Thermal stability and phase transformations of martensitic Ti-Nb alloys

Aiming at understanding the governing microstructural phenomena during heat treatments of Ni-free Ti-based shape memory materials for biomedical applications, a series of Ti-Nb alloys with Nb concentrations up to 29 wt% was produced by cold-crucible casting, followed by homogenization treatment and water quenching. Despite the large amount of literature available concerning the thermal stability and ageing behavior of Ti-Nb alloys, only few studies were performed dealing with the isochronal transformation behavior of initially martensitic Ti-Nb alloys. In this work, the formation of martensites (alpha' and alpha '') and their stability under different thermal processing conditions were investigated by a combination of x-ray diffraction, differential scanning calorimetry, dilatometry and electron microscopy. The effect of Nb additions on the structural competition in correlation with stable and metastable phase diagrams was also studied. Alloys with 24 wt% Nb or less undergo a alpha'/alpha '' -> alpha + beta -> beta transformation sequence on heating from room temperature to 1155 K. In alloys containing >24 wt% Nb alpha '' martensitically reverts back to beta(0), which is highly unstable against chemical demixing by formation of isothermal omega(iso). During slow cooling from the single phase beta domain alpha precipitates and only very limited amounts of alpha '' martensite form.