Study On Thermal Stability Of NiMnGaX Shape Memory Alloys

Ni-Mn-Ga-based high-temperature shape memory alloy has the advantages of thermo-controlled shape memory effect,good shape memory performance and large range of martensite phase transition temperatures,but it is limited by the intrinsic brittleness of intermetallic compounds.It was reported that the mechanical properties can be improved by introduce the dual-phase structures due to the addition of the fourth component Cu,Fe,Nd,V,etc.Dual-phase NiMnGaX alloys have attracted much attention as a candidate material for high temperature shape memory alloys.When high-temperature shape memory alloys are used,they need to wait at a higher temperature for a long time and transformed repeatedly from martensite to austenite.Therefore,it is necessary to have acceptable oxidation resistance and thermal stability of microstructure and performance during thermal cycles.However,related research has rarely been reported.The oxidation resistance and thermal cycling stability of Ni56Mn2iGai9Cu4,Ni56Mn25Ga15Fe4,Ni54Mn25Ga20.5Nd0.5 and Ni56Mn25Ga17V2 high temperature shape memory alloys were studied systematically.Phase structure and microstructure before and after oxidation,phase structure,phase transiformation,microstructure,mechanical properties and shape memory properties before and after thermal recycling are tested and analyzed by X-ray diffractometer,metallographic microscope,scanning electron microscope,energy spectrometer,differential scanning calorimeter and room temperature compression tester.The quenched NiMnGaX(X=Cu,Fe,V)alloys are composed of a non-modulated tetragonal martensitic matrix phase and a face-centered-cubic phase.Ni54Mn25Ga20.5Nd0.5 is composed of tetragonal martensite and hexagonal phase.The martensitic transformation start temperature(Ms)of NiMnGaX(X=Cu,Fe,Nd,V)alloy are 4711℃,390℃,223℃ and 181℃,respectively.They are both higher than 100℃,satisfying the requirement of high temperature shape memory alloys.An oxide layer were formed on the surface of NiMnGaX(Cu,Fe,Nd,V)alloy at 550℃ and 700℃.The oxidation products are mainly composed of Mn.The oxidation resistance of the alloy is rated according to the average oxidation rate constant.At 550℃,the Ni54Mn25Ga20.5Nd0.5 alloy is secondary oxidation resistance,and the others are oxidation resistant.At 700℃,Ni56Mn21Gai9Cu4 alloy is oxidation resistant,and the others are secondary oxidation resistance.The NiMnGaX(Cu,Fe,Nd,V)alloy have certain oxidation resistance at 550℃ and 700℃.The Ni56Mn21Gai9Cu4 alloy which always rated for oxidation resistance has especially good oxidation resistance.The phase structure remains unchanged in NiMnGaX(X=Cu,Fe)alloy before 50 thermal cycles.The phase structure is stable.A new cube structure precipitates in the Ni56Mn2iGa19Cu4 alloy after 100 thermal cycles,and a new structure precipitates in the Ni56Mn25Ga15Fe4 alloy after 50 thermal cycles.The martensitic transformation start temperature of Ni56Mn21Ga19Cu4 alloy and Ni56Mn25Gai5Fe4 alloy are increased by 19℃ and 2℃,respectively.The phase transition characteristics of NiMnGaX(X=Cu,Fe)is stable at high temperatures and it has thermal cycle stability.The compressive strength and compressive deformation rate of the cyclic NiMnGaX(X=Cu,Fe)alloys change little.The average compressive strength of Ni56Mn21Ga19Cu4 alloy and Ni56Mn25Ga15Fe4 alloy are 1646MPa and 1943MPa,respectively.The mechanical properties are good.The shape recovery rate of the two alloys is basically unchanged,and the shape memory performance is slightly lowered.