Study On The Crystallization Kinetics And Physical Properties Of Ni-Mn-Sn-Fe Alloy Thin Films

Ni-Mn-Sn magnetic shape memory alloy thin films can be subjected to a metamagnetic transition from the weak magnetic martensite to strong ferromagnetic austenite under an external magnetic field.Ni-Mn-Sn thin films have a high response frequency and a large output stress.Meanwhile,Ni-Mn-Sn thin films exhibit multifunctional properties and physical phenomena due to the magnetic-field-induced phase transformation.However,Ni-Mn-Sn based alloy thin films which sputtered at room temperature are normally amorphous alloys in as-deposited state.Shape memory effect and other multifunctional properties can be shown after crystallizing.However,the crystallization mechanism of Ni-Mn-Sn thin films have not been clarified yet.Moreover,high brittleness of Ni-Mn-Sn thin films limits its application and development seriously.We hope to enhance the mechanical properties of Ni-Mn-Sn thin films by doping Fe element and clarify the crystallization kinetics of Ni-Mn-Sn thin films systematically,in order to make Ni-Mn-Sn based thin films can be better applied in MEMS.Ni_50-xMn₃₉Sn₁₁Fe_x（x=0,0.5,2,4 at.%）free-standing thin films are prepared by magnetic sputtering.The microstructures are investigated by SEM,AFM and XRD.The crystallization kinetics is studied by DSC,and the reason for the difference of crystallization temperatures of Ni-Mn based MSMAs is studied by the first-principles methods.Finally,martensitic transformation and mechanical properties are investigated by PPMS and nanoindentation.Non-isothermal DSC shows that the crystallization peak temperatures of Ni_50-xMn₃₉Sn₁₁Fe_x（x=0,0.5,2,4 at.%）thin films are 542.7 K,557.8 K,560.1 K and 568.0K respectively.The crystallization peak temperatures increase gradually with increasing Fe content.The apparent activation energy increases with increasing Fe content.The local activation energy is changeable with increasing crystallization volume fraction.The average of Avrami exponents for Ni_50-xMn₃₉Sn₁₁Fe_x（x=0,0.5,2,4 at.%）thin films is all about 1.2.The variational value of local Avrami exponent n（x）indicates that the crystallization mechanism of thin films is from diffusion-controlled one-dimensional growth to diffusion-controlled two-dimensional and three-dimensional growth,and the nucleation rate increases gradually.Moreover,the vacancy formation energy of Ni₂MnGa（In,Sn）alloys by Ni-vacancy is 1.12 eV,0.91 eV,0.51 eV.It can be found that the crystallization peak temperatures of Ni-Mn based MSMAs increase with their increasing vacancy formation energy with Ni-vacancy.The results have important guiding significance for obtaining the crystallization temperatures of Ni-Mn based shape memory alloy thin films with different components.M-T curves shows that the reverse martensitic transformation temperatures of Ni_50-xMn₃₉Sn₁₁Fe_x（x=0,0.5,2 at.%）thin films decrease with increasing Fe content.The austenitic Curie temperature of austenite increases with increasing Fe content.Nanoindentation tests show that the hardness and elastic modulus of Ni_50-xMn₃₉Sn₁₁Fe_x（x=0,0.5,2,4 at.%）thin films increase with increasing Fe content.H/E and H³/E²increase with increasing Fe content,indicating that Fe doping improve the wear resistance and toughness of thin films.The creep resistance of thin films increases with increasing Fe content.Therefore,doping Fe in Ni-Mn-Sn thin films can improve mechanical properties of thin films well.