The Martensitic Transformation And Elastocaloric Effect Of Ti-Ni-Cu-V Shape Memory Alloys

Since the conventional vapor compression refrigeration technology can no longer meet the demand of sustainable development,elastocaloric refrigeration has attracted a lot of attention from researchers.Among them,Ti-Ni-based shape memory alloys have become one of the research hotspots due to their large adiabatic temperature change.However,the Ti-Ni alloy itself has a large transformation hysteresis,which leads to poor cyclic stability and becomes a bottleneck for its wide application.In order to break through the bottleneck,researchers have conducted a series of researches,however,most of them have been conducted by experimental trial-and-error method.The trial-and-error method has the problems of low efficiency and long cycle time,and cannot design the alloy with narrow hysteresis and large adiabatic temperature change quickly and efficiently.In this thesis,a Ti-Ni-Cu-V alloy with a narrow hysteresis at room temperature is quickly predicted by combining first-principles calculations and machine learning,and its hysteresis change law is verified by preparation and experimental characterization,and its elastocaloric properties are systematically studied.Finally,the Ti-Ni-Cu-V alloy with narrow hysteresis and large adiabatic temperature change at room temperature is designed.In this thesis,the prediction models of phase change temperature and thermal hysteresis of Ti-Ni-Cu-V alloy are established by machine learning and first principles calculation,and the models are evaluated by R²（coefficient of determination）.The R2of all prediction models is higher than 0.9,which indicates that the models have high accuracy and excellent prediction ability.Based on the prediction results,narrow hysteresis Cu and V doping ranges around 10 at.%and 1 at.%,respectively,at room temperature were determined.To verify the accuracy of the predicted results,Ti_49.2-xNi_40.8Cu₁₀V_x（x=0.5,1,1.5）and Ti_49.2Ni_40.8-xCu_x（x=0,2.5,5,7.5,10,12.5）alloys were prepared in this paper,and it was found by DSC experiments that compared with Ti-Ni and Ti-Ni-Cu alloys,Ti-Ni-Cu-V alloys have lower hysteresis.The theoretical study shows that theλ₂（eigenvalue）between Ti-Ni-Cu-V parent phase and martensitic phase is closer to 1,indicating that the compatibility between the parent phase and martensitic phase of the alloy gradually increases and the macroscopic manifestation of thermal hysteresis becomes narrower.The experimental results of the elastocaloric properties show that the adiabatic temperature change does not decrease after the introduction of Cu and V elements,and the cyclic stability is greatly improved.The maximum adiabatic temperature changes of 6.5 K and 7.8 K were measured at room temperature,and the adiabatic temperature changes were higher than those of the intrinsic alloy.In the cycling stability test,Ti_48.2Ni_50.8 alloy decayed by 40%after 4 cycles.However,the adiabatic temperature change and stress-strain curves of Ti_48.2Ni_40.8Cu₁₀V₁and Ti_47.7Ni_40.8Cu₁₀V_1.5 alloys did not change significantly in 30 cycles.The above study shows that Cu and V doping at room temperature can improve the elastocaloric properties of Ti-Ni based shape memory alloys by reducing the hysteresis.This paper proposes a new idea for the optimization of hysteresis of Ti-Ni-based shape memory alloys and provides a reference for the research and development of new shape memory alloys with narrow hysteresis and large adiabatic temperature change at room temperature.