Elimination of oxygen sensitivity in α-titanium by substitutional alloying with Al

Abstract

Individually, increasing the concentration of either oxygen or aluminum has a deleterious effect on the ductility of titanium alloys. For example, extremely small amounts of interstitial oxygen can severely deteriorate the tensile ductility of titanium, particularly at cryogenic temperatures. Likewise, substitutional aluminum will decrease the ductility of titanium at low-oxygen concentrations. Here, we demonstrate that, counter-intuitively, significant additions of both Al and O substantially improves both strength and ductility, with a 6-fold increase in ductility for a Ti-6Al-0.3 O alloy as compared to a Ti-0.3 O alloy. The Al and O solutes act together to increase and sustain a high strain-hardening rate by modifying the planar slip that predominates into a delocalized, three-dimensional dislocation pattern. The mechanism can be attributed to decreasing stacking fault energy by Al, modification of the “shuffle” mechanism of oxygen-dislocation interaction by the repulsive Al-O interaction in Ti, and micro-segregation of Al and O by the same cause.

Grain refinement in titanium prevents low temperature oxygen embrittlement

“…In the Ti-0.3O alloy, the CG microstructure had a very limited elongation ( ε f ~ 1.5%) at 77 K (Fig. 2b ), similar to the value we previously reported 7 , 8 . Surprisingly however, the uniform elongation greatly increased from 1.5% to 14.0% with decreasing grain size (Fig.…”

“…In region B , several planar slip bands were observed (Fig. 3c, e ), a typical dislocation slip mode in Ti-0.3 O at low temperature 7 , 8 . The planar slip bands also led to localized deformation, but associated with much lower strain magnitude and an alternating distribution characteristic (Fig.…”

“…Interstitial oxygen has long been considered as a double-edged sword in pure titanium with a hexagonal close packed (hcp) structure. Oxygen can significantly increase the strength 1 – 8 , but it can also severely deteriorate the ductility 4 – 8 , rendering titanium extremely brittle at 77 K when the oxygen content merely exceeds 0.3 wt.% 7 , 8 . The dramatic decrease in ductility with increasing oxygen content is also accompanied with a transition from transgranular ductile fracture to intergranular brittle fracture 7 .…”

Dwell fatigue in titanium and the connection between slip intermittency and alloying

“…There are also noticeable amounts of dislocation interaction within slip bands, and several dislocations extending between them. Chong and Zhang et al [35] explained their observations of similar behaviour in Ti-Al-O alloys at cryogenic temperatures as due to hardening within slip bands that promotes the emission of slip into relatively softer surrounding material, promoting spatial homogenisation of slip within a grain. Slip band formation due to crystallographic ordering of aluminium is well known and occurs in materials with either shortrange order (SRO) or long-range order in Ti 3 Al α 2 precipitates.…”

The influence of alloying on slip intermittency and the implications for dwell fatigue in titanium

“…There are also noticeable amounts of dislocation interaction within slip bands, and several dislocations extending between them. Chong and Zhang et al 28 explained their observations of similar behaviour in Ti–Al–O alloys at cryogenic temperatures as due to hardening within slip bands that promotes the emission of slip into relatively softer surrounding material, promoting spatial homogenisation of slip within a grain.…”