Metastable phase diagram on heating in quenched Ti-Nb high-temperature shape memory alloys

Metastable phase diagrams of β (BCC)-Ti high-temperature shape memory alloys (HTSMAs) have been investigated extensively, where however β→isothermal ω (iso-ω, hexagonal) transition upon heating has not been accessed. Following α” (orthorhombic)→β reverse martensitic transformation on heating, iso-ω precipitation is commonly encountered. These two transitions may overlap within certain composition range, but have not been clearly differentiated. It is of vital importance for the understanding of the subsequent transition behaviors. In this paper, phase transformations upon heating at various heating rates were characterized in quenched Ti-(16–25 at.%)Nb HTSMAs. In contrast to the linear increase in As (the starting temperature of α”→β transition) with decreasing Nb-content, ωs (the starting temperature of β→iso-ω transition) exhibits normal decrease firstly and shows abnormal increase below 20Nb. It is because iso-ω precipitates only in the reversed β phase but not in α” martensite proved by transmission electron microscopy observations. Namely, β→iso-ω transition is postponed to higher temperature due to the suppression of α” martensite below 20Nb. On this basis, the characteristics of both transformations can be determined for Ti-Nb below 20Nb by proper peak deconvolution. New metastable phase diagrams of Ti-Nb are formulated, including both α”→β and β→iso-ω transitions upon heating. Moreover, effective activation energies for β→iso-ω transition during isochronal annealing are determined by the Kissinger method.