More than 40 years of outer space explorations have made space flights look like an ordinary event. Only a few experts know that it is the metal alloys with shape memory effect that are applied to assemble and unfold large space structures.

SHAPE MEMORY ALLOYS: STATE OF AFFAIRS AND PROSPECTS

Grigory ZATULSKY, Gennady ZAK, the National Technical University of Ukraine/Kiev Polytechnic Institute

More than 40 years of outer space explorations have made space flights look like an ordinary event. Only a few experts know that it is the metal alloys with shape memory effect that are applied to assemble and unfold large space structures.

Substances with this remarkable effect are taken advantage of on the Earth just as well. The broad scope of shape memory effect’s application spreads from making of eyeglass frames to safety systems of nuclear reactors. So, what are the shape memory materials?

This is a new type of substances that reverse high inelastic strains, have rubbery elasticity, transform thermal energy into mechanical work, etc. All these features together are usually referred to as the shape memory effect.

Back in 1949 G. Kurdyumov and L. Handros from the Metals Physics Institute of the Academy of Sciences of the Ukrainian Soviet Socialistic Republic discovered the phenomenon of thermoelastic phase equilibrium appearing in the course of martensite transformation. This effect is a cornerstone of the shape memory properties.

German metallographer Adolf Martens (1850-1914) was the first to discover martensite transformations in meteorite iron. Now the name of martensite refers to a wide range of non-diffusion transformations that show weak dependence of the transformation start and end temperatures on the rate of temperature’s change; most often reversible nature of transformations; noticeable hysteresis of the forward and backward transformation temperatures; considerable influence of mechanical strains on these temperatures, and some other features.

Shape memory alloys have the following principal properties that determine their potential and possible future applications:

operation in the narrow-temperature-range heating;

making of high efforts when transforming backwards to the original shape;

shape redemption when heated after being subject to various types of pre-strains, such as extension, compression, torsion, and bending;

high damping capacity (up to 30-40%);

superelasticity of up to 6%, whereas the regular spring steels have an 0.2% superelasticity at most.

As this type of materials advanced further, it became possible to accumulate and transform efforts. If a shape memory material is pre-strained under a temperature below that of the end of forward martensite transformation, then heating in condition of strong outer counteraction causes a reactance effort, which is fivefold or tenfold as strong as the strain effort.

It is rather important that shape memory materials do not require much heat to generate the shape memory effect. Most frequently, solar heat, climatic or daily temperature difference, hot water, human body warmth, etc. are enough to set these materials going.

Extensive efforts are made to create shape-memory-alloy-based transducers to covert thermal energy into mechanical work. If this problem is settled, it will become possible to design environment-friendly transducers that would run on low-calorie fuel. There are so many sources of low-temperature thermal energy in the world that making use of even a couple of percents of their total number would furnish the global balance of power with considerable gains.

The so-called martensite engines, i.e. cyclical devices based on shape memory materials, take advantage of the metals’ multiple shape recovery properties. In the simplest case, shape memory materials convert heat into mechanical work in a double-stroke operation mode. With the first stroke, which can be referred to as a preparatory one, work is applied to deform an alloy. With the second stroke (a working one), heating transforms the strain backwards. In the most common modes, the first-stroke strain is completely restored during the second stroke, while the pre-strain force is much lower than the one that opposes shape redemption. Therefore, the work at the first stage is smaller than that required for subsequent heating. The work difference shows the degree of thermal energy’s conversion into mechanical work.

By combining strength and strain properties of shape memory metal substances, one can design extraordinarily simple and efficient execution robots, driving gear, transmission amplifiers, and the like.

Creation of various thermomechanical fasteners is among the significant and worthwhile technical applications of the shape memory effect. It is a good way to fasten together tubes and rods with shape memory couplings; to do pressure testing of various parts with shape memory tools; to assemble units made of different materials in out-of-the-way places or in specific surroundings, e.g. under water, in the outer space, and so on.

Application of shape memory materials has made it possible to design revolutionary assemblage and unfolding technologies for large space structures. These technologies have been successfully tested on the Mir space station in the course of the Crab, Sofora, and Rapana experiments. Russia’s rocket-space corporation Energiya assigns big tasks to shape memory alloys in the international space station Alpha that is currently being built.

It is a rather promising way to use shape memory alloys as final control elements (actuators) in advanced technical equipment instead of electromechanical, thermistor, electromagnetic, thermoelectric, and other devices. Owing to their tiny architecture, shape memory effect actuators find application in the automotive, electrotechnology, and medicine.

By taking advantage of shape memory effect actuators, one can create fundamentally new safety systems for nuclear power plants that would respond to temperature changes in the nuclear reactor zone. These safety systems are highly reliable and can function in an off-line unmanned mode.

Shape-memory-alloy-based devices that enhance serviceability of the power grid’s high-voltage contact components have shown very good performance. So far, operation of these devices is based on pre-strained shape washers’ ability to generate considerable effort when electric contacts become heated over the certain temperature values. Thus, they eliminate the contact pressure release, which appears in the course of service and frequently causes complete a contact burnout.

One of the greatest commercial successes is the application of shape memory alloys in making of eyeglass frames that easily revert back to the original shape in case of accidental deformation. It is easy to tackle this problem – shape memory effect frames should simply be immersed in hot water and that’s all.

The amazing properties of shape memory alloys have called forth a true euphoria among various scientific and technical experts, with the apex of high spirits registered in the second half of the 1980s (see table 1).

Table 1. Total number of patents registered in the world in 1985-1994 as regards creation and application of shape memory alloys

It even came to ridiculous things. Quite a few inventors were not really familiar with special properties and technical abilities of shape memory alloys, thus getting into serious debate on application of shape memory alloys in making of car bodies. It seemed that these cars would have been easy to repair after the road accidents. The advocates of this idea simply neglected the fact that heating of the best shape memory alloys can transform backwards only up to a 9% strain and not more.

Such misunderstanding of the nature of this phenomenon and ignorance of the materials’ properties have severely damaged the ideology of shape memory alloy application because only a handful out of the hundreds of seemingly pretty inventions actually proved efficient.

Today, there are several factors that impede widespread application of shape memory alloys. There have yet to be generated the standard approaches to how to determine origination of shape memory effects of these alloys. Moreover, there is still no common terminology needed to describe behavior of these substances. Each school of shape-memory-material specialists in different parts of the former USSR makes use of its own scientific terms, which frequently fall beyond understanding of experts from the other scientific centers. This hampers exchange of information among specialists. Besides, designers and engineers who want to use shape memory alloys run into serious troubles with these scientific terms as well. All the numerous attempts to introduce a common terminology and uniform approaches to shape memory effect’s testing have so far fallen short of reaching their goal. Things are pretty much the same in Western Europe and the USA. Each manufacturer of shape memory effect goods practices its own methods of testing thermomechanical properties and quality of these materials. Quite often, such manufacturing firms have to employ designer engineers and various skilled specialists who understand well the material side of shape memory articles’ performance and can give a feasible assessment of whether these materials can be applied in specific structures. As a rule, these people have to work with consumers and create prototypes of potential devices that take advantage of shape memory effect elements. This means that designers employed by the customer firm pretty much have nothing to do. They can word the general principles of how the future device should operate and the functions of the shape memory effect element employed, but they have no opportunity to take part in the engineering calculations on the full structure. In this case, the manufacturer of shape memory goods needs to make much effort and have very good contacts with potential clients. In turn, this drives up the cost of these goods. In the mid-1990s, 1-mm-diameter wire (semi-finished product) made of NiTi shape memory alloys cost $/kg 2,400 at the Western markets, wire made of CuZnAl and CuAlNi alloys was priced $/kg 1,300-1,600, and wire made of MnCu alloys was worth $1,100 per kg.

It is noticeable that CIS prices for shape memory alloys and the relevant semi-finished products are a few times below the mentioned rates. Moscow-based VILS company, which offers a wide range of semis (mainly the ones made of NiTi alloys) is among the well-known large manufacturers of shape memory alloys in the former USSR.

At this point of time, out of the extensive pack of shape memory alloys, only the NiTi and copper-based alloys can find industrial commercial application (see table 2 for details).

Table 2. Properties of commercially applied shape memory alloys

Nickel-titanium alloys feature the best combination of properties. Besides to high thermomechanical performance, these alloys have low density, superb corrosion resistance, high plasticity, and durability. Without any doubt, copper-based shape memory alloys benefit from high electrical and thermal conductivity. Besides, copper-based alloys can be made of regular materials on the ordinary metalmaking equipment, which makes the manufacturing technology much easier and cheaper, as well as lowers the cost of alloys themselves. Meanwhile, to make NiTi alloys, one has to use costly and scarce components and apply one-of-a-kind complex technologies, such as melting in inert gas mediums, vacuum induction, and electron beam melting. Meanwhile, copper-based shape memory alloys can be manufactured in a simple induction furnace.

One should also keep in mind that high performance of nickel-titanium alloys mainly originates from the good studies of these materials’ properties. This has made it possible to optimize the principal elements of the manufacturing technology, and plastic and heat treatment of these alloys. Consequently, it looks like titanium nickelide’s properties mentioned in reference literature are very much alike the highest attainable for this group of alloys. Special properties of copper-aluminum-manganese alloys, which can be substantially enhanced by conditioning, refinement, and heat treatment, are the most similar to those of titanium nickelide. This makes manganese-alloyed aluminum bronze the most worthwhile copper-based shape memory material that rivals titanium nickelide, especially in devices, which do not run in numerous repetitive heat cycles.

According to western mass media, Japan leads the way in the number of registered relevant patents with 64% of the world total, being far ahead of the other developed economies that successfully deal with the shape memory material business. Apparently, this figure based on the western data takes no consideration of investor’s certificates and patents issued in the USSR and the CIS (our estimates show that the number of such patents and certificates comes to about a thousand, i.e. roughly 40% of the total relevant inventions in the world).

Unlike their overseas colleagues, the Japanese specialists already take advantage of common standards that relate to making and testing of shape memory materials, thus tackling pretty much all the problems that bother widespread application of these alloys in the competitor countries.

Therefore, Japan’s Smart Materials seem to be much smarter today than their foreign congeners.

Ukraine has already accumulated much scientific research information on the nature and mechanisms of martensite transformations and has invented the technologies needed to manufacture copper-based alloys and certain semi-finished articles.

Recovery and further progress of competitive production of these innovative materials require relatively small outlay to standardize and regulate alloys and their control and testing methods, to develop manufacturing technologies for shape memory effect products, and to run marketing research in this field. Our country already has all the necessary scientific potential and the longstanding experience of such a work.

the Metal