​Titanium and zirconium, two crystalline metals used in industrial applications (particularly nuclear and aeronautical applications), have very similar electronic structures (same number of electrons in the outer shell). They also crystallize in a similar manner, i.e. when the atoms assemble into a crystalline structure, they adopt a similar geometry.

Despite the many similarities between these two metals, the researchers have unexpectedly shown that they respond differently to mechanical stress. By stretching a pure sample of each metal under a transmission electron microscope at various temperatures ranging from -170°C to +20°C, they have managed to observe and compare apparent line defects, i.e. dislocations evolving as a function of mechanical stress. Two types of dislocation behavior have been observed: dislocations passing jerkily through different planes (in the case of titanium), and dislocations slipping continuously on a single plane (in the case of zirconium). 

Simulation in support of experimental observations

In order to understand this difference in dislocation mobility, the researchers have modeled the dislocation core at the atomic scale using GENCI's Curie supercomputer[1]. These simulations show that the dislocations may adopt two different configurations: one slipping easily and continuously, the other with difficulty. Each of these two configurations exists in both metals, but with a different degree of stability (or recurrence): the most stable dislocations observed in titanium are of the easily slipping type, as opposed to zirconium.

With this new understanding of plasticity in pure titanium and zirconium, it is now possible to model the plastic deformation behavior of corresponding alloys based on robust physical principles. Regardless of whether titanium or zirconium alloys are considered, the alloying elements used (particularly oxygen) have a significant effect on the material's plastic deformation behavior. The next step is therefore to investigate how the alloying elements interact with the different dislocation configurations and modify both their stability and mobility. There are important technological stakes involved, since zirconium and titanium alloys are structural materials commonly used in the nuclear and transport industries.

This research has benefited from 800 000 hours of computing time on GENCI computing resources in 2014, and 12 million hours on the Curie supercomputer from March 2014 to March 2015, within the framework of the PRACE project.