Young's Modulus of Austenite and Martensite Phases in Superelastic NiTi Wires

Young’s modulus of the B2 austenite 40-70 GPa and B19’ martensite 20-40 GPa appear as elastic properties of NiTi in material property tables. Such wide ranges, however, are unacceptable for engineers. The problem is that stress induced B2-R transformation can be easily introduced during elastic loading of austenite, that transformation and twinning processes may proceed beyond the superelastic range and that no reliable experimental data exist for elastic constants of the B19’ martensite. Elastic constants of the B19’ martensite obtained by first principle calculations suggest that martensite shall be the harder phase. This, however, clearly contradicts the majority of experimental data from stress-strain tests.  What are the correct values and why the martensite modulus is so low?

In this work, Young’s moduli of austenite and stress induced martensite in superelastic NiTi wires were studied by three complementary experimental methods (tensile testing, dynamic mechanical analysis and in-situ synchrotron X-ray diffraction) and confronted with values calculated from elastic constants. It is found that Young’s modulus of the B2 austenite is about 70GPa regardless of the wire texture and can’ t be much higher. If a smaller value is measured in stress-strain tests, it is due to the stress induced B2-R transformation, as confirmed by in-situ electric resistance and x-ray diffraction measurements. The unusually low elastic modulus of the stress induced B19’ martensite observed in tensile test on NiTi wire is partially due to the fact that transformation and twinning processes continue even beyond the transformation range. However, as DMA studies clearly show, this is not the main reason for it. The main reason is, as found from the in-situ x-ray diffraction studies, the unique elastic anisotropy and texture in the stress induced martensite. The long term puzzle of the unusually low Young’s modulus of the B19´ martensite was thus finally rationalized.