Research On Thermal Cycling Properties And Alloying Of NiMnGa-based Shape Memory Alloys

The study of high temperature shape memory alloys(HTSMAs)has attracted much attention due to the control requirements of high-temperature drive(>100?)and overheating warning in aerospace,oil exploration,high voltage transmission,automotive and other engineering fields.The Ni-Mn-Ga-based HTSMAs show high martensitic transformation temperature range,excellent shape memory effect and low cost.However,Ni-Mn-Ga polycrystalline alloys are too brittle for practical application.Besides good mechanical and shape memory properties and higher transformation temperatures,the thermal cycling stability of microstructures and properties at high temperatures and after cycling transformations is also the important basis for evaluating the practicability of HTSMAs.In recent years,alloying with the fourth element to enhance the ductility of Ni-Mn-Ga HTSMAs has become a new research hotpot.On the basis of the preliminary study,the Ni₅₆Mn₂₅-xGa₁₉Yx(x=0-2),Ni₅₅Mn₂₅Ga₁₈Ti2 and Ni₅₆Mn₂₅Ga₁₇V₂ HTSMAs were chosen and prepared.In this paper,the effect of Y substitution for Mn on the microstructures and properties of the Ni-Mn-Ga HTSMAs and the thermal cycling stability of Ni56Mn24.5Gai9Y0.5,Ni₅₅Mn₂₅Ga₁₈Ti2 ? Ni₅₆Mn₂₅Ga₁₇V₂ HTSMAs were studied by using Scanning Electron Microscope,Energy Dispersive Spectrometer,Optical Microscope,Simultaneous Thermal Analyzer,X-ray Diffraction Analyzer,and room-temperature compression testing machine.The results show that for Ni₅₆Mn₂₅-xGai9Yx(x=0?2)HTSMAs at room temperature,when x=0,the alloy has a single structure with non-modulated tetragonal martensite.When x>0.2,the alloys show a dual-phase microstructure including hexagonal secondary phase and martensite matrix.With Y content increasing,the volume fraction of secondary phase increases and the grain are refined.All of the alloys show one-step thermoelastic martensitic transformation.Accompanied by Y rising,the phase transition temperature of the alloy varies from increasing to decreasing.The compressive strength and strain were remarkablely improved due to grain-size strengthening and precipitation strengthening.While the shape memory strain of the alloy reduces with the Y addition.The as-quenched and thermal-cycled Ni₅₅Mn₂₅Ga₁₈Ti2 and Ni₅₆Mn₂₅Ga₁₇V₂ HTSMAs show dual-phase microstructure containing face-centered cubic structure ? phase and tetragonal martensite matrix.After 500 thermal cycles,no remarkable phase structure change was observed on the Ni56Mn24.5Ga₁₉Y0.5 and Ni₅₅Mn₂₅Ga₁₈Ti2 HTSMAs.The martensitic transformation temperatures and thermal hysteresis keep stable.The compressive strength and strain change little.Both of them show good thermal cycling stability.There is no microstructure and phase structure change taking place in the thermally-cycled Ni₅₆Mn₂₅Ga₁₇V₂ HTSMAs.The reverse martensitic transformation temperature remains stable.While the forward martenstitic transformation were weak.The shape memory and mechanical properties drops with the increase of the number of thermal cycles.The thermal cycling stability of Ni₅₆Mn₂₅Ga₁₇V₂ alloy is inferior to the above two alloys.