Extreme hardening of titanium with colossal interstitial contents of nitrogen and oxygen

Abstract

Homogenous thin foils of titanium with nitrogen and oxygen in interstitial solid solution were characterized with X-ray diffraction, light optical microscopy and electron backscatter diffraction. Samples covering the broad solubility ranges of O and N were synthesized. The mechanical properties were assessed using nanoindentation and related to the interstitial content, h.c.p. lattice expansion and grain orientation. Effects of surface pile-up and sink-in during nanoindentation were adequately accounted for, yielding hardness and indentation modulus values for nearly the full range of concentrations, finding consistency with theoretical expressions and very limited literature values. The hardening/strengthening achieved by colossal interstitial alloying of O and N correlates with the change in c/a ratio of the h.c.p. Ti lattice as caused by interstitial occupancy.