Metallurgists have designed an extraordinary titanium alloy that is light, strong and flexible, and which recovers its original shape after large amounts of deformation — even when as cold as liquid helium or hotter than boiling water.

Nearly all metals can be strengthened, yet it is a struggle to make them stretch or bend flexibly and reversibly like rubber. Why? When a metal is stretched, defects form in its crystal lattice and thereby dissipate the strain energy (the energy stored from applied stress). This limits the amount of deformation that can be recovered (the recoverable strain) to less than 0.5% and causes permanent shape change. Metals are therefore good at permanently retaining their shape and are resistant to breaking, but they are not suitable for applications in which a material cycles through large shape changes. Writing in Nature, Song et al.1 report an impressive lightweight titanium alloy that has high mechanical strength and yet recovers strain of more than 5% across an extensive temperature range — with potential applications for biomedical devices, energy infrastructure and space exploration.

In 1932, a seminal discovery2 was made that informed subsequent efforts to make metallic alloys with rubber-like flexibility. This was the discovery of mechanically assisted phase transformation — a phenomenon in which the atoms in an alloy move collectively in response to an external mechanical load, reversibly altering the crystal structure. Mechanically assisted phase transformation enables the recovery of an immense amount (several per cent) of strain while producing minimal strain-energy-dissipating defects. Alloys that have this appealing mechanical functionality are said to be superelastic.

Superelasticity is useful for a multitude of uses in which materials must withstand substantial deformation yet return smoothly to their original shape. However, superelasticity often occurs in a narrow temperature range, imposing stringent constraints on the operating conditions of potential applications. For this reason, only a handful of superelastic alloys are currently used...