Interdependencies between graphitization of carbon nanotubes and strengthening mechanisms in titanium matrix composites

Titanium (Ti) metal matrix composites (TMCs) reinforced with multi-walled carbon nanotubes (MWCNTs) were fabricated by powder metallurgy. Different concentrations (0.5 and 1.0 wt.%) of MWCNTs were dispersed into Ti powder by high energy ball milling (HEBM) and solution ball milling (SBM) processes. The solid-state interfacial reactions between dispersed MWCNTs and Ti matrices were controlled through altering the ball milling parameters and conditions. The powder mixtures were consolidated and sintered at 1100 °C in a vacuum furnace. The graphitization of MWCNTs and its strengthening efficiency in the fabricated TMCs were quantitatively characterized. The relationships between the graphitization and the key strengthening mechanisms were determined. The Ti-0.5 wt.% MWCNTs composites with in-situ TiC nanorods during HEBM and debundling of MWCNTs during SBM processes exhibited compressive yield strength of 882 and 920 MPA which demonstrated 18 and 32% increase compared to commercially pure titanium (CP-Ti), respectively. The key strengthening mechanisms included grain refinement, dispersion strengthening of homogenously dispersed MWCNTs and in-situ TiC particles, solid solution strengthening of carbon, oxygen and nitrogen in Ti matrix, and load-bearing strengthening in TMCs.