Effect of O addition on microstructure and mechanical properties of Ti-Nb alloys with various β stability

To study the effect of O on β-Ti alloys with various β phase stability, Ti–xNb–0.4O (x = 30, 32, 34, 36, 38, 40, 42, mass%) alloys were prepared by arc melting followed by homogenization, hot rolling, and solution treatment. To ensure the O content of the alloys, melting and hot rolling were performed under an Ar atmosphere, and homogenization and solution treatment were performed under vacuum conditions. The mass fractions of O in the Ti–Nb–O alloys were finally accurately controlled in the range of 0.4–0.5%. The phase composition of the alloys was determined by analyzing the X-ray diffraction patterns, the microstructure was observed by optical microscopy, and the mechanical properties were measured by tensile tests. The ω phase and α" phase were found in the solution-treated Ti–30Nb–0.4O. Ti–30Nb–0.4O showed the lowest yield strength owing to the existence of the α" phase. O suppressed ω phase and α" phase transformations during quenching, and showed an obvious solution-strengthening effect. Because the O content of the alloys was essentially the same, the Ti–xNb–0.4O alloys showed similar tensile strengths. The Young's moduli of the alloys ranged from 60 to 69 GPa, and the elongations were all higher than 15%. Particularly, Ti–36Nb–0.4O exhibited desirable mechanical properties, including a Young's modulus of 61 GPa, a tensile strength of 760 MPa, and an elongation of approximately 35%.