Research On Martensitic Transformation And Microstructure Of Cu-doped Ni₅₂Fe₁₈Ga₂₇Co₃ And Ni-Mn-Sn Ferromagnetic Shape Memory Alloys

Ferromagnetic shape memory alloy is more and more attracting people's attention as a new type of intelligent materials, the characteristics of magnetic shape memory and the magnetic field induced strain have good prospect in engineering applications. Ni-Mn-Ga was the first alloy discovered ferromagnetic shape memory effect and Ni-Fe-Ga alloy has become another hotspot of ferromagnetic shape memory alloy. For these two alloys have some limiting disadvantages, people have done a lot of related research about Ni-Mn-Ga alloy and Ni-Fe-Ga alloy systems. This thesis has chosen undoped and Cu-doped Ni52Fe18Ga27Co3 and Co, Al doped Ni50Mn37Sn13 alloy as research objects, effects of heat treatment and alloying on the microstructure, matensitic transformation features have been investigated by metalloscope, DSC, EDS and TEM methods.The results show that the composition has an important effect on microstructure of Ni-Fe-Ga alloy system. For Ni52Fe18G27-xCo3Cux(x=?,2,3)alloys, the phase is martensite and ?' phase at room temperature, the content of ?' phase increases as decrease of Ga content and increase of Cu content. The valence electron concentration is 7.49-7.75. Martensitic transformation temperature increases with increase of Cu content and decrease of Ga content; ?' phase hinders the martensitic transformation and broaden the peak on DSC curves, hysteresis is nearly not changed. The precipitated phase in Ni52Fe18G27-xCo3Cux(x=1,2,3) alloy is ordered y'phase, it has L12 superstructure. The microstructure of martensite is non-modulate detragonal structure, twins relationship has been found between the variants. The substructure of twins variants is nano-twins.For Ni52Fe18-yG27Co3Cuy (y=1,2,3) alloys, the phase is still martensite and ?' phase at room temperature, ?' phase has a larger content when y=1 and a less content when y=2 and 3. The valence electron concentration is 7.52-7.58. Martensitic transformation temperature decreases and then increases as increase of Cu content and decrease of Fe content. The valence electron concentration is lower than Cu replacing Ga samples but martensite transformation temperature is higher, which shows that martensite transformation temperature is not only relating to the valence electron concentration. The microstructure of martensite is non-modulate detragonal structure, the precipitated phase is ordered y'phase.The microstructure of Ni41Co9Mn37Sn13 alloy are a few retained austenite, non-modulate tetragonal structure martensite and mass of modulate martensite(5M,6M), and mass of dislocations exist inside; The microstructure of Ni50Mn37Sn12Al1 are austenite, mass of NM martensite which has lots of dislocations inside and modulate martensite; The microstructure of Ni50Mn37Sn11Al2 is similar with Ni41Co9Mn37Sn13 sample but without retained austenite, which indicates that Ni50Mn37Sn11 Al2 sample has a good performance.