Effect of Nitrogen Addition on Superelasticity of Ti-Zr-Nb Alloys

Abstract

Recently, the Ti-Zr-Nb alloys have been developed as Ni-free shape memory and superelastic alloys. In this study, the effects of Nb and nitrogen (N) contents on martensitic transformation behavior, shape memory effect and superelasticity in Ti-18Zr-(12∼16)Nb-(0∼1.0)N (at%) alloys are investigated using loading and unloading tensile tests, optical microscopy and X-ray diffractometry. The shape memory effect is observed in Ti-18Zr-(12∼13)Nb and Ti-18Zr-12Nb-0.5N alloys at room temperature. The superelastic behavior appears by the increase of Nb or N content. The Ti-18Zr-(14∼15)Nb, Ti-18Zr-(13∼14)Nb-0.5N and Ti-18Zr-(12∼14)Nb-1.0N alloys exhibit the superelasticity at room temperature. The martensitic transformation start temperature decreases by 75 K with 1 at% increase of N content for the Ti-18Zr-13Nb alloy. The critical stress for slip deformation and the stress for inducing the martensitic transformation increase with increasing N content. The superelastic recovery strain is also increased by adding N. The maximum recovery strain of 5.0% is obtained in the Ti-18Zr-14Nb-0.5N alloy.