The Elastocaloric Effect Of Ni-Mn-Ti All-d-metal Heusler Alloys

In this project,high-purity Ni,Mn,Ti metal elements are selected as raw materials,and high-vacuum arc melting furnace,directional solidification furnace and other equipment are used to prepare the<001>strong texture Ni50Mn50-xTix(x=17?20)all-3d metal Heusler polycrystalline alloys.The martensitic transformation behavior,crystal structure,microstructure,magnetic and thermal properties have been investigated by means of differential scanning calorimetry apparatus(DSC),scanning electron microscope(SEM),X-ray diffraction(XRD),electronic universal testing machine and magnetic measurement system.Firstprinciples calculations was carried out on the experimental phenomena in the process of experiment.It is found that the martensitic transformation temperature of the Ni-Mn-Ti series alloys prepared with the change of electron concentration follows the rule of the traditional Ni-Mn based heusler alloy.In addition,the phase transition temperature of Ni-Mn-Ti series alloys is very sensitive to the content of Ti.When the content of Ti increases by 1%(on the basis of Ti=17),the characteristic temperature of martensite phase transition decreases by?6.8k.The austenite of Ni50Mn50-xTx(x=17?20)alloy at room temperature is an ordered structure of B2 phase,and fine dispersion of the second phase(Ni,Mn)3Ti was precipitated at the grain boundary and within the grain.The occupation mode of Mn in the ordered structure of B2 phase makes the alloy show antiferromagnetic.Therefore,the unique magnetic properties of this series of alloys significantly reduce the contribution of magnetic entropy in the martensite phase transformation process.The maximum thermal entropy and maximum enthalpy of the alloys are up to?70 J/(kg-1·K)and?20 J g-1 respectively.In terms of properties,Ni50Mn50-xTix(x=17?20)alloy shows excellent mechanical properties,superelasticity and elastocaloric effect.Due to d-d atomic orbital hybridization between 3d metal atoms,the interatomic forces in Ni-Mn-Ti series alloys are much lower than those in traditional Ni-Mn based hassler alloys.The theoretical maximum elastocaloric effect of Ni50Mn31.75Ti18.25 alloy is up to?36.5 K.The actual elastocaloric effect of projectile is as high as?-20.4 K.The higher loading rate and strain are,and the more significant elastocaloric effect of the alloy is.When the strain is 5%and the loading rate and unloading rate are 0.005s1,the elastocaloric effect of Ni50Mn31.75Ti18.25 alloy is?3.2 K,and the elastocaloric effect performance of the alloy does not decrease after 150 cycles.The entropy change and adiabatic temperature change are close to Ni-Ti alloy and much higher than the traditional Ni-Mn based Heusler alloy.Therefore,the research on the elastocaloric effect of Ni-Mn-Ti alloy has great application potential in the field of elastocaloric refrigeration.