The world of steel is constantly evolving. Although we are used to working with structural, corrugated, or rolled steels for construction, many more advanced alloys seem straight out of science fiction. One of the most surprising is the so-called ‘shape memory steel’, a material capable of deforming and automatically returning to its original shape when heated.

This behaviour makes these alloys smart materials, with applications in sectors as diverse as medicine, aerospace engineering, and robotics. But… how does this type of steel work and why is it so special?

It is a special metal that can be deformed in a reversible manner. If it is bent, crushed, or stretched within certain limits, it returns to its original shape when subjected to heat. This phenomenon is due to changes in the crystalline structure of the material, which “reorganises” its atoms when it reaches a certain temperature.

In other words, shape memory steel “remembers” its initial shape and returns to it when activated by heat, as if it had a small internal spring that is awakened.

Steel commonly used in construction or industry retains the shape given to it through deformation or machining processes. However, these smart alloys have two key characteristics:

– Superelasticity: they can be deformed more than usual without breaking.
– Memory effect: they return to their original shape when heated.

These properties are not present in traditional steels, making them unique materials.

The secret lies in two phases of the material:
– Martensite (‘flexible’ phase): when the steel is in this phase, it can be easily deformed.
– Austenite (‘rigid’ phase): when heat is applied, it changes phase and returns to its original shape.
This reversible change is what creates the memory effect.

Although they are not used in construction or metalwork, they do have very specialised applications:
– Medicine: stents and surgical devices that unfold at body temperature.
– Aerospace: components that change shape in flight.
– Robotics: actuators that work without motors, only with heat.
– Electronics: tiny mechanisms that are activated by temperature.
– Advanced engineering: systems that ‘self-correct’ in the event of vibrations or deformations.
One of the best-known alloys is Nitinol, composed of nickel and titanium, famous for its elasticity and shape memory.

It is not currently common in conventional construction due to its cost, delicacy, and particular applications. However, there are research projects studying its use to create structures that absorb vibrations, bridges that adjust automatically, or elements that partially “repair” themselves with heat.

Although it will be some time before it is in widespread use, it opens the door to very interesting possibilities in civil engineering.