Martensitic transformation behavior and superelasticity of Ni50Ti49Nb1 alloy
Ni50Ti49Nb1 and Ni50Ti50 alloy wires with different grain sizes were prepared by wire drawing at room temperature and annealing process.The differential scanning calorimetry(DSC),electrical resistivity(ER)measurement,X-ray diffraction(XRD)and transmission electron microscope(TEM)were used to characterize the martensitic transformation behavior during cooling and heating and the microstructure.Superelastic proper-ties were examined at different temperatures using a universal tensile machine.Results show that the phase transition of both types of alloys were increasingly suppressed with the reduction of grain size.When the grain size is nanocrystalline,the B19'phase was completely suppressed.The suppression of R phase in nanocrystal-line Ni50Ti49Nb1 was particularly severe at low temperature.As tensile test results,nanocrystalline Ni50Ti49Nb1 alloy exhibited tensile superelasticity over 218-398 K,being significantly superior to nanocrystalline Ni50Ti50.Moreover,the critical stresses and stress hysteresis of the former changed with temperature in a non-linear manner,in sharp contrast to that of the latter.The results of this study may shed light in the understanding of the role of Nb on the phase transformation behavior and superelasticity of NiTi based alloys,and in the develop-ment of new superelastic alloys.
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