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