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

Metastable beta Ti alloys can be strengthened by controlling alpha 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 alpha phase precipitation, thereby affecting its mechanical properties. Accordingly, the aim of this study was to evaluate the alpha 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 beta transus and aged at 600 degrees C for 600 min) or HT2 (heat-treated at temperatures below beta transus, air cooled, heated to 600 degrees 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 alpha 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 degrees C. (C) 2016 Elsevier B.V. All rights reserved.