Formation of thermally induced α″martensite after β solutionizing is suppressed by addition of N more than 0.1mass%. All the alloys are deformed by stress-induced martensitic transformation at room temperature. Forward and reverse stress-induced martensitic transformations during tensile tests are observed in the alloys with nitrogen addition, confirming the occurrence of superelasticity. The addition of N also increases the amount of elastic deformation of β matrix by solid solution strengthening.

Decreasing the cooling rate after β solutionizing is effective in enhancement of superelasticity. In the N-free alloy, there is no superelasticity after cooling at 270/K whereas superelasticity with recovered strain of about 1% appears after cooling slower than 50K/s. In the 0.2N alloy, the maximum recovered strain exceeds 3% when cooled at 50K/s.