A Eutectic Dual-phase Design Of Ni-Mn Base Heusler-type Ferromagnetic Shape Memory Alloys

Heusler magnetic shape memory alloy is a new type of shape memory material.It not only has the thermoelastic shape memory effect controlled by temperature,but also has a magnetic shape memory effect controlled by a magnetic field.These alloys also exhibit other novel properties,such as large recovery strain,high response frequency,and precise control that can be applied in the fields of magnetic brakes,high-efficiency sensors,and magnetic refrigerators.However,these intermetallic compounds generally show high brittleness and low strength,which seriously hinders their processability and practical application.In order to overcome the shortcomings of the traditional toughening methods for Heusler alloys in the literature,this thesis proposes a new microstructure design and magnetic property control strategy.In this design stragety,an ultra-fine two-phase Heusler/?phase composite utilizing the liquid-solid eutectic reaction was prepared.This aims to solve the key problems of poor mechanical properties,uncontrollable phase transformation and poor magnetic properties of the traditional dual phase Heuselr alloys.As demonstrations,Ni₅₀Mn_40-xSn₁₀Fe_x?x=3,4,5,6,7,8,9,10,11?and Ni_51.5Mn_40-xSn_8.5Fe_x?x=5,6,7?alloys were prepared and systematically studied by means of scanning electron microscopy?SEM?,transmission electron microscopy?TEM?,X-ray diffraction?XRD?,in-situ synchronous high energy X-ray diffraction?he-xrd?,differential scanning calorimetry?DSC?and magnetic properties analysis?SQUID?.It is found that nano-lamellar?phase is formed in Heusler alloy by replacing Mn with Fe,which effectively improves the compressive fracture strength and plasticity of Ni₅₀Mn_40-xSn₁₀Fe_xalloy.With the increase of Fe content,the?phase of the eutectic layer formed in the alloy also increases.According to the characteristics of microstructure,the alloys can be divided into three categories:when Fe%<4 at%,the alloy is single-phase;when 4 at%<Fe%<8 at%,the alloy is hypoeutectic;when 8at%<Fe%,the alloy is hypereutectic.When the Fe content reaches 8 at%,that is,the composition of Ni₅₀Mn₃₂Fe₈Sn₁₀,the alloy forms a fully eutectic dual-phase structure.The alloy shows high strength of 1950 MPa,high plasticity of 19.5%,large elastic strain of 3.2%and stable mechanical cycling performance.Under the change of the magnetic field intensity of 5T,the alloy has a good magnetocaloric effect.Its magnetic entropy change is 10.2 J/kg K,and its magnetic refrigeration capacity is168 J/kg.The martensitic transformation temperature of Ni₅₀Mn₃₂Fe₈Sn₁₀is about 120 K,which is not suitable for near room temperature applications.At the same time,the transformation temperature is lower than the Curie temperature of the martensite,resulting in the limited magnetization difference??M=43 emu/g?in the transformation process,which makes the magnetic field induced transformation driven by Zeeman energy difficult to occur.In order to solve this problem,the alloy composition is further fine tuned to Ni_51.5Mn₃₄Sn_8.5Fe₆by adjusting the e/a value and Fe concentration of the alloy composition,while still having a fully eutectic dual phase structure with 1645 MPa high strength and 14.8%high plasticity.At the same time,the martensitic transformation meets the two key conditions of T_M?room temperature and T_C^M<T_M<T_C^A,so its practical application potential is the largest.