The Martensitic Transformation And Elastocaloric Effect Of Ni-Mn-Sn-Fe Magnetic Memory Alloys

With the increasingly serious energy crisis and environmental problems,solid-state refrigeration technology that saves,improves efficiency,cleans and consumes less energy has become a hot research topic.Ni-Mn-Sn magnetic shape memory alloy has both magnetocaloric and elastocaloric effects.At the same time,the coupling effect between its magnetism and elasticity can also achieve the Doka effect,so it is considered as a highly potential refrigeration material.However,Ni-Mn-Sn alloys are brittle and have poor mechanical properties,which severely restrict the practical application of such materials in the field of elastocaloric cooling.For this reason,this article proposes the idea of??doping Fe element in Ni-Mn-Sn alloy to improve the brittleness of the alloy on the basis of ensuring its elastocaloric effect.The thesis systematically studied the influence of different Fe content on the microstructure,martensitic transformation behavior,mechanical properties and elastocaloric effect of Ni-Mn-Sn alloy and its micro-mechanism.The study found that with the increase of Fe content,the structure of the alloy gradually changed from martensite phase to austenite phase at room temperature.The doping of Fe element will significantly reduce the martensite transformation temperature of the Ni-Mn-Sn alloy.When the Fe doping content is 2 at.%,the phase transformation temperature of the Ni₄₈Mn₃₇Sn₁₃Fe₂alloy is reduced to 24?.First-principles calculations show that the replacement of Ni by Fe increases the total energy difference between the paramagnetic state and the ferromagnetic state of the Ni-Mn-Sn-Fe parent phase,and the total energy difference between the parent phase and the martensite phase decreases,resulting in the martensite transformation temperature decreases.The test results of mechanical properties show that Fe doping can significantly enhance the mechanical properties of Ni-Mn-Sn alloy.Doping Fe can increase the compressive fracture strength of the alloy.When the Fe doping content is 6 at.%,the compressive fracture strength of Ni₄₄Mn₃₈Sn₁₂Fe₆alloy is as high as about 600 MPa and the strain reaches 8.3%.Combining the microstructure and fracture morphology,it is found that a large number of second phase particles are precipitated at the grain boundary,and the fracture mode changes from intergranular fracture to transcrystalline cleavage fracture.The results of the elastocaloric effect test show that the replacement of Ni with an appropriate amount of Fe ensures the elastocaloric effect of the alloy.At room temperature,the rapid unloading rate is 0.1%s^-1,and the temperature change value of Ni₄₈Mn₃₇Sn₁₃Fe₂alloy is around 2.0?.Under a constant unloading stress of 500MPa,the adiabatic temperature change of Ni₄₄Mn₃₈Sn₁₂Fe₆alloy reaches the maximum value,and after 10 cycles of training,the adiabatic temperature change value is stable at about 2.3?,which is higher than the adiabatic temperature change of other alloy components.Comparing the calculated isothermal entropy value of Ni-Mn-Sn and Ni-Mn-Sn-Fe alloys,the increase after doping is 23.24 J/mol/K.The above results indicate that Fe doping provides a reference and reference for the development of Ni-Mn-Sn magnetic shape memory alloys with excellent mechanical properties and obvious elastocaloric effect.