Face centered cubic titanium in high pressure torsion processed carbon nanotubes reinforced titanium composites

Abstract

Carbon nanotube reinforced titanium (CNT/Ti) composites with high tensile strength (872 ± 5 MPa) were synthesized by the high pressure torsion process (HPT). Cs-corrected high resolution transmission electron microscopy (TEM) revealed face centered cubic titanium (FCC–Ti) distributed in the grains and at the grain boundaries of a hexagonal close packed titanium (HCP–Ti) matrix, with an orientation relationship of [0002]HCP//[111¯]FCC and {21¯1¯0}HCP//{011}FCC between FCC-Ti and HCP-Ti. The phase transformation from a HCP-Ti structure to a FCC-Ti structure in the CNT/Ti composites during the HPT process made a significant impact on the mechanical properties. Combining experimental results with density functional theory calculations, the stability of FCC-Ti and HCP-Ti under external pressure were analyzed. Moreover, the nucleation and growth mechanisms of FCC-Ti phase in HCP-Ti matrix were identified. The results showed that the stress-induced phase transformation from HCP-Ti to FCC-Ti in the CNT/Ti composites during the HPT process is associated with the source of Shockley partial dislocations. This work helps to the better understanding of the phase transformation mechanism from a HCP-Ti structure to a FCC-Ti structure in CNT/Ti composites.