Nanocomposites and nanomaterials

Thermal stability of deformation microstructure of ultrafine-grained and nanocrystalline titanium
Pohribnaya Yu.M., Smolianets R.V., Moskalenko V.A., Braude I.S. 
B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Science of Ukraine. 47 Nauky Ave., Kharkiv-61103, Ukraine. 
E-mail: Plotnikova@ilt.kharkov.ua

In most cases, to obtain of nanocrystalline (NC) and ultrafine-grained (UFG) materials large plastic deformation modes, both at elevated and at cryogenic temperatures, are used. Therefore, the issue of thermal stability of microstructures that form during technological processes is very important.
The influence of annealing at a gradual increase in the temperature 423  523  723  943 К on the sizes of crystallites L and the level of microdeformations <2>1/2 in samples with a micron (CG), submicron (UFG) and nanoscale (NC) grain size of titanium VT1-0 was studied. The initial CG, UFG and NC states of the samples by repeated rolling at the temperature of liquid nitrogen under the action of high deforming stresses were obtained [1]. The microstructural elements of the emerging structural states and their parameters were determined by the activity of deformation modes - slip and twinning. Their activity depended on the degree of cryorolling.
A different effect of the initial structural state on the change of the microstructural parameters during annealing has been found. It is suggested that their thermal stability is mainly due to the presence of the corresponding substructure elements in the microstructure. In CG samples, they are randomly arranged screw dislocations, annihilation of which leads to a noticeable increase in the crystallite size L and a decrease in the microdeformations <2>1/2. In UFG samples, annealing increases L compared with the initial value, and doesn't lead to a change in the value <2>1/2. The observed annealing effect in this case is due to activity the twinning at formation of the initial state. Twinning is favourable to removing internal stresses to sufficiently low values. The observed independence of the crystallite size from the annealing temperature to T = 723 K is apparently related to the absence of deformation defects inside the nanoscale crystallites/grains. At the same time, a decrease in the value <2>1/2 indicates microstructural changes occurring in the grain boundaries.

1.	Moskalenko V.A., Smirnov A.R., Moskalenko A.V. // Low Temp. Phys.- 2009. -35.-P. 1160-1164