Oxidization on phase transformations in Ti-Nb high temperature shape memory alloys

Properties of metastable β-Ti alloys can be optimized by tuning the rich phase transformations. Oxygen (O) introduction, inevitable during melting and processing, affects greatly on phase transformations and the associated properties in β-Ti. We investigated the effects of oxidation on phase transformations in Ti-Nb alloys. Based on microscopic characterizations, the as-quenched microstructure of oxidized Ti-20 Nb is revealed to contain 3 distinct layers with respect to the phase constitutions and subsequent transformations, i.e. oxide, oxygen influenced (OI), and center (C) layers. On subsequent heating, β→ ω _iso occurs in OI layer, and in C layer undergo successive two step α”→β and β→ ω _iso transformations. In C layer of low O-content, both the starting temperatures of α”→β ( A s C ) and β→ ω _iso ( ω s C ) transitions increase slightly with O-content. In OI layer of high O-content, since the β→α” transition is suppressed upon quenched to 273 K, the retained β starts to precipitate ω _iso far below ω s C upon subsequent heating. In summary, with the increase of O-content, the A _s and ω _s slightly increase at low O-content and then undergo a drastic decrease. The underlying mechanisms are further discussed. Our work may shed lights on the high temperature thermomechanical processing and designing high performance metastable β-Ti alloys.