| Magnetic shape-memory alloys(MSMA)exhibit both thermoelastic martensite and ferromagnetic transformations,making them versatile materials that can be used as drivers,sensors,actuators,and refrigeration working materials.Besides large output strain and fast response frequency,MSMAs also possess diverse functional properties such as magnetothermal effect,elastothermal effect,and hyperelasticity.The low-cost and non-toxic Ni-Mn-Sn series of MSMAs has shown great potential for practical applications.However,intermetallic compounds in these alloys result in weak bond strength between grain boundaries and reduce machinability,making them brittle and limiting their application.This dissertation investigates how element doping can improve the mechanical properties and corrosion resistance of the alloy.The Ni45Co5Mn38Sn12-x Gdx(x = 0,0.3,0.6,1,1.5at%)magnetic shape-memory alloys were synthesized via a melting process in a WK-II vacuum arc furnace,where rare earth element Gd was used to substitute partially for Sn.The resultant alloy was then sealed in a quartz tube filled with argon gas and heat-treated in a furnace for 24 hours at 1173 K before being quenched by water cooling.The microstructure,crystal structure,martensitic phase transformation behavior,mechanical properties,magnetic properties,and seawater corrosion resistance of the Ni-Co-Mn-Sn-Gd alloy were analyzed using various techniques such as scanning electron microscopy,energy spectrum analysis,X-ray diffractometry,differential scanning calorimetry,universal mechanical testing,multi-parameter magnetic measurement,comprehensive physical property measurement,and Gamry electrochemical workstation.The study discusses the impact of Gd substitution for Sn on the microstructure,martensitic transformation,mechanical properties,and the mechanism of substitution.It was observed that Ni45Co5Mn38Sn12 alloy has a single-phase structure,and the addition of rare earth element Gd affected the microstructure of the alloy.At 0.3 at% Gd,gray speckle precipitates appeared with a small number of white particles precipitated phase attached to the gray mottled phase.As the amount of Gd increased,the volume fraction of the two precipitates gradually increased,and they interconnected to form a network distribution around the grain boundary.Compared to the matrix,the content of Co increased to ~13 at%,which was more than twice its previous content.The content of Sn decreased to ~2%,and the content of Ni and Mn fluctuated slightly.In the white granular phase,the content of Ni reduced by 10 at%,while the content of Co and Mn reduced by ~80%.The content of Mn was only ~1 at%,and the content of Co was stable at ~6 at%.Gd existed only in the white granular phase with a maximum solubility of28.9 at%.At room temperature,Ni45Co5Mn38Sn12-x Gdx(x = 0,0.3,0.6,1,1.5at%)alloy presented 10 M orthogonal martensite phase;however,when the Gd content rose to 1.5at%,the alloy comprised both 10 M orthorhombic martensite and 14 M monoclinic martensite.The increase in Gd doping amount caused the lattice constant to become anisotropic with a and c decreasing and b increasing,and the corresponding alloy cell volume also decreased.The heating and cooling processes of Ni45Co5Mn38Sn12-x Gdx(x= 0,0.3,0.6,1,1.5at%)alloy revealed a typical first-order martensite transformation.Gd doping decreased the cell volume and increased the valence electron concentration e/a in the matrix,leading to a rise in the phase transition temperature.Additionally,as the Gd content increased,the thermal hysteresis of the alloy decreased.The study investigated the effect of Gd doping on various properties of Ni45Co5Mn38Sn12-x Gdx(x = 0,0.3,0.6,1,1.5at%).The results showed that the magnetic properties of the alloy were influenced significantly by Gd doping.The Curie temperature of the alloy decreased with increasing Gd content.The magnetic saturation strength difference between martensitic transformation and inverse transformation increased with increasing Gd content and reached a maximum of 41.5 emu/g under 5T magnetic field for Ni45Co5Mn38Sn11.4Gd0.6 alloy.The magnetic entropy change value of Ni45Co5Mn38Sn11.4Gd0.6 alloy increased with increasing magnetic field and reached a maximum of 7.24 J/kg · K at 315 K.The cooling capacity of the alloy also increased significantly and reached 98.6 J/K.The mechanical properties of the alloy improved with increasing Gd content.The compressive fracture strength and strain increased rapidly from 348 MPa to 1258 MPa and from 4.34% to 13.72%,respectively,with increasing Gd content.The alloy also showed more plastic behavior with Gd doping,changing from along-crystal fracture to through-crystal deconvolution fracture.The alloy’s seawater corrosion resistance improved with Gd doping as well.The self-corrosion potential of the alloy increased,and the self-corrosion rate and current decreased.The capacitance arc radius and charge transfer resistance also slightly improved,indicating improved corrosion resistance.Therefore,the study suggests that Gd-doped Ni-Co-Mn-Sn alloys could be potentially useful in magnetic refrigeration and for developing high-strength and corrosion-resistant materials. |
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