Nanocomposites and nanomaterials

Positive deviation from a Hall- Petch relationship in           the nanocrystalline titanium
Smolianets R.V., Moskalenko V.A., Smirnov A.R.
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: smoljanets@ilt.kharkov.ua

The nature of polycrystals strain hardening with grain size d decreasing is described mathematically by the empirical Hall-Petch relation: σy= σ0 + kyd-1/2 (1), where σy is the yield strength, σ0 is the resistance strain to the dislocation motion and ky is the coefficient of grain boundary (GB) hardening.
The influence of the grain size variation from micron to nanoscale values on the yield strength σ0,2 of technical purity titanium VT1-0 in the temperature interval 4.2-395 K has been studied in the present work. Nanocrystalline titanium with an average grain size of ≈ 35, 45, and 80 nm was obtained by cryomechanical grain fragmentation and subsequent annealing [1].
The obtained values of σ0,2 , were plotted in coordinates σ0,2 – d–1/2 (Fig. 1). It can be seen that the GB hardening effect with a decrease in the average grain size to the nanoscale level at temperatures of 293 and 395 K is well described by the Hall-Petch relation (1). On the other hand, at the temperatures T ≤ 180 K on the dependences 0,2(d–1/2) (Fig.1) in the transition from the ultrafine-grain structural state to the nanocrystalline structural state (d <dcr) ) is observed a deviation of the experimental values of 0,2 from linearity in the direction of their increase in accordance with the relation (1). In this case, the yield strength describes by the dependence: σy= σ0 + kyd-1, where 0 has such a value as in (1), and the coefficient ky characterizes the difficulty of generation of dislocations by the grain boundary. The controlling role of the grain boundary dislocation sources in the low-temperature plastic deformation of titanium is confirmed by the results of the optical and electron microscopy. The mechanism of the observed GB hardening is discussed proceeding from the concept of the role of grain boundary dislocation sources, whose activity depends both on the grain size and the mobility of the edge and screw components of the dislocation loop.

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