resulting in transformation due to stress.
Researchers are working to enhance the performance of SMAs, especially to increase their bandwidth, fatigue life and stability.
Researchers figure out how to cheaply grow large shape memory alloy crystals.
Shape memory alloys work best when they are grown as single crystals. But single crystals are difficult to manufacture, so current applications use poly crystal forms of the materials, which have limitations. “These shape memory materials, when in the poly crystal form, tend to crack along grain boundaries,” said Richard James, a materials scientist from the University of Minnesota in Minneapolis who was not involved in the study. Just like a particle board is weaker than a solid plank of wood, a poly crystal material has its weak points between the grains.
According to Toshihiro Omori, co-author of the Nature Communications paper and a materials scientist from Tohoku University in Sendai, Japan, current manufacturing methods can barely produce a single crystal grain of shape memory alloys much larger than a grain of rice. Looking to change that, Omori and his colleagues developed a new technique and grew a single grain of copper-based shape memory alloy more than 2 feet long. The technique they used, known as abnormal grain growth, sets off an uneven competition between the individual crystal grains and causes them to merge and form bigger and bigger grains.
“I think soap bubble is a good metaphor,” said Omori, referring to the way a pile of smaller soap bubbles pop and merge together to form larger bubbles. Omori and his colleagues have discovered that if they repetitively heat and cool a bunch of tightly packed shape memory alloy crystals, the smaller crystals, just like the soap bubbles, can merge and form a larger, single grain crystal. The trick is to figure out the right temperature and speed to heat and cool the material, which is one of the main findings of their paper.
The researchers claim that the size of their 2-foot-long crystal was only limited by the size of their laboratory equipment, which is like saying that your wallet is too small to hold all your cash — not a bad problem to have. Even larger single crystals of this alloy can be made with larger industrial size equipment, Omori said.
Shape Memory Alloy Materials
The two main types of 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. Although iron-based and copper-based SMAs, such as Fe-Mn-Si, Cu-Zn-Al and Cu-Al-Ni, are commercially available and cheaper than NiTi, NiTi based SMAs are preferable for most applications due to their stability, practicability and superior thermo-mechanic performance. SMAs can exist in two different phases, with three different crystal structures (i.e. twinned martensite, detwinned martensite and austenite) and six possible transformations.
Recently, uses of shape memory alloys for seismic devices such as dampers and isolators have attracted considerable attention since they can dissipate energy by stress-strain hysteresis, recover deformation upon unloading and limit force transmission. Ti-Ni shape memory alloy bars showing self-centering capability due to superelasticity are being used on a trial basis as a part of bridge columns to reduce permanent deformation by earthquakes. However, the low machinability and cold-workability of the conventional Ti-Ni alloy are obstacles to its widespread use.
Since Cu-Al-Mn shape memory alloys have high machinability and cold-workability, their application to seismic devices has been investigated. The superelasticity of Cu-based shape memory alloys is drastically enhanced by increasing the grain size relative to the cross-sectional size of materials and in particular, an ideal superelastic response can be obtained in a single crystal. If the fabrication of large single crystal parts can be realized by simple heat-treatment, applications to seismic devices are expected to increase
High Temperature Shape Memory Alloys
Extensive research for HTSMAs with other ternary additions to the NiTi SMA (e.g. Au, Hf, Pd, Pt and Zr) has been undertaken, due to the increasing demands for high-temperature applications. Practically, HTSMAs are defined as SMAs that are operating at temperatures above 100 _C, and can be categorized into three groups based on their martensitic transformation ranges.
Magnetic Shape Memory Alloys
Magnetic shape memory alloys (MSMAs), which are also known as ferromagnetic shape memory alloys (FSMAs) can actuate at higher frequencies (up to 1 kHz) because the actuation energy is transmitted via magnetic fields and is not hindered by the relatively slow heat transfer mechanism . FSMA strain rate is quite comparable to magnetostrictive and piezoelectrics active elements, but at strains as large as SMAs. FSMA can also provide the same specific power as SMAs, but deliver it at higher frequencies.
Shape Memory Material Thin Films
SMM thin films evolved from the advancement of fabrication technology, where SMMs are deposited directly onto micromachined materials or as stand-alone thin films to become micro- actuators. Moreover, in the rapidly growing field of micro-electro-mechanical systems (MEMSs), NiTi thin films have become the actuator of choice at the micro-scale level, due to the attributes as described earlier (i.e. higher actuation force and displacement), but at relatively low frequency (up to 100 Hz) and efficiency as well as the non-linear behaviour The versatility of NiTi thin film with multiple degrees of freedom and compact structure, expand the potential of NiTi in biomedical, aerospace, automotive, and consumer products applications.
Shape Memory Polymers
Shape memory polymers (SMPs) are relatively easy to manufacture and fast to train (or program) as well being able to be tailored for a variety of applications. SMPs are claimed to be a superior alternative to SMAs for their lower cost (at least 10% cheaper than SMAs), better efficiency, biodegradable and probably by far surpass SMAs in their mechanical properties.
When one considers the vast commercial application of polymer products, it is apparent that SMPs have significant commercial application, such as smart fabrics, self-repairing (or seal-healing) plastic components, spacecraft sails, biomedical devices and intelligent structures.
The biodegradable nature of certain SMPs provide advantages over metal implants, where the removal of the implant after regeneration can be avoided, thus gentler, more effective and more economical treatments can be offered. However, despite the advantages described above, SMAs are still preferred for applications that require higher actuation forces and faster response