Abstract:

The evolution of phase constitution and mechanical properties of a low-modulus metastable β-type Ti-30Nb-1Mo-4Sn(wt%) titanium alloy subjected to thermo-mechanical treatment, is systematically investigated using X-ray diffraction apparatus (XRD), transmission electron microscopy (TEM) and mechanical testing. The results indicate that the solution-treated alloy exhibits low structural stability and undergoes stress-induced martensitic transformation, leading to a quite low yield stress. Even though the solution-treated alloys are subjected to aging treatment, the amount of precipitates is small due to the lack of heterogeneous nucleation sites and relatively short duration, leading to the low stability of the alloy. As a result, martensitic transformation occurs during quenching and the strength of the specimens is not improved significantly. After cold rolling, grain refinement takes place and large amount of dislocations is induced which provides sufficient heterogeneous nucleation sites for the α precipitation, therefore, the yield strength and tensile strength of the cold rolled alloy are significantly improved during aging treatment. It is worth noting that the maximum strength can be obtained in the cold rolled plus annealed alloys with a proper annealing temperature (350 ℃). Moreover, the strength of the alloy declines sharply with further increased annealed temperature, which can be attributed to the decrease in the density of dislocations due to recrystallization.

Key words: metastable &beta, titanium alloy, elasticity modulus, aging, martensitic transformation, mechanical properties