Abstract
Two different types of martensites, i.e. the α'- and α"-phases, are known to form in (α+β)-titanium alloys by a diffusionless transformation of the β-phase. The material properties of α'-martensite include a comparably high Young’s modulus, high strength and lower ductility so that deformation at room temperature is not possible whereas the α"-martensite has a low Young’s modulus, low strength at high ductility, meaning that room-temperature deformation with strong work hardening might be feasible. In addition, in metastable β-alloys, athermal ω-phase as a third non-equilibrium titanium phase can develop during quenching. The presence of ω-phase in titanium alloys in general leads to an increase in the Young’s modulus, strong hardening and a pronounced decrease in ductility.
Depending on the exact composition of the alloys, in classical (α+β)-alloys containing comparably low amounts of β-stabilising elements, the α'-martensite can form during quenching. In contrast to this, in β-rich (α+β)-alloys with higher amounts of β-stabilisers, a martensitic β-to-α"-transformation can occur. In some β-rich (α+β)-alloys, the α'-phase can form as well. In case the amount of β-stabilising elements is further increased, in the so-called solute-lean metastable β-alloys, a martensitic transformation is supressed during quenching but a transformation of the β-phase to athermal ω-phase is possible. Independent of the type of non-equilibrium phase being formed during quenching, neither martensite- nor ω-containing microstructures can be used in safety-critical applications as the fatigue performance is typically reduced compared to conventional microstructures. On the other hand, the non-equilibrium phases can be used for microstructural refinement and thus an improvement of the properties is achieved if a decomposition by a subsequent heat-treatment is performed.
In the present paper, microstructure and properties improvement by non-equilibrium phase decomposition is discussed at different examples, namely, (1) decomposition of α"-martensite in Ti-6Al-2Sn-4Zr-6Mo, an alloy primarily used as a fan disk material at higher strength levels in some newer and larger aircraft engines, leading to an increase in fatigue performance of about 15%, (2) α"-decomposition in Ti-13Nb-13Zr, a second-generation titanium alloy for dental implants, leading to a nanostructured material with antibacterial properties, improved biocompatibility, well-adjusted Young’s modulus at high strength and sufficient ductility and (3) the application of an ω-to-α-transformation in Ti-15Mo, a second-generation medical titanium alloy for larger implants, to improve the properties of additively manufactured parts and develop equiaxed, nano-sized α-precipitates in a β-matrix. The results of the different investigations show that non-equilibrium phase decomposition can be a powerful tool for microstructure and properties optimisation in titanium alloys.