Preparation And Elastocaloric Effect Of Directionally Solidified Ni-Mn-Sn Magnetic Shape Memory Alloys

Under the background of increasingly serious environmental pollution and energy shortage,elastocaloric refrigeration,as a new solid-state refrigeration technology,has attracted great attention.Ni-Mn-Sn magnetic shape memory alloys,as the alternative material for elastocaloric working medium,have the advantages of low cost,simple preparation,low driving stress and rich external field.In this work,the ternary Ni42+xMn48-xSn10(x=0,1,2,3)series alloys and the quartemary Ni41-xMn42Co7+xSn10(x=0,1,1.5,2)series alloys were designed and prepared,and the related properties of martensitic transformation were studied systematically.The components with phase transition temperature close to room temperature were selected from the above alloys for directionally solidified treatment,then the mechanical properties,the elastocaloric effect and the magnetocaloric effect of the alloys were measured and analyzed.With the substitution of Mn by Ni,the valence electron concentration of Ni42+xMn48-xSn10(x=0,1,2,3)alloys increases gradually,which leads to the increase of the transformation temperatures,so the phase composition of alloys changes from austenite to martensite at room temperature.The elastocaloric effect of Ni43Mn47Sn10 alloy and Ni44Mn46Sn10 alloy was studied.By directional solidification,the alloys form a strong<001>A orientation,and the coarse columnar-shaped grains in the alloys are arranged along the directionally solidified direction,which significantly improves the mechanical properties of the alloy:the fracture strain of Ni43Mn47Sn10 alloy and Ni44Mn46Sn10 alloy can reach 13.6%and 12.5%respectively,and the fracture strength of the alloys can reach 980 MPa and 970 MPa respectively.In addition,the transformation strain of the alloys reaches up to 5.3%and 5.6%respectively,which is greater than the transformation strain of the single crystal Ni-Mn-Sn alloys reported in the literatures.Curie temperatures of the alloys decrease as the antiferromagnetism of the alloys is enhanced with increasing content of Mn,as a result the martensite transformation of the alloys is carried out between the paramagnetic austenite and the weak magnetic martensite during loading and unloading compressive stress,which greatly weakens the negative contribution of magnetic entropy change to elastocaloric effect.At 295 K and 320 K,the remarkable adiabatic temperature variation of-17.3 K and-18 K can be obtained in Ni43Mn47Sn10 alloy and Ni44Mn46Sn10 alloy,respectively.This result has exceeded the maximum adiabatic temperature variation of Ni-Mn based magnetic shape memory alloys reported in the literatures so far,which makes the alloys have the potential to become the alternative material for elastocaloric refrigeration working medium.In Ni41-xMn42Co7+xSn10(x=0,1,1.5,2)series alloys,the valence electron concentration of the alloys decreases with the substitution of Co element for Ni,which leads to the decrease of the transformation temperatures of the alloys.Ni39.5Mn42Co8.5Sn10 was selected for directionally solidified treatment.After directional solidification,the transformation strain of the alloy is as high as 6.7%,the fracture strength increases from 150 MPa to 900 MPa,and the fracture strain increases from 4.2%to 12%.The volume fraction of the martensite in the alloy increases with the increase of the strain,so the measured temperature change is also increasing.When the unloading speed is 0.3 s-1 and the strain is 9%,a giant adiabatic temperature variation up to-8 K can be achieved during unloading the compressive stress.In the cyclicity test,with the strain of 5%unchanged,the sum of positive and negative temperature variation of the alloy after 50 cycles is still greater than 5 K,which indicates that the elastocaloric effect of the alloy has a good stability of cyclicity.Due to the introduction of ferromagnetic element Co,the magnetization difference between the two phases of the alloy increases,which makes the alloy exhibit excellent magnetocaloric effect at the same time.Under the magnetic field of 1.5 T,the adiabatic temperature variation of-5.1 K and-1.5 K can be obtained at 275 K and 274 K when the field was applied for the first time and the second time,respectively.Under the same intensity of the external magnetic field,this value is much higher than that of the same kind of Ni-Mn-Sn alloys.Ni39.5Mn42Co8.5Sn10 directionally solidified alloy can exhibit excellent elastocaloric effect and magnetocaloric effect at low external field,which makes it have the potential to become a working medium of multicaloric refrigeration.