The effect of Nb content on the superplasticity of Ti–25Al–xNb alloy

The superplasticity of three alloys with composition of Ti–25Al–xNb (x=7,10,13) was investigated in this study. The widmanstattan microstructure of the alloys was refined by the addition of Nb element. Throughout the two-stage hot rolling processes and designed recrystallization treatments, the microstructures of the three Ti–25Al–xNb alloys were manipulated to be very similar. The superplastic tests show that the superplasticity of the alloy increases and the flow stress of the alloy decreases as the Nb content raises. It means that the addition of Nb element could improve the properties of grain boundaries and make the grain boundary sliding easy. The dislocation networks in the α2 grains were observed in the TEM microscopy of a superplastically deformed specimen. It could be concluded that the superplasticity of Ti–25Al–xNb alloy can be explained by the superplastic model of grain boundary sliding accommodated by dislocation motion. These alloys may develop an (0 0 0 2)[2 0 2̄ 0] rolling texture after a series of rolling processes. All the textured Ti–25Al–xNb alloys exhibit anisotropic superplasticity. Since the preferred orientation may be disturbed by the superplastic grain motion, Ti–25Al–7Nb alloy with less Nb content shows less superplasticity than the other two alloys, and the degree of anisotropic superplasticity of Ti–25Al–7Nb alloy is higher than those of other two alloys.