Effect of Nb Addition on Martensitic Transformation Behavior of AuTi-15Co Based Biomedical Shape Memory Alloys

The effect of quaternary addition of Nb being from 0 to 2mol% on the phase constitution, phase transformation behavior and shape memory behavior of Au-50mol%Ti-15mol%Co (AuTiCo) alloys is studied. X-ray diffraction analysis shows that, with increasing Nb concentration, the primary phase appeared at room temperature changes from B19 orthorhombic martensite phase to B2 cubic parent phase, and that all the lattice parameters are not largely changed by Nb addition. The maximum transformation strain evaluated in 2mol%Nb-added AuTiCo is 7.4%. Differential scanning calorimetry revealed that the reverse martensitic transformation finish temperature (Af ) decreases from 414K in Nb-free AuTiCo to 303K by 2mol%Nb addition. The ratio of Af change by Nb addition is evaluated to be 155K/mol%Nb, that is comparably larger than 131K/mol%Nb in NiTi. These results indicate that Nb addition is effective to stabilize B2 parent phase. AuTiCo alloy containing less than 2mol%Nb exhibits shape memory effect with 90% shape recovery ratio. Besides, AuTiCo containing 2mol%Nb exhibits 2.8% superelastic shape recovery at room temperature as well as 7.4% maximum transformation strain. Therefore, Nb addition to AuTiCo is effective to decrease the martensitic transformation temperature and to improve superelasticity, although the transformation temperature hysteresis increases by Nb addition, similar to Nb-added NiTi. [doi:10.2320/matertrans.M2014418]