Evidence for a transition in deformation mechanism in nanocrystalline pure titanium processed by high-pressure torsion

Abstract

Nanocrystalline titanium with an average grain size of about 60–70 nm was prepared by high-pressure torsion. The results of hardness and structural evolutions indicate that a strain-induced hardening–softening–hardening–softening behaviour occurs. For coarse-grained titanium, 〈a〉-type dislocation multiplication, twinning and a high pressure-induced α-to-ω phase transformation play major roles to accommodate deformation, leading to a significant strain hardening. As deformation proceeds, dynamic recrystallisation leads to a decrease in dislocation density, especially for 〈a〉-type dislocations, leading to a slight strain softening. The 〈c〉-component dislocation multiplication dominates the deformation when the grain size decreases to 100 nm and 〈c〉-component dislocation multiplication, grain refinement and the α-to-ω phase transformation contribute to the second strain hardening. The following strain softening is attributed to dynamic recovery.

Keywords:

• Nanocrystals
• deformation mechanisms
• dislocations
• transmission electron microscopy
• high-pressure torsion
• grain refinement

Funding

Financial support from the National Natural Science Foundation of China [grant number 51301207], the China Postdoctoral Science Foundation [grant number 2013M531806], [grant number 2015T80882]; Fundamental Research Funds for the Central Universities of Central South University [grant number 2015zzts183], and the China Postdoctoral International Exchange Program is appreciated. This work was supported in part by the National Science Foundation of the United States under [grant number DMR-1160966] (SS and TGL).