The applied stress thermal cycle test of the memory alloy spring was carried out on a self-made thermal cycle tester. The applied external force is O. 7N and 1. ON, the upper and lower limits of the thermal cycle are 10 Â° C and 15 Â° C, respectively, at the upper and lower limit temperature dwell time 305, the total number of thermal cycles is 12000 times. The length of the memory alloy spring as a function of temperature is measured by the displacement sensor and recorded by the X-Y instrument.
The operation (elongation) start temperature and the end temperature of the memory alloy spring during heat cycle heating are represented by Ts and T, respectively, and L is used to indicate the shape recovery rate R when the Nth thermal cycle passes the Nth thermal cycle. The relative length of the spring recorded during the cycle is measured as the relative ratio of the maximum length of the spring at the 1st cycle.
Experimental Results and Analysis Under the free-state thermal cycling condition without external force, the relationship between the shape memory performance of the CuznAI memory alloy spring and the number of thermal cycles is shown in Fig. 1. It can be seen that the memory alloy spring in the free state has a small change in the shape recovery rate at the initial stage of the thermal cycle. As the number of thermal cycles increases, the shape memory performance decreases rapidly, and when cycled to 12 for the second time, the shape recovery rate is only about 43%.
The shape memory property of the memory alloy spring has no change in the memory performance of the cuznAI alloy in the side-by-side thermal test. The change of the L of the memory alloy spring element with the number of thermal cycles is exactly opposite to the change of the shape recovery rate.
Since the difference between the initial length of the memory alloy spring at a certain thermal cycle and the initial length at the second thermal cycle shows an unrecoverable cumulative residual elongation, the intestinal curve in 1 also reflects the memory alloy yellow Variation characteristics of cumulative residual elongation during thermal cycling. In the study, it was found that the maximum elongation of each memory cycle without an external force is almost a fixed value, and the cumulative residual elongation increases with the number of cycles, which means that the memory is free of external force. The shape recovery rate attenuation of the alloy spring is mainly caused by the change in the cumulative residual elongation of the memory alloy spring.
It is known that the elongation of the memory alloy spring element and the shrinkage during cooling during thermal cycle heating correspond in principle to the two processes of martensite reverse transformation and Markov transformation in the alloy. The increasing increase of the cumulative residual elongation of the spring during thermal cycling reflects that the thermal cycle process has a great influence on the martensitic transformation in the memory alloy. 'This influence law and the study on the filament or flake specimen of the memory alloy The results obtained are the same.
The phenomenon that the maximum elongation of the spring is almost constant when there is no external force thermal cycle is obvious that the martensite reverse transformation process occurring therein is less affected by the heat. The shape recovery rate R of the no-load memory alloy spring, the relationship between the initial length of the intestine and the number of thermal cycles 2 is that the memory alloy springs are respectively applied with 0.7 N and l. ON constant external force thermal cycle, the shape recovery rate R, the starting length of the island when the cycle changes with the number of cycles.
Compared with the memory alloy spring in free state, especially when the number of cycles is high, the memory performance of the memory alloy spring subjected to 0.7N and 1.0N forces not only does not accelerate the attenuation, but also attenuates The degree is obviously smaller. It can be seen that the shape recovery rate is still as high as 82% after loading the 0.7N memory alloy spring through 12 (X) 0 thermal cycles. While applying 1. In the case of ON force, the shape memory performance also decreases slightly, but it is much higher than 43% when there is no load.
In addition, after the memory alloy spring is subjected to an external force, the initial length of the spring during the thermal cycle is also constant. The influence of the external force on the shape memory performance and the operating temperature of the CuznAI spring is greatly reduced. The results of these experiments show that the martensitic transformation process which causes the spring shrinkage during the thermal cycle becomes easier due to the effect of the outward force, and the process is more thorough, so the shape memory performance associated with it can be maintained. High level.
Applying the same external force as the direction of spring contraction promotes the associated phase change process, but at the same time creates resistance to the reverse transformation of the martensitic body that induces spring elongation, which is clearly related to the applied external force. Under the load state of the memory alloy spring, the starting temperature boundary and the end of the operating temperature are related to the number of thermal cycles. There is no external force, 0 T and N, and N, under the action of external force, the initial temperature of the action of the thermal recovery of the memory alloy spring The action termination temperature T is as shown in 3 as the number of thermal cycles changes.
In the absence of external force, Ts decreases slightly with the number of cycles, while the bounds increase significantly, which is basically consistent with the variation of A: and A, points of Baiyi nAI alloy itself. The memory alloy spring subjected to external force has almost no change in the range of small cycle times, T, and T; only when the number of cycles is large, å…€ and T are slightly increased or decreased. It is worth mentioning that the initial temperature Ts of the shape memory alloy spring during the initial thermal cycle is basically consistent with the starting point A of the martensitic reverse transformation measured by the electric resistance method, but the action termination temperature T of the memory alloy spring, There is a big difference between the apricot and the martensite reverse phase transition temperature.
Conclusion 1) When the heat-extension and shrink-type cuznAI memory alloy spring has no external force thermal cycle, as the number of cycles increases, the redemption memory performance decays faster, and the cumulative residual elongation of the spring also increases faster. 2) Under the thermal cycling conditions of 0.7N and 1.0N constant external force, the shape recovery rate of the memory alloy spring is significantly improved compared with the external force due to the external stress. The greater the external force, the more the shape recovery rate is improved. Significantly; at the same time, the applied constant external force greatly reduces the cumulative residual elongation of the memory alloy spring during thermal cycling.
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