Smolianets R. Mechanisms of plastic deformation in nanocrystalline titanium obtained by cryomechanical grain fragmentation

The thesis is devoted to the study of the regularities plastic deformation of the bulk nanocrystalline (NC) commercial purity titanium VT1-0 with different of grain size distribution in the temperature range T = 4.2 - 395 K. The aim this thesis is clarify of the plastic deformation mechanisms of NC titanium. The NC structural state with a grain size in the range of 80 – 35 nm was first obtained by the employ of an original method of cryomechanical grain fragmentation using cryo-rolling at T = 77 K and further annealing. At the first time the deformation diagrams under the quasi-static tension of titanium NC was obtained. This allowed us to determine the dependence from temperature, grain size and grain size distribution of their main characteristics: the yield strength σ0.2, strain rate hardening  and elongation to failure f from temperature, grain size and grain size distribution in the temperature range 4.2 – 395 K. It was found that the dependence of the yield strength on grain size d in the range of 35 nm – 20 μm due to grain boundary (GB) hardening corresponds to the classical Hall-Petch relation σ0,2(d–1/2) at room and elevated temperatures and is violated at low temperatures. The scientific substantiation of performance of the Hall-Petch relation in a wide range of temperatures is offered. The physical mechanisms of GB hardening at low temperatures are associated exclusively with the activity of GB dislocation sources. The revealed low-temperature feature of GB hardening in the case of NC titanium (d ~ 80 – 35 nm), which is observed as a "positive" deviation from the Hall-Petch relation towards higher values of σ0.2, is explained by the nanoscale grain dependence of the diameter of the dislocation loop of GB source, and the nucleation stress of it is inversely proportional to its size σ ~ 1/d. Thermoactivation analysis of experimental temperature dependence of yield strength and velocity sensitivity of NC titanium with a grain size of several tens of nanometers (35 nm and 45 nm) and their monomodal distribution by value in the temperature range 4.2 – 395 K based on the theory of thermally activated motion of dislocations in the sliding plane. The relative influence of local barriers (impurity atoms) and internal stresses due to grain boundaries on the kinetics of such motion has been established. The controlling mechanism of plastic deformation of titanium NC is determined. The physical mechanism of increasing ductility of NC titanium with heterogeneous (bimodal) microstructures at low temperatures, which is not observed in the case of monomodal NC structural state, is determined. The observed phenomenon is explained by a combination of several processes: dislocation sliding, extraordinary dynamic grain growth induced of tensile stresses, and activated nanotwinning in the resulting grains of submicron and micron sizes. The obtained results indicate the important role of nanotwinning in the process of deformation of the NC HCP metals with a heterogeneous (bimodal) structure.