Thermal stability and phase transformations of martensitic Ti–Nb alloys

Abstract

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 (α′ and α″) 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 transformation sequence on heating from room temperature to 1155 K. In alloys containing >24 wt% Nb α″ martensitically reverts back to β0, which is highly unstable against chemical demixing by formation of isothermal ωiso. During slow cooling from the single phase β domain α precipitates and only very limited amounts of α″ martensite form.

Factors influencing the elastic moduli, reversible strains and hysteresis loops in martensitic Ti–Nb alloys

“…Heating of the initial martensitic microstructure results in either decomposition of the martensites directly into the equilibrium α and β phases or displacive reversion of the orthorhombic martensite α″ to the parent β-phase. This is followed by diffusional formation of isothermal ω iso and α precipitates [34]. The modulus minimum in solute lean alloys containing 15-16 wt.% Nb corresponds very well to the limit from which on further addition of Nb causes the hcp α′ martensite to lose its hexagonality and to distort into the orthorhombic lattice of α″ [13].…”

“…The phase formation in Ti-Nb-alloys is strongly affected by the prevailing cooling conditions [31][32][33][34]. Fig.…”

“…2a, which shows the distribution of Nb mapped in a section of as-cast Ti-28.5Nb. Slower cooling rates such as those occurring during cooling in a furnace may give rise to pronounced precipitation of α [32,34]. A homogenization treatment in the single-phase β-zone for sufficient amount of time followed by quenching eliminates the microscopic elemental fluctuations present after casting, as shown by Fig.…”

Development of Ti-Nb-Zr shape memory alloys

“…With reference to Fig. 6 and based on the previous observations [32,33] a sharp endothermic peak corresponds to the α''→ β and precedes an exothermic reaction indicating the formation of the ω iso phase. While the athermal ω phase (ω ath ) forms on quenching, its isothermal correspondent (ω iso ) forms on aging in a temperature range that can be between 100 to 500 ˚C.…”

Structure and corrosion resistance of Ti-Nb layers obtained by non-vacuum electron-beam cladding on CP titanium substrates

“…The conventional technology of Ti-Nb alloy fabrication consists in multiple arc remelting of the components in vacuum or shielding gases [8][9][10]. It should be noted that continuous ingot inverting is carried out between remeltings for the uniform distribution of alloying elements over a cross section.…”