Isothermal omega assisted alpha phase precipitation and microstructural evolution of an aged Ti-30NB-3Fe alloy

Abstract

The mechanical behavior of metastable β Ti alloys can be controlled through heat treatments. Thus, the relationship between the precipitation of α phase and the mechanical properties of these alloys is of special interest. In this work, the microstructure evolution of Ti-30Nb-3Fe alloy during aging heat treatments was evaluated using optical microscopy, scanning electron microscopy and X-ray diffraction. Moreover, Vickers hardness and elastic modulus were measured as a function of aging time. Finally, the ultimate strength and ductility of the alloy aged at 500 °C was assessed by tensile tests. In comparison to a Ti-30Nb alloy, the addition of Fe lowered the β-transus temperature, decreased the martensite start temperature to a value below room temperature, increased the precipitation temperature and reduced the dissolution temperature of ω phase, and lastly, decreased the α phase precipitation temperature. Low heating rates enabled isothermal ω phase precipitation and growth, providing favorable conditions for α phase precipitation and increasing the amount of α phase precipitates. Compared to the solution heat-treated and water-quenched condition, aging heat-treated Ti-30Nb-3Fe alloy presented higher Vickers hardness and mechanical strength, without significant loss of ductility.