Mechanical behaviour of Ti-Nb-Hf alloys

Abstract

Ti-(24-26)Nb-(2-4)Hf at.% alloys were designed by assuming that hafnium has a similar effect to zirconium in the Ti-Nb-Zr system. Alloy specimens were produced using vacuum arc melting and subsequently hot-rolled. Uniaxial tensile testing was then performed both at ambient temperature and in liquid nitrogen at −196°C. While the alloys showed no obvious superelastic behaviour, they exhibited pronounced strain hardening and could achieve high elongations before failure (>30% engineering strain). Post-mortem examination revealed that the mechanism of strain hardening was extensive {332} and/or {211} deformation twinning. Twinning was found to be more prevalent in alloys with 2 at.% Hf compared to those with 4 at.%. The cryogenic temperature deformation also promoted deformation twinning when compared to ambient temperature results. As is the case with other metastable β-Ti alloys, maintaining control over the precipitation of ω phases was found to be crucial for attaining desirable mechanical behaviour. Further, microstructural engineering and alloying may be used to develop strong, lightweight alloys based on the Ti-Nb-Hf system with beneficial strain hardening characteristics for energy absorption, cryogenic and biomedical applications.