α phase precipitation and mechanical properties of Nb-modified Ti-5553 alloy

Metastable β Ti alloys can be strengthened by controlling α phase precipitation during aging heat treatments. An example of this is Ti-5553 alloy (Ti-5Al-5Mo-5V-3Cr-0.5Fe wt%), which has been extensively applied in the aerospace industry due to its high strength-to-weight ratio. The substitution of V with Nb in Ti-5553 enhances the alloy's hardenability and may alter the mechanisms of α phase precipitation, thereby affecting its mechanical properties. Accordingly, the aim of this study was to evaluate the α phase precipitation and mechanical behavior of Nb-modified Ti-5553. Samples were prepared by arc melting and hot working by swaging, and then subjected to HT1 (solution heat-treated above β transus and aged at 600 °C for 600 min) or HT2 (heat-treated at temperatures below β transus, air cooled, heated to 600 °C and aged for 600 min). Compared to the base Ti-5553, the Nb-modified alloy exhibited improved mechanical strength in the HT1 condition due to nonhomogeneous α phase precipitation, but lower ductility in both aging conditions. An examination of the fracture surface revealed that the substitution of V with Nb did not change the fracture behavior and all the samples showed a quasi-cleavage fracture mode with some evidence of intergranular features. The Ti-5553 samples subjected to HT1 presented more dimples and hence, higher ductility. On the other hand, the HT2 samples showed fracture surfaces with predominantly transgranular features. Lastly, with regard to the pseudospinodal mechanism in the Nb-modified alloy, it was found that the substitution of V with Nb reduces the temperature at which pseudospinodal decomposition occurs by roughly 75 °C.