Effects of double aging heat treatment on the microstructure, Vickers hardness and elastic modulus of Ti–Nb alloys

Aging heat treatment can be a good way to optimize mechanical properties, changing the microstructure, and hence, the mechanical behavior of Ti alloys. The effects of aging heat treatments on β-type Ti–30Nb alloy were investigated to evaluate the kinetics of α″ → α + β transformation. The results obtained from differential scanning calorimetry and high-temperature X-ray diffraction experiments indicated the complete decomposition of orthorhombic α″ phase at close to 300 °C, followed by α phase precipitation at 470 °C. The aging heat treatments also enabled us to observe a transformation sequence α″ → β + ω → β + ω + α, indicating martensite decomposition and ω phase precipitation at 260 °C after 2 h, followed by α phase nucleation after heating at 400 °C for 1 h. The elastic modulus and Vickers hardness of Ti–30Nb alloy were found to be very sensitive to the microstructural changes caused by heat treatment.

► DSC and XRD shed light on the α″ decomposition and nucleation of ω and α phases.
► Aging allows for α″→β transformation and nucleation of ω dispersed in the β matrix.
► During aging, α″ interplanar distances are reduced to enable β phase nucleation.
► Mechanical behavior is dependent on the microstructure and the phases in the alloy.
► It is not possible to obtain high strength and low elastic modulus by applying aging.