Superelasticity in β titanium alloys with nitrogen addition

Deformation behavior at room temperature was examined in near-β titanium alloys, Ti-10V-2Fe-3Al, containing small amounts of N up to 0.2 mass%. After a β solution treatment, thermally induced α″ martensite is formed in the N-free alloy but no martensite is formed in the 0.1 and 0.2% N alloys. In the N-free alloy, stress-induced α″ martensite transformation occurs during deformation at room temperature, and a shape memory effect by heating after deformation is exhibited, as was previously reported. In the N-containing alloys, shape recovery over 90%, which is much larger than that in the N-free alloy, was obtained by unloading after a bending deformation. In the 0.1 % N alloy, forward and reverse martensite transformations are observed during tensile deformation by in-situ optical microscopy, confirming the occurrence of superelasticity in this alloy. On the other hand, a large elastic strain over 1 % is obtained in the 0.2% alloy by strengthening of β matrix, although the α″ martensite introduced by further deformation does not disappear by unloading.