The Martensitic Transformation And Elastocaloric Effect Of NiTiCuX(x=Co,Fe,Al) Shape Memory Alloy

NiTi-based shape memory alloy has become one of the most widely studied shape memory alloys because of its excellent mechanical properties,large elastocaloric effect,good corrosion resistance and commercial properties,and is considered to be the most promising elastocaloric material.However,there are some problems in NiTi-based alloy,such as large thermal hysteresis and low fatigue life,which limit the application of NiTiCu alloy in elastocaloric field.In NiTi alloy,the addition of appropriate amount of Cu element can reduce the sensitivity of phase transformation temperature to composition change,reduce hysteresis and improve fatigue properties.However,there are still some shortcomings,such as poor mechanical processability,high phase transition temperature and so on.Therefore,the development of NiTiCu alloy which can be well used under room temperature with low hysteresis and good mechanical properties is of great significance for the application of this alloy.In this paper,a series of NiTiCuX(X=Co,Fe,Al)alloys were designed and prepared.By means of X-ray diffractometer,differential scanning calorimeter,field emission scanning electron microscope and electronic universal testing machine,the effects of different mole fractions of Co,Fe and Al on the microstructure,phase transformation behavior and mechanical properties of NiTiCu alloy were studied systematically.The components with phase transition temperature close to room temperature and good mechanical properties were selected for cold rolling and low temperature short-term annealing,which affected the microstructure,phase transformation behavior and elastocaloric effect.In Ni50-xTi50Cux(x=0,5,7.5,10 and 12.5)alloys,the addition of Cu reduces the sensitivity of phase transformation temperature to composition,reduces the transformation hysteresis and reduces the strength of NiTi alloy.With the increase of Cu mole fraction,the phase transformation type of NiTiCu alloy changes from B2→B19’ to B2→B19→B19’.In Ni42.5-xTi50Cu7.5Cox(x=0,1.5,3,3.5,4.5 and 6)alloys,with the increase of the mole fraction of Ni substituted by Co,the valence electron concentration,phase transition temperature and enthalpy change of the alloy decrease gradually,and the mechanical properties are obviously improved.The cold-rolled N Ni39Ti50Cu7.5Co3.5 alloy has excellent mechanical properties,superelasticity and elastothermal properties after 10mins recrystallization annealing at 400℃.When the strain is 9%,the elastocaloric effect reaches-12.2K.In Ni42.5-xTi50Cu7.5Fex(x=0,1,1.5,2,3,4 and 5)alloys,with the increase of the mole fraction of Ni substituted by Fe,the valence electron concentration,phase transition temperature and enthalpy of phase transition decrease gradually,and the type of phase transition changes.The cold-rolled Ni41Ti50Cu7.5Fe1.5 alloy has excellent superelasticity and elastothermal properties after 10min recrystallization annealing at 400℃.When the strain is 10%,the elastocaloric effect can reach-9.2K.In Ni42.5Ti50-xCu7.5Alx(x=0,0.5,1,1.5,2 and 3)alloys,with the increase of the mole fraction of Ti substituted by Al,the valence electron concentration,phase transition temperature and enthalpy of phase transition decrease gradually,and the type of phase transition changes.The addition of appropriate amount of Al can obviously improve the mechanical properties of the alloy.The cold-rolled Ni42.5Ti48.5Cu7.5Al1.5 alloy has excellent elastothermal properties after 10min recrystallization annealing at 400℃.When the strain is 10%,the elastocaloric effect can reach-14.8K.