Shape Memory Alloys - Basics

There are numerous active materials with a wide variety of mechanisms of action. We primarily deal with magnetic and thermal shape memory alloys as well as shape memory steels.

All active materials have in common the inherent sensor and actuator function in a material. This qualifies this group of materials for both sensory and actuator tasks and enables a new degree of integration as well as extreme functional consolidation. Conventional construction materials such as steel, aluminum, or fiber-reinforced plastic composites become more intelligent through the application or integration of active materials and can be made lighter as overall components, thus saving weight and resources.

Thermal shape memory alloys (SMA) are thermosensitive materials that change their geometry when they reach defined temperatures and exhibit elastic properties in certain temperature ranges. The so-called shape memory effect is known since the 1930s, when Swedish chemist Arne Ölander was able to demonstrate it in gold-cadmium alloys. However, products that specifically exploit this effect have only entered the market in recent decades. Materials with shape memory effect primarily include alloys, i.e., combinations of two or more metals. Nickel-titanium alloys, often referred to as Nitinol, are particularly well known. This name is derived from the two metals and the US Naval Ordnance Laboratory, where the properties of this alloy were discovered.

In the past, material and application development always went hand in hand when developing applications based on SMA. Both the improvement of material properties and a better understanding of crucial design aspects led to new applications. In the smart materials group, SMA has the highest degree of maturity alongside piezoceramics. There are applications on the market, sometimes established for decades, that consistently exploit the special properties of SMA. Examples include SMA spring applications in fluid circuits and thermostats (more than 5 million units per year), pneumatic valves for seat comfort systems (more than 10 million units per year), and autofocus and image stabilization systems in smartphone cameras (more than 40 million units per year).

Fraunhofer IWU has been researching SMA for more than 15 years. In addition to fundamental questions relating to materials science, design, and simulation, the focus is particularly on production engineering. This ensures that the entire value chain, from SMA semi-finished products to mass-produced products, is always considered as part of development, thereby minimizing risks. 

The development of magnetic shape memory alloys (MSM) dates back to the mid-1990s. The outstanding properties of MSM are the magnetic field-induced strains, which can be more than 10 % for single-crystal Ni-Mn-Ga sample material. The main difference compared to thermal SMA is that the magnetic shape memory effect is not caused by a temperature effect, but by the application of a magnetic field in the low-temperature martensite phase, which enables a significantly higher repetition rate in the work cycles. The process can be controlled by varying the magnetic field strength and can be used in numerous actuator and sensor applications in various industrial sectors, such as for grippers in robotics, in fluid technology (pumps, valves) and for devices for generating and damping vibrations (tools, sound measuring devices).

Fraunhofer IWU has many years of experience in the field of MSM materials and their behavior. The know-how for characterization was largely gained within the framework of BMBF projects - also in the smart³ consortium with the Fraunhofer IWU as consortium leader. This involved numerous investigations into cutting and removal behavior as well as tests on the production of secondary mold elements on actuator sticks for fastening options that are relevant for actuator applications.

Shape memory steels (SMS) represent a relatively new material group of iron-based alloys with shape memory properties. The effect is based on different material mechanisms than thermal SMA, which also results in other areas of application. SMS are particularly suitable for applications where the material price plays a decisive role, for example in the construction industry or for mass-produced fasteners. On the other hand, they are less suitable for typical SMA applications such as in medicine or micro and small actuators, which explains their low prevalence.

Fraunhofer IWU is conducting research into both the materials engineering principles of shape memory steels and their application. As an example, the use of SMS for intelligent fasteners in the construction industry was investigated in order to assess their potential for making the assembly process safer, more sustainable and more economical. Specifically, the work focused on a smart fastener for façade elements made of innovative carbon concrete. The results show that connections for large and heavy components, such as façade elements, are also possible with SMS. A special feature is the possibility of blind assembly without mechanical access to the connection, e.g. with a wrench or similar. The connection is closed remotely by activating the SMS at the push of a button.

Current research questions deal with innovative fasteners for the circular economy across a range of applications. Here, SMS-based fasteners offer the potential to make assembly and, in particular, non-destructive disassembly easier and more economical, thereby enabling reuse and recycling on a large scale.