Effect of stress-induced α″ martensite on Young's modulus of β Ti-33.6Nb-4Sn alloy

The effect of stress-induced α″ martensite on Young's modulus was investigated by cyclic tensile tests, optical microscopy and X-ray diffraction analysis to determine the low Young's modulus of severely rolled or swaged, unstable β Ti alloys. An unstable β Ti-33.6%Nb-4%Sn alloy and a stable, slightly Nb-rich Ti-36%Nb-4%Sn alloy used as a reference were groove rolled and swaged after hot-forging. The groove rolling and swaging of Ti-33.6Nb-4Sn rods formed a fiber structure composed of 〈010〉_α″ and 〈110〉_β textures along the rolling direction, and stress-induced α″ martensite was stabilized by groove rolling, swaging and cyclic tensile deformation. Almost completely stabilized α″, which was attained by a 91% reduction in rod diameter via rolling followed by swaging, exhibited linear behavior in the initial stage of a stress-strain curve. Nb-rich Ti-36Nb-4Sn rods that were 91% rolled and swaged showed 〈110〉_β textures and linear behavior. From the slope of the linear portion of the stress-strain curves, Young's moduli along the rolling/swaging direction were determined to be 40GPa for Ti-33.6Nb-4Sn and 48GPa for Ti-36Nb-4Sn. Assuming that the rule of mixture is applicable to 91% rolled/swaged Ti-33.6Nb-4Sn with volume fractions of 0.81 for α″ and 0.19 for β, a low Young's modulus of 38GPa was calculated for the 〈010〉 orientation of α″ martensite. It is concluded that the fiber texture development of 〈010〉_α″ and 〈110〉_β with a low modulus is responsible for the low Young's modulus in rolled or swaged, unstable Ti-33.6Nb-4Sn.