The Study Of Elastocaloric Effect In Ni-Mn-Ti And Ti-Ni-Cu Shape Memory Alloy

With the increasingly serious environmental and energy problems,it has become very urgent to explore new refrigeration technology to replace the traditional vapor-compression refrigeration which has shown a lot of drawbacks.At present,because of its significant advantages in environmental protection,high efficiency and energy saving,the refrigeration technology based on the caloric effect of solid-state phase transition has become a strong candidate for the new refrigeration technology.Among solid-state refrigeration technology,the refrigeration technology based on elastocaloric effect have became the research hotspot due to its significant thermal effect,wide window temperature and demand for cheap equipment.The Ni-Mn based shape memory alloy is considered as a potential refrigerating material because it has multiple thermal effects of solid-state refrigeration.However,the intrinsic brittleness of Ni-Mn based shape memory alloys seriously limits the application of alloys.In addition,the Ti-Ni based shape memory alloy has become a focus in the research of elastocaloric due to its remarkable temperature change.However,its large driving stress requirements and low cycling stability also limit the application of Ti-Ni base shape memory alloy.Therefore,the Ni-Mn-Ti based alloy with excellent mechanical properties and Ti-Ni-Cu based alloy with high cycle stability were studied in this paper.(1)The influence of Ti content on the elastocaloric properties and mechanical properties of Ni51Mn31-xTi18+x(x=0,1,2)alloy was studied.The XRD shows that Ni51Mn31-xTi18+x(x=0,1,2)alloy is composed of B2 phase(Austenite)and 5M(Martensite)phase at room temperature.With the increase of Ti element,the Martensitic phase transformation temperature gradually decreased,the analysis shows that the decrease of martensite temperature results from the decrease of valence electron concentration.At the same time,the mechanical properties of the sample have been improved with increasing Ti element at room temperature,the fracture strength of the sample increased from 1000 Mpa to 1500 Mpa.The elastocaloric tests show that the Ni51Mn30Ti19 exhibits the best elastocaloric effects,in which the adiabatic temperature change is up to-8.8 K under 500 Mpa,which is superior to most polycrystalline Ni-Mn based shape memory alloys.In addition,the alloy exhibits excellent reversibility,and the decrease of temperature change almost can't be observed in the process of up to 350 loading and unloading cycles.(2)The elastocaloric and magnetocaloric effect in Fe and Co Co-doped Ni-Co-Fe-Mn-Ti alloy were studied.The XRD result confirm that Ni3 7Co9Fe4Mn35Ti15 alloy is composed of B2 phase and 5M phase at room temperature.By the analysis of M-T and DSC tests,the Martensitic transformation temperature is closed to room temperature.Therefore,the analysis of temperature change and entropy change are conductedt near room temperature.The result shows that the temperature change is up to 6.3 K under 400 Mpa and the magnetic entropy change is 8.4 J·Kg-1 K-1 with the application of the magnetic field of 3 T.In addition,the elastocaloric properties keeps almost constant even after 1000 cycles.(3)The elastocaloric and mechanical property have been studied in Ti50-xNi35+xCu15(x=1,2,3).The result of XRD shows that the alloys are the mixture of f B2 phase,B19 phase and the Ni-rich precipitate at room temperature.Ti48Ni37Cu15 alloy show a temperature change of 8.6 K under 500 Mpa,and the temperature change keep almost constant during the next 300 cycles for the trained samples.In addition,the sample showed strong mechanical properties,and the fracture strength of all the samples are greater than 1700 Mpa.By introducting the Nb element,the enhenced temperature change has been realized in(Ti50Ni38Cu12)100-xNbx alloys.The temperature change is as high as 10.6 K under 500 Mpa,which is superior to most polycrystalline shape memory alloys..The temperature change of sample which has been trained keep unchanged in the next 300 cycles.The above results show that this kind of alloy is a good candidate for elastocaloric refrigeration materials at room temperature.