Research On Martensitic Transformation And Microstructure Of Cu-doped Ni₄₉Fe₁₈Ga₂₇Co₆ And Ni_51.5Mn_26.5Ga₂₂ Magnetic Shape Memory Alloys

Ni-Fe-Ga and Ni-Mn-Ga magnetic shape memory alloys (MSMAs) have received great attention due to their fast dynamic response and giant magnetic shape memory effect. However, there still remain many deficiencies about the research of alloying and microstructure. Consequently, research on Cu-doped Ni-Fe-Ga-Co alloys'and Ni51.5Mn26.sGa22 alloy's martensitic transformation and microstructure has great significance. This thesis has choose Ni49Fe18-xGa27Co6Cux (x=0,1,2,3,4,5), Ni49Fe1sGa27-xCo6Cux (x=0.5,1,3,5) and Ni51.5Mn26.sGa22 alloy as research objects, and the influence of composition on martensitic transformation features and microstructure has been investigated by means of metalloscope, EDS, DSC, TEM and HRTEM.For Ni49Fe18-xGa27Co6Cux (x=0,1,2,3,4,5) alloys, the matrix phase is austenite when x=0, and the matrix phase is martensite when x=1-5 at room temperature. With the increase of Cu content, the content of y phase increases, moreover, the starting temperature Ms and the end temperature Mf of martensitic transformation also increase. Increase of Cu content also results in the increase of thermal hysteresis. The valence electron concentration of matrix is 7.71-7.81, and the valence electron concentration of y phase is 8.24-8.29. The structure of martensite in Ni49Fe1sGa27Co6Cu3(x=3) alloy is non-modulated tetragonal structure. Besides, the substructure of twins variants is nano-twins, the inner of nano-twins exists the strain contrast modulated along<112>A direction. For Ni49Fe13Ga27Co6Cus(x=5) alloy, the structure of martensite is also non-modulated tetragonal structure and the substructure is nano-twins.For Ni49Fe18Ga27-xCo6Cux(x=0.5,l,3,5) alloys, the phase composition is martensite and y phase at temperature. With the increase of Cu content, the content of y phase increases. The valence electron concentration of matrix is lower than nominal composition's, while the valence electron of y phase is higher than nominal composition's. At the same heat treatment conditions, the values of Ms and Mf increase,while the value of As, Af, Th decrease with the increase of Cu content. At the condition of same nominal composition, with the longer the annealing time and the slower cooling rate, the value of As, Af, Th decrease and Ms, Mf increase. The structure of martensite in Ni49Fei8Ga26Co6Cu1(x=1) alloy is non-modulated tetragonal structure.For Ni51.5Mn26.5Ga22 alloy, the matrix phase is martensite at room temperature, moreover, the intermediate martensitic transformation exists in the process of phase transformation. The structure of Ni51.5Mn26.sGa22 alloy is long-period modulated structure, including 5M,6M,7M and incommensurate 5M.