Shape Memory Alloys Continue to Improve the Future
Proven technology is advancing next generation appliance and hardware solutions.
Shape memory phenomena in metals are some of the most intriguing in material science. Since the discovery of Shape Memory Alloy (SMA) effects, they have often been called solutions looking for problems. When people see the effect, they are almost compelled to think of ways to apply these materials. This is certainly true when considering the more elusive side of the technology commonly called shape memory actuators. Simply defined, an “actuator” produces desired movements. Smaller and lighter SMA-based devices are replacing commodity solutions such as electric motors, solenoids, wax motors, bi-metals, and piezo electric actuators. One spool of SMA wire can replace thousands of solenoids. As well as simple movements, and perhaps most valuable, SMAs enable new types of motion which cannot be accomplished with conventional technologies.
Recently new applications reaching volume production have been geometrically growing. Multiple sources of materials, better overall technical understanding, lower costs, an increasing number of successful products in the marketplace, and an overall product maturity are a few of the growth factors. While the SMA costs are generally low (in most devices usually pennies), the primary advantage is uniqueness. Today one can visit a local toy store and purchase products using SMAs to perform simple tasks like morph wings or release projectiles. One can also find FDA approved medical devices using SMAs to pump insulin inside surprisingly small, coin cell operated designs that provide lifestyle improvements. SMAs are in a growing number of consumer items like automobiles—not to mention, in specialty devices like vehicles on Mars.
How it works
The underlying principle with SMAs is their ability to change their crystal form, or atomic structure, depending on the temperature of the material. When the material is in the cool martensite phase it has a B19 or monoclinical crystal phase and when in the hot austenite phase an ordered B2 form. For a specific alloy and manufacturing process, the material can predictably change from one phase and related form to the other, have predictable temperatures of transformation, and with predictable force levels. The cool martensite phase can be deformed with less force than the recovery forces of the hot austenite phase allowing a usable work delta between them. This primary phenomenon tied with the notion of Joule heating the materials results in a viable shape memory actuator with advantages over existing commoditized technologies in smaller applications (inch pounds or centimeter kilograms), but not efficient enough for large tasks (foot pounds or newton meters).
In wire form, a given wire length can change up to 3% to 5% switching between these two phases. The contraction phenomenon of SMAs is in many respects akin to a human muscle, and can achieve similar movements in a wide variety of embodiments. Very small diameter wires heat and cool quickly, creating usable forces over many cycles. Continuous tests over 25 years indicate that, with proper usage, these materials will go hundreds of millions of cycles.
This was not always so. The discovery of the SMA phenomena in the 1960s by the Naval Ordnance Lab introduced potential solutions, theories and ideas. One early application was to use the resulting work from the material phase change as a heat engine. These are engines, which produce usable energy from simple temperature differentials. They showed exciting promise, but the two major problems for long-term viability were material stability and quality. The experiments would work in a lab environment, but would not have the longevity or cost efficacy needed to sustain an energy replacement solution.
As a result, instead of high cycle-life applications, the industry turned towards one-time applications like high performance aircraft tubing couplings. Standard metal tubing fittings or welded joints were replaced with SMA couplings. A Nickel/Titanium alloy with very low transformation temperature was used, and chilling the machined tubular couplings in liquid nitrogen transformed them to martensite so they could be temporarily deformed. The couplings were expanded in diameter and stored in liquid nitrogen until they were to be installed. When the chilled couplings were quickly installed on the metal tubing and allowed to warm to room temperature, the SMA transformed to strong austenite and the tube couplings contracted in diameter to efficiently connect the tubes. While viable for high value, high performance aircraft (the entire fleet of F-14 Tomcat fighter planes were assembled using these couplings), this solution is cost prohibitive for everyday implementation.
Advances of both the shape memory alloy materials and how to use them have completely changed the expected reliability, reopening the potential for next generation products.
Certain design considerations need to be applied to commercialize SMA products successfully. Like most materials SMAs need to be used within the correct boundaries. The areas to focus on first are ensuring the material is working inside the correct heating/cooling and stress/strain ranges. Overheating the material begins to reanneal the internal structure resulting in performance degradation. For most NiTi alloys this starts to occur at 250C. The correct stress level is a more complex question because it is multivariable. In other words, the correct stress levels are determined by material strain, cycle requirements, overall stress profile (most devices are not a constant force), and also material temperature during heating and cooling. Most applications have heating stresses
< 340MPa (the majority < 200MPa), and cooling stress levels between 70MPa and 130MPa.
The shape taken during the heating phase is set during the manufacturing process of the materials, but the recovery shape is heavily affected by usage and is often referred to as “training.” As a result, it has often been referred to as notoriously history dependent. A properly trained material is sometimes referred to as having a two-way memory. However, a word of caution is in order, shape memory materials and how they perform at this level vary from supplier to supplier so working closely with the vendor is always recommended.
A simple example of Joule heating is a latch. Figure 2 shows an SMA wire anchored with brass crimps, going out around a post on a lever. The crimps are electrically isolated from each other, but physically secure the wire while providing an electrical connection. Applied current then travels through the wire generating heat, like an Edison light bulb (but not nearly as hot). The heat in turn actuates the wire, which retracts the latching mechanism freeing the lid. In this configuration the SMA wire is generating twice the force of a single wire, but with half the stroke. The “action” takes place at the lever, which comes towards the anchor points as the wire gets hot and shortens in the hot austenite phase. The second plastic extension opposite the lever provides a spring-loaded opening of the enclosure. The volume taken up by the SMA actuator itself is small, but its force is over two pounds. This same concept of SMA latching is used in a variety of commercialized latches and locks.
Using ambient temperature changes is another method to actuate SMA materials. A simple example of this is an air register, Figure 3, which is based on either heated air reaching the SMA or cooled air reaching the SMA, which will direct the register louvers to predetermined positions. For example pushing hot air towards the floor to spread and rise, and cooled air towards the ceiling to spread and fall. Similar to this application of ambient temperature controlled actuators is the commercial application, which uses SMA as a safety shut-off in showers as a scald protection device that stops the water if its temperature rises too high.
On The Horizon
Advances in both the understanding of these alloys and new types of shape memory alloys continue to open the door to new opportunities in both appliances and general product designs. Applications that continue to be explored range from refrigerator dampers, to gas valves for burners, to coffee and tea maker valves, to emergency appliance shutoffs, locks and latches in dishwashers and laundry machines, along with many more.
Current commoditized solutions are widely used, understood and accessible to most everyone. SMAs, on the other hand, are newly emerging and more difficult to implement, but that fact alone makes the products they are used in highly unique and competitive, oftentimes, making these products stand out in their field.