The mechanism of negative linear thermal expansion behavior of cold-rolled Ti-34Nb alloy

The mechanism of negative linear thermal expansion (NLTE) of Ti-34Nb (wt.%) alloy after 90% cold rolling is investigated by X-ray diffraction, thermal expansion and transmission electron microscopy. From the results, it is observed that 90% cold-rolled Ti-34Nb alloy is composed of β and α″ (Martensite) phases with the existence of β and α″ textures along rolling direction (RD). The cyclic thermal expansion, XRD and TEM studies show that when the thermal cycle temperature is at 100 °C, the RD of 90% cold-rolled Ti-34Nb alloy performs a reversible NLTE, which gradually weakens when thermal cycle temperature is at 300 °C, attributing to the gradual decomposition of α″-phase. When thermal cycle temperature rises to 380 °C, the reversible NLTE disappears and turns into positive linear thermal expansion, meanwhile, α″-phase decomposes completely. Based on the formation of β and α″ textures by cold rolling and α″ ↔ β thermo-reversible transformation mechanism, the NLTE mechanism of 90% cold-rolled Ti-34Nb alloy is successfully explained. Moreover, according to the present results, a novel strategy is proposed to tailoring the negative coefficient of linear thermal expansion by changing α″ content, which improves the application potential of Ti alloys.