Intermartensitic Transformation Of Ni-Mn-Sn Ferromagnetic Shape Memory Alloy

Heusler type Ni-Mn-Z based（Z=Sn,In,Al,and Sb）ferromagnetic shape memory alloys have received widespread attention due to their various functional properties such as shape memory effect（SME）,magnetothermal effect（MCE）,and magnetoresistance（MR）.This type of Heusler alloy has great application potential in drivers,sensors,information storage,and magnetic refrigeration.Many functional characteristics of Heusler alloys stem from the magnetic structure transition from weakly magnetic martensite to strongly ferromagnetic austenite during the martensitic transformation（MT）process induced by temperature or magnetic field,which results in significant difference of magnetization（ΔM）and resistivity variation,etc.Similar to martensitic transformation,intermediate martensitic transformation is also a primary structural transformation that can lead to changes in the structure and properties of martensite.Heusler alloys with intermartensitic transformation are considered to be of great significance for studying the mechanism of martensitic transformation and magnetic induction transformation,and these alloys may also achieve certain specific application functions.Intermartensitic transformation often occurs in Ni-Mn-Ga Heusler alloys.Intermartensitic transformation with different crystal arrangements caused by internal stress（changes in chemical composition）,temperature,field,and external pressure has become the most common and interesting phenomenon in Ni-Mn-Ga ferromagnetic Heusler alloys.However,compared to Ni-Mn-Ga Heusler alloys,Ni-Mn-Sn Heusler alloys are considered to have the most promising application prospects among similar Heusler alloys due to their low cost of constituent elements,non-toxicity,and low brittleness.Therefore,this paper focuses on the study of the intermartensitic transformation behavior in Ni-Mn-Sn Heusler alloys.The main research contents are as follows:（1）The intermartensitic transformation process was observed in Ni₄₃Fe₁Mn₄₅Sn₁₁and Ni_43.5Cu_0.5Mn₄₅Sn₁₁Heusler alloys by doping Ni₄₄Mn₄₅Sn₁₁with transition group elements Fe and Cu instead of Ni sites.There are relatively few reports of intermediate martensitic transformation in Ni-Mn-Sn Heusler alloys.We have demonstrated the existence of intermartensitic transformation through magnetic measurements and electrical resistance measurements.We have observed two consecutive changes in the slope of the measurement curve,indicating that during the cooling process,the parent phase（austenite）first transforms into intermartensitic transformation,and further cooling will transform into martensitic phase at lower temperatures.After Ni is replaced by Cu,Fe,and Ga,the lattice parameters decrease,and the resulting intermartensitic transformation is gradually suppressed with the increase of the applied magnetic field strength.When the applied magnetic field strength reaches a certain value,the intermediate martensitic transformation completely disappears,and the sample directly transforms from austenite to martensite.In addition,two peaks can be observed in the magnetic entropy change curve,broadening the temperature range of the material.（2）The main group elements Al and Ga are used to apply chemical pressure instead of Sn and Ni sites respectively.The reason is that compared with Sn and Ni atoms,the atomic radius of Al and Ga is smaller,which can generate enough chemical pressure and obtain enhanced magnetoelastic coupling.Thus,the intermartensitic transformation behavior is observed in Ni₄₄Mn₄₅Sn_10.5Al_0.5and Ni_43.5Ga_0.5Mn₄₅Sn₁₁Hassler alloys.As a result,intermediate martensitic transformation behavior is observed in Ni₄₄Mn₄₅Sn_10.5Al_0.5和Ni_43.5Ga_0.5Mn₄₅Sn₁₁Heusler alloys.We have demonstrated the existence of intermediate martensitic transformation through magnetic and magnetoresistance measurements.The results of magnetic and resistance measurements indicate that the sample undergoes a structural transformation from austenite to intermediate martensite,and then to martensite during the martensitic transformation process.We have found that the intermediate martensitic transformation behavior of Ni₄₄Mn₄₅Sn_10.5Al_0.5Heusler alloy is very stable and will not be suppressed with the increase of the applied magnetic field.In addition,the magnetic moment of Ni₄₄Mn₄₅Sn_10.5Al_0.5Heusler alloy during martensitic transformation varies greatly,and the applied magnetic field will lower the transformation temperature point.