"Nanocomposites and nanomaterials"

Low-temperature dynamic relaxation in nanostructured metals
Vatazhuk O.M.
B .Verkin Institute for Low Temperature Physics and Engineering of NAS of Ukraine,47 Nauky Ave., Kharkiv, 61103, Ukraine. 
E-mail: vatazhuk@ilt.kharkov.ua

Materials with nano structure attract considerable attention because of their promising mechanical and physical properties and also the damping capacity they possess. As a result of severe plastic deformation, a microstructure with individual elements (grains, subgrains or dislocation cells) a few hundred nanometers in size is formed. The new properties achievable in this way open up novel applications, in particular, in equipment for use at low temperatures down to the cryogenics range.
Experimental study of dynamic modulus of elasticity and dislocation internal friction in face-centered cubic and hexagonally closed packed nanostructured metals is a source of valuable information on their dislocation structure. A systematic analysis of the influence of changes in the dislocation structure (during plastic deformation, thermal cycling, aging and annealing) and frequency of vibrations on the parameters of the low-temperature dynamic relaxation has been carried out.
It was found for the first time that the peaks of internal friction found in nanostructured copper and highly fragmented nanostructured fibrous composite Cu-Nb are determined by the low-temperature dynamics of the movement of dislocation segments in the primary and secondary copper and copper matrix Peierls. It is also shown that the relaxation of internal friction near 250 K, detected earlier in the coarse-grained annealed Zr, remains after severe plastic deformation, but its height increases by about 10 times, and the localization temperature shifts to low temperatures. In addition, after severe plastic deformation, a new peak of internal friction was registered for the first time in the region of moderately low temperatures near 80 K. The insufficient stability of the structures of the nanostrutured and coarse-grained of samples created by applying various severe plastic deformation schemes was established. The temperature dependence of the Young's modulus of the nanostructured samples revealed a characteristic of low-temperature elastic moduli of glasses as a result of the interaction of acoustic oscillations with dislocation two-level systems, relaxation systems, and harmonic oscillators. It is hypothesized about the dislocation nature of the glass subsystem (or “phase”) in nanostructured metals obtained by severe plastic deformation.