Experimental Research on Inhibition of temperature to inverse martensitic phase transformation of NiTi Shape Memory Alloy

The effects of temperature changes, load cycles, and amplitude changes on the mechanical properties of NiTi shape memory alloys such as the attenuation of energy dissipation capacity, equivalent damping ratio, and residual strain were studied experimentally. Experimental results show that: under different temperature and amplitude, the shape memory alloy achieves stable performance after 10 cycles, and the attenuation of energy dissipation capacity mainly occurs in the first 6 cycles; as the temperature decreases or the number of cycles increases, equivalent damping ratio tends to decrease; When the temperature decreases near the completion temperature of austenite transformation, the residual strain is significantly affected. Even if the strain amplitude is small, residual deformation accumulation will occur, inhibiting the occurrence of reverse martensite transformation, and it is not good for the super-elasticity of SMA.


Introduction
Shape memory alloy (SMA) is a functional material widely used in electronic equipment, automotive industry, medical equipment and other fields [1] . Normal temperature austenite phase shape memory alloy has super-elasticity at this temperature and is a good material for project of energy dissipation. Regarding the research on the properties of shape memory alloys, scholars at home and abroad have made a series of achievements in terms of temperature influence, cyclic loading, stress (strain) amplitude, and loading rate (frequency) [2][3][4][5] . Around shape memory alloy austenite transformation completion temperature f A , the increase in residual strain will inhibit the occurrence of inverse martensite transformation. The energy dissipation capacity of SMA decays rapidly in the first few cycles and becomes stable after several cycles [6] .
The residual strain of SMA is closely related to temperature and loading amplitude [7] .The increase of strain amplitude will lead to the increase of residual strain. Increased residual strain will inhibit the occurrence of reverse martensitic phase transformation. The larger the residual strain of the shape memory alloy, the less its supere-lasticity will be performed, and the less its energy dissipation performance will be exerted [8] .
Regarding the study of the influence of temperature, most of the set temperature points exceed the austenite transformation temperature f A by 15°C, and the research within 15°C is not thorough. Therefore, through experimental methods, three temperature points were set up to study the performance characteristics when the temperature was close to f A . The test is mainly to study the performance characteristics of shape memory alloys near the austenite transformation temperature f A . The main research contents are: the effect of temperature on the stress-strain relationship, the effect of temperature on residual strain, the effect of temperature on energy dissipation attenuation, and the effect of temperature on equivalent damping ratio.   W  is the energy consumption of a single cycle of SMA, which is an important parameter for SMA to make engineering energy-consuming equipment.;and W is the total strain energy in a single cycle of SMA. The calculation method of the equivalent damping ratio is: In this paper, the effects of temperature change and load cycle on the energy dissipation capacity attenuation range, equivalent damping ratio, and inverse phase transformation of SMA martensite are studied.  Fig.3, Fig.4 and Fig.5 are the stress-strain curves of SMA at 24°C, 18°C and 12°C. After 10 cycles, the SMA's performance has stabilized, defining the attenuation of the energy dissipation capacity of the nth cycle as:   Fig.6. Attenuation of energy dissipation capacity with the cycle Fig. 6 is the attenuation of energy dissipation capacity with the cycle, the attenuation of the energy dissipation capacity of the SMA is large in the first few cycles. At 24℃, 18℃, and 12℃, the energy attenuation of the second cycle is 44.74%, 28.24% and 22.15%, the energy attenuation of the first six cycles is 71.70%, 61.31%, and 74.12%，so the energy attenuation mainly occurs in the first six cycles; the energy change of the two adjacent cycles is within 5% when it is stable,. Therefore, in order to obtain a stable energy-dissipating SMA, a certain number of training is necessary. Fig.7. Variation of the equivalent damping ratio with the number of cycles As shown in Fig. 7, the equivalent damping ratios of 24℃ and 18℃ differ by about 0.05 under the same strain amplitude, and the energy dissipation capacity of SMA is better at 24℃, as the number of cycles increases, both it gradually decreases, but the reduction is not obvious. At 12 ℃, the residual strain accumulation speeds up and the equivalent damping ratio rapidly decays. After 15 cycles, the equivalent damping ratio eq  is only 0.021,this is because of the accumulation of residual strain inhibits the inverse phase transformation of SMA martensite. As a result, the yield platform disappears

Inhibition of low temperature cycling on reverse phase transformation of SMA
According to the tensile-unloading cycle test with an amplitude of 6%, the strain corresponding to the normal martensitic transformation at 12℃ is about 4.5%. Therefore, the tensile-unloading test with an amplitude of 4.12% is performed. After 10 cycles, the result is shown in the Fig.. As shown in the Fig.8, the shape memory alloy still shows a large residual strain under a load cycle of 4.12% amplitude, and the relationship between the residual strain and the cycle is shown in Fig.9. Fig.8. 4.12% amplitude stress-strain curve at 12℃ As shown in Fig. 8, the shape memory alloy still shows a large residual strain under a load cycle of 4.12% amplitude, and the relationship between the residual strain and the cycle is as shown Fig. 9. In the first three cycles, there is little difference in the residual strain under the two amplitude cycles. In the fourth cycle, the residual strain with a magnitude of 4.12% strain is stabilized in advance, and the stable residual strain value is 2.46%. It can be seen that when the temperature is close enough to the completion temperature of martensite phase transformation, even if the strain amplitude is small, SMA will also show large residual deformation, and the accumulation of residual deformation will make the stress-strain curve long and narrow.

Conclusion
At different temperatures, the attenuation of SMA's energy dissipation capacity is mainly concentrated in the first 6 cycles. After the performance is stable, the energy dissipation capacity of the adjacent two cycles changes within 5%, which can provide stable energy-dissipating for the project; under low temperature conditions, the material is more prone to the accumulation of residual strain, inhibits the occurrence of reverse martensite transformation, and the equivalent damping ratio rapidly decays; under low temperature conditions, large residual deformation will occur even if the strain amplitude is small, and residual strain is closely related to strain amplitude.