Fundamentals of titanium nanocrystalline structure creation by cryomechanical grain fragmentation

The evolution of the CP titanium microstructure at deformation by rolling at the temperature of 77 K in the range of 0.06-3 true strain has been investigated using light microscopy, transmission electron microscopy and X-ray analysis. Role of deformation twinning as the main mechanism responsible for the fragmentation of grains and formation of the bulk nanocrystalline state is determined. We discovered a three-stage character of the grain refinement with increase in cryorolling reduction. It corresponds to stages of twinning process development. The limit of grain refinement in the processing of nanocrystalline titanium using the method of cryomechanical grain fragmentation is ~ 40 nm. It is due to a low probability of occurrence of twins in the grains with a size less than ~ 100 nm. The coincidence of the grain size and the crystallite size (coherent scattering regions) indicates that grains have a sufficiently perfect internal structure. It demonstrates that dislocations known to be a source of lattice distortion cannot be accumulated in nanoscale grains. It was shown that cryomechanical grain fragmentation (CMGF) is the effective method of processing a bulk nanocrystalline titanium (as well as zirconium). The mechanical properties of titanium have been characterized in three structural states: coarse-grained, ultrafine-grained and nanocrystalline.