Research On Martensitic Transformation And Microstructure Of Ni-Fe-Ga-Co Ferromagnetic Shape Memory Alloys

Ni-Fe-Ga ferromagnetic shape memory alloys as a kind of intelligent materials with great application prospect, has attracted more and more attention. Co addition can make the martensitic transformation temperature Tm and magnetic transformation temperature Tc change in the opposite directions, which will improve the service temperature and magnetic performance of the materials. Moreover, increase the content of Co can make the alloys have superelasticity or change the alloys into strain glass state. Consequently, research on Ni-Fe-Ga-Co alloys’martensitic transformation and microstructure has great significance. This thesis has choose Ni49+xFe18Ga27-xCo6 (x=0,1,2,3,4), Ni53-xFe17+xGa27Co3 (x=0,1,1.5) two alloy systems as research objects, influence on martensitic transformation features and microstructures by composition and heat treatment condition has been investigated, by means of metalloscope, EDS, DSC, TEM, and HRTEM.For Ni49+xFe18Ga27-xCo6 (x=0,1,2,3,4) alloys, the matrix phase is austenite when x=0, and the matrix phase is martensite when x=1,2,3,4 at room temperature. With increase of Ni content and decrease of Ga content, the content of y phase increase. The valence electron concentration of matrix is 7.59-7.69, the valence electron concentration of y phase is 8.16-8.23, the system adjust itself by means of increase the content of y phase. Martensitic transformation temperature Tm increased with increase of Ni content and decrease of Ga content; y phase impeded the martensitic transformation and broaden the peak on DSC curves; increase of y phase content result in the increase of hystersis. The precipitated phase in Ni51Fe18Ga25Co6 alloy is ordered γ phase, it has L12 superstructure. Tweed-like pre-martensite phase has been found in the alloy. The microstructure of martensite is non-modulated tetragonal structure, twins relationship has been found between the variants. The substructure of twins variants is nano-twins, the inner of nano-twins exist the strain contrast modulated along<112>A direction.For Ni53-xFe17+xGa27Co3 (x=0,1) alloys, martensitic phase exist in as-cast specimens at room tempreature; for the specimens homogenizing annealed at 1000℃ for 72h and furnace cooled, the matrix is martensite at room tempreature, and y phase precipitated along the grain boundary. For Ni51.5Fe18.8Ga27Co3 alloy, martensite, austenite, and y phase co-existed in as-cast specimen; austenite and y phase co-existed in heated specimen. As the parent phase ordered degree SL21 of homogenizing annealed Ni52Fe18Ga27Co3, Ni51.5Fe18.5Ga27Co3 alloys has increased, the martensitic transformation temperature Tm has decreased, conform to the general rules; for heated Ni53Fe17Ga27Co3 alloy, Tm has increased, show the abnormal phenomena; Tm of heated alloys increased as valence electron concentration increased. Hystersis decreased when Fe content increased. The microstructure of as-cast alloy is non-modulated tetragonal martensite. Homogenizing annealed alloy has more complex microstructure. Twin variants of some areas modulated along <110>A direction, its periodicity is not distinct, and form the multi-modulated martensite; some twin variants of other areas modulated along <110>A, and form the 7M modulated martensite.