Towards A New Generation Of Materials

The atoms of metals are organized in ordered structures denominated crystal lattices. The geometry of the latter depends of the nature of the material as well as of temperature and pressure. At room temperature and atmospheric pressure, pure metals like gold, aluminium and copper have cubic lattices, and others like magnesium, titanium and zirconium have hexagonal structures (called alpha phases, a).

Increases in pressure occasionally cause changes in the geometry of the crystal lattice, resulting in the appearance of new phases. For example, in the case of titanium, the hexagonal a lattice, stable at 1 atm, transforms into a cubic structure (beta phase) when a hydrostatic pressure of approximately 1 million atmospheres is applied. If, once the cubic phase has been generated, the pressure is reduced down to 1 atm, the reverse transformation takes place, giving rise to the original hexagonal a phase. Due to the extreme pressure conditions needed to generate these new phases, the practical applications of these materials are very limited.

Scientists from (IMDEA Materiales) – in collaboration with the Centro Nacional de Investigaciones Metalúrgicas [National Centre for Metallurgical Research] (CENIM) and the Iskra institute of Ufa, Russia – have developed a mechanical method to stabilise at room temperature and atmospheric pressure crystalline phases of metals that until now have only been stable at very high pressure. The method is based on simultaneously applying compression and shear strains. It has been proven that shear enhances the transformation kinetics significantly, eliminating the need for very high pressures. This technique has been successfully applied to pure titanium and zirconium and a patent application has been filed.

The high-pressure phases could have properties of great technological interest. For example, cubic titanium (beta phase) is very attractive for manufacturing bone implants, since its elastic modulus is more similar to bone than hexagonal titanium. Moreover, it is known that the critical superconducting temperature of beta titanium is also higher. This research therefore represents a first step towards manufacturing a new generation of materials with as yet unknown properties and opens the doors to their practical application.