Cyclic deformation behaviors of nano-grained superelastic Nitinol tubing (Ni55.6Ti (wt%)) of 5.18mm OD x 3.76mm ID was experimentally studied.  Under load controlled fully reversed tension-compression condition at the stress below the plateau stress, the stress-strain exhibits hysteresis loops that are very different from that of a regular NiTi based superelastic alloy under cyclic tension-tension loading.  The loading portion of the stress-strain response resembles the typical elastic-plastic deformation such as regular steel.  However, upon unloading, the stress-strain relationship appears to be approximately linear.  Under a tension-tension loading with the stress amplitude much smaller than the plateau stress of the material, the material displayed stress-strain hysteresis loops that are similar to those under fully reversed tension-compression.

When the material was subjected to a fully reversed cyclic torsion, the shear stress-shear strain behavior resembles that of a regular NiTi based shape memory alloy.  The measurements of the local strains suggested that the phase transformation occurred simultaneously in the gage section of the specimen under fully reversed torsion.  It is also observed that there is a stress-strain asymmetry under fully reversed cyclic torsion.  This asymmetry is repeatable on several samples and therefore is suspected to be a texture effect resulted from tubing drawing processes.