Micro-Structure And Properties Of Ni-Mn-Ga-Sr High Temperature Shape Memory Alloy

Ni-Mn-Ga shape memory alloy can be used as high temperature shape memory alloy because of its wide range of martensitic transformation temperature and good shape memory effect.Especially,the alloy has excellent thermal stability and is a kind of new functional materials with broad prospects.However,the shortcomings of polycrystalline brittleness limit the application of the alloy in practice.In this paper,the method of Sr doping alloying is adopted to improve the mechanical properties of Ni₅₆Mn₂₅Ga_19-xSr_x high temperature shape memory alloy and improve the corrosion resistance of the alloy.The influence law and microscopic mechanism of Sr element content on the microstructure,martensitic transformation,mechanical properties,shape memory effect and corrosion resistance of Ni₅₆Mn₂₅Ga_19-xSr_x high temperature memory alloy were systematically explored through metallographic microscope,scanning electron microscope,transmission electron microscope,differential thermal analyzer,X-ray diffractometer,mechanical testing machine,electrochemical workstation and other equipment.It is found that the matrix of Ni₅₆Mn₂₅Ga_19-xSr_x alloy is martensitic lath.When Sr content is greater than 0.3at.%,the second phase appears in the matrix of Ni₅₆Mn₂₅Ga_19-xSr_x alloy.With the further increase of Sr content,the number of the second phase increases.Combining the results of X-ray diffraction,transmission selected area electron diffraction and transmission energy spectroscopy,we write the second phase similar to the cubic Ga Sr phase as the cubic(Ni,Mn,Ga)Sr phase.The content of Sr element has a great influence on the martensitic transformation of Ni₅₆Mn₂₅Ga_19-xSr_x alloy.With the increase of Sr content,the martensitic transformation temperature increases.When the Sr content is 1.0at.%,the M_p(peak temperature of martensitic transformation)and A_p(peak temperature of reverse martensitic transformation)of the alloy reach 451?and 506?,respectively.The Ni₅₆Mn₂₅Ga_19-xSr_x alloy has excellent thermal stability,and the martensitic transformation temperature does not change significantly after 150 thermal cycles.The experimental results show that Sr doping refines the grain size of Ni₅₆Mn₂₅Ga_19-xSr_x alloy and increases the plastic Sr content.With the increase of Sr content,the fracture strain of the alloy first increases and then decreases.The compressive fracture strain of the undoped Sr alloy is only 10.3%;when the Sr content is 0.3at.%,the compressive fracture strain of the alloy reaches a maximum of13.2%,and the fracture strength is 784Mpa;with the further increase of Sr content,the hard and brittle phase of(Ni,Mn,Ga)Sr in the alloy increases,and the compression fracture strain reduction.The shape memory effect of Ni₅₆Mn₂₅Ga_19-xSr_x alloys is enhanced by Sr doping,and the shape memory effect of Ni₅₆Mn₂₅Ga_19-xSr_xalloys first increases and then decreases with the increase of Sr content.When the composition of the alloy is Ni₅₆Mn₂₅Ga_18.7Sr_0.3,the alloy can recover 100%after 10%pre-deformation,and the reversible strain is up to 8.33%.The reason is that Sr doping has a solid solution strengthening effect on Ni₅₆Mn₂₅Ga_19-xSr_x alloy,which increases the critical slip stress.The corrosion resistance of Ni₅₆Mn₂₅Ga₁₈Sr_1.0 alloy is improved by doping Sr.The maximum free corrosion potential and minimum free corrosion current are 0.19V(vs.SCE)and 7E-8 A/cm~2,respectively,for the Ni₅₆Mn₂₅Ga₁₈Sr_1.0 And that maximum impedance value obtain in the EIS impedance spectrum is 5 times of that of the undoped Sr alloy.The reason for this result is that when the Sr content is 1.0 at.%,a dense passive film is formed to hinder the corrosion of the alloy.