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The mechanical properties and shape memory behavior of Ti–26Nb–(0–1.0)B and Ti–27Nb alloys were investigated. The stress for inducing martensite and the critical stress for slip deformation were greatly affected by increasing B content up to 0.1 at%, and less affected by further increase in B content up to 0.7 at%. The martensitic transformation start temperature decreased suddenly with a slope of 350 K/at% B when B content was less than 0.1 at% and decreased with a smaller slope of 15 K/at% B with further increase in B content up to 1.0 at%. In all samples tested, only superelasticity was observed at room temperature. TEM investigations revealed that the solubility of B in Ti–26Nb alloy was around 0.1 at%. In spite of the low solubility limit, B was effective to stabilize the superelastic behavior by effectively increasing the critical stress for slip deformation. On the other hand, while the stress for inducing martensite increased effectively by the addition of 1.0 at% Nb to the Ti–26Nb alloy, the critical stress for slip deformation remained almost constant and hence superelasticity was deteriorated. [ABSTRACT FROM AUTHOR]

While the stress for inducing the martensitic transformation in a Ti–27Nb alloy changed only slightly on aging, the stress at which the martensite phase starts to plastically deform and the ultimate tensile strength increased on aging due to the formation of thermal ω phase or α phase. An anomalous temperature dependence of the shape memory behavior was also observed and explained by the fact that the thermal ω phase encouraged the formation of the athermal ω phase during cooling. [ABSTRACT FROM AUTHOR]

Abstract: The effect of room temperature (RT) aging on the superelasticity of Ti–Nb–Mo-based superelastic alloys is investigated. The results show that annealing at relatively low temperatures such as 973K after severe cold rolling results in poor resistance to the effect of RT aging. The transformation stress increases considerably due to the formation of an isothermal ω phase at RT. Addition of Sn is partially effective in suppressing the RT aging effect in the specimens annealed at 973K. The RT aging effect is suppressed by increasing the annealing temperature, due to the annihilation of lattice defects or non-equilibrium vacancies introduced during cold rolling, which are responsible for accelerating the diffusion process, however, superelasticity is reduced by annealing at higher temperatures, due to a decrease in the critical stress for slip deformation (σ CSS). The specimen annealed at 1173K followed by aging at 773K exhibits stable superelasticity with a high resistance to the effect of RT aging. Annealing at 1173K causes the annihilation of lattice defects or non-equilibrium vacancies, while aging at 773K induces precipitation of the α phase, which in turn causes an increase in σ CSS, and further enhances the resistance to the RT aging effect by enriching the matrix with β-stabilizing elements. [Copyright &y& Elsevier]