Study On Martensitic Transformation And Functional Behavior Of Ti-Zr-Hf-Ni-Cu(Co) High-entropy Shape Memory Alloys

As a kind of functional material for sensing and driving,shape memory alloys not only exhibit superelasticity,shape memory effect and elastocaloric effect,but also have the comprehensive characteristics of giant output strain,high response frequency and precise control.As a result,this kind of alloys,as sensing,driving,damping and elastocaloric refrigeration materials,play an important role in aerospace,automobile,energy,construction,solid-state refrigeration fields.However,shape memory alloys have many disadvantages such as narrow operating temperature window,poor cycle stability and high-temperature thermal softening,which limit their development and applications.Based on the above research background,a series of Ti-Zr-Hf-Ni-Cu(Co)novel high-entropy shape memory alloys have been developed by introducing the concept of high-entropy alloy into shape memory alloys,and martensitic transformation behavior,microstructure and related functional properties of this kind of alloys have been studied.Based on Ni-Ti-Hf-Cu shape memory alloys,a novel Ti25Zr10Hf15Ni25Cu25 high-entropy shape memory alloy with thermoelastic martensitic transformation has been developed by employing high-entropy alloy concept.The temperature-induced reversible martensitic transformation of high-entropy shape memory alloy was proved by DSC and in-situ synchrotron X-ray diffraction technique.The present alloy exhibits superelasticity with a recoverable strain up to 3%,and the superelasticity originating from stress-induced reversible martensitic transformation was revealed by using in-situ synchrotron X-ray diffraction technique during loading and unloading.In addition,martensitic transformation is a first-order phase transformation,thus the high-entropy shape memory alloy exhibits excellent elastocaloric effect,and the adiabatic temperature change is up to 9.3 K.This work not only provides a new idea for the development of novel functional materials for sensors,actuators and solid-state refrigeration applications,but also broadens the application range of high-entropy alloys.The cryogenic and wide-temperature-range superelasticity and elastocaloric effect in(TiZrHf)50Ni25Cu12Co13 high-entropy shape memory alloy were investigated.The results show that the present alloy exhibits excellent superelasticity at temperature span of 77 K?423 K.Especially,at 77 K and 123 K,the superelastic recoverable strains are 4.3%and 4.7%,respectively.In addition,because of stress-induced martensitic transformation with a great isothermal entropy change,(TiZrHf)50Ni25Cu12Co13 high-entropy shape memory alloy shows a huge elastocaloric effect in different temperature and stress conditions.Under 1600 MPa,the maximum adiabatic temperature change can reach as high as 15.2 K,and adiabatic temperature change is more than 4 K in the temperature range from 123 K to 423 K.Based on the first-principle research and TEM observation,the mechanism of wide-temperature-range superelasticity and elastocaloric effect is mainly attributed to the solid solution strengthening induced by severe lattice distortion effect.Therefore,this study reveals that(TiZrHf)50Ni25Cu12Co13 high-entropy shape memory alloy exhibits excellent functional properties at cryogenic temperature and wide-temperature-range conditions.Employing the lattice distortion effect and sluggish diffusion effect effectively inhibiting thermal softening phenomenon of high-entropy alloys at high temperatures,a Ti20Zr15Hf15Ni25Cu25 high-entropy high-temperature shape memory alloy with excellent high-temperature superelasticity and shape memory effect was designed.Ti20Zr15Hf15Ni25Cu25 high-entropy high-temperature shape memory alloy exhibits superelasticity with the recoverable strain up to 4%in the temperature range of 458 K-558 K.Furthermore,in three-point bending mode,the present alloy exhibits shape memory effect with a recoverable strain up to 2.6%.This work has guiding significance for the design of functional materials with excellent high-temperature superelasticity and shape memory effect.A(TiZrHf)50Ni25Cu17Co8 high-entropy shape memory alloy with strong texture was prepared by directional solidification method.Due to<011>columnar crystalline microstructure,the present alloy exhibits superior superelasticity,shape memory effect and high cyclability of elastocaloric effect.The present alloy shows superelasticity with the recoverable strain up to 5%in the temperature range of 293 K?393 K.Under 600 MPa,the alloy exhibits excellent shape memory effect with a recoverable strain as high as 3.7%.In addition,compared with the reported high-entropy shape memory alloys,the present alloy shows a large adiabatic temperature change under smaller stress.At the same time,after 1500 cycles,this alloy exhibits high cyclability of elastocaloric effect.