IPMash RAS physicists, together with colleagues from Ufa and Nanjing (China), studied nanostructured technically pure titanium synthesized previously, which is promising for the creation of dental implants, and explained the mechanism of its high strength.
The results of the study were published in the scientific journal Metals. The scientists studied nanostructured technically pure titanium (with a small amount of impurities), created at the Ufa University of Science and Technology. The material was processed in a special way – by twisting twice under high pressure followed by annealing. As a result, the size of titanium grains decreased to 100 nm (100-1000 times less than the thickness of a human hair), and their boundaries were saturated with impurities. Such nanostructured titanium showed record strength, which cannot be explained by known hardening mechanisms. The scientists from Ufa found that, after special treatment, nanostructured titanium demonstrated strength characteristics: the yield strength (stress at which the material begins to deform) reached 1340 MPa, and the ultimate strength (maximum stress before fracture) was 1510 MPa. For comparison, ordinary hot-rolled titanium with large grains (10 microns) has much lower strength — 500 MPa and 680 MPa, respectively. In other processing modes, the values ranged from 600 to 1200 MPa (yield strength) and from 720 to 1340 MPa (ultimate strength), which confirms the key role of the chosen technology in strengthening the material. The researchers suggested that super strength is associated with the accumulation of impurities at the grain boundaries. In turn, the scientists at Nanjing University of Science and Technology confirmed this experimentally by finding an unusually high concentration of iron atoms in thick layers along the boundaries.

Mikhail Gutkin, Head of the IPMash RAS Laboratory of Nanomaterial Mechanics and Defect Theory offered the following explanation: «We assumed that the super strength of nanostructured titanium is related to the effect of impurities on the defective grain boundary structure. To test this idea, I proposed using a theoretical model developed earlier for aluminum alloys nanostructured in the same way. It explains how the impurities at the grain boundaries interact with the dislocations present there, blocking their mobility and increasing the strength of the material.”

In nanostructured metals obtained by intense deformation, plasticity depends largely on the behavior of grain boundaries. There are special defects on them – introduced dislocations which accumulated at the boundaries during the deformation process. Under load, these dislocations rearrange themselves, forming clusters that create high local stresses and help the grains deform.

However, if impurities (for example, iron) appear at the boundaries, they „fix“ dislocations, preventing them from accumulating in clusters. Because of this, the stresses are not concentrated, and the plasticity inside the grains decreases – the material becomes stronger.