• Adding Nb to Ti-5552 alloy reduces the α/β phase transformation temperature, decreasing α phase volume fraction and affecting superplasticity.
• Ti-5552-9Nb alloy exhibits optimal tensile strength (307.2 MPa) and superplasticity (106%) at 923 K.
• Superplastic deformation at 923 K is governed by β grain boundary sliding and dislocation movement.
• Achieving superplasticity at lower temperatures (below 0.5Tm) can reduce energy consumption and shorten production processes.