Research On Microstructure Of NiMnGa-based Shape Memory Alloys

Ni-Mn-Ga shape memory alloy,with both thermoelastic martensitic transformation and magnetic transition,can be developed into magnetic shape memory alloy and thermal shape memory alloy.The Ni-Mn-Ga high temperature shape memory alloy has attracted much attention of researchers due to its large phase transition temperature range,good thermal stability,high performance of shape memory,low cost of raw materials and preparation,but the brittleness of polycrystalline materials limits the practical application of the alloy.It has become one of the hot topics in this field to improve its toughness and maintain good shape memory properties by means of alloying.In this paper,the effect of Ti substitution for Mn or Ga on microstructure,phase transition behavior,mechanical properties and shape memory properties of Ni55Mn25Ga20 alloy has been studied systematically using X ray diffraction,optical microscopy,scanning electron microscopy,transmission electron microscopy,energy dispersive spectroscopy,differential scanning calorimetry and room temperature compression test machine.The microstructures of Ni-Mn-Ga-V,Ni-Mn-Ga-Nd and Ni-Mn-Ga-Fe alloys were studied by transmission electron microscopy.The results of the study show that:Ti substitution for Ga:when x?1,Ni55Mn25Ga20-xTix alloy still retains a single tetragonal martensite phase,when 2<x<6,the alloy exhibits a dual-phase structure composed of a martensitic phase and a face-centered cubic ? phase.? phase is Ni-rich and Ti-rich phase,and its volume fraction increases with the increase of Ti content.One step thermo-elastic martensitic transformation occurs during the heating and cooling of Ni-Mn-Ga-Ti alloy.With the increase of Ti content,the martensitic transformation temperature of the alloy gradually decreases,and the starting temperature of martensitic transformation(Ms)changes from 317? of x=0 to 175? of x=6,which is mainly attributed to the combined effect of martensitic phase size and electron concentration.Ti substitution for Mn:when x?1,Ni55Mn25-xGa20Tix alloy still retains a single tetragonal martensite phase,when 2<x<4,the alloy exhibits a dual-phase structure composed of a martensitic phase and y phase.? phase is Ni-rich and Ti-rich phase,and its content increases with the increase of Ti content.When x=6,the alloy is a dual-phase structure composed of austenite and y phase.With the increase of Ti content,the martensitic transformation temperature of the alloy gradually decreases,and the starting temperature Ms changes from 317? for x=0 to 65? for x=4.And the phase transition temperature drops below room temperature when x=6.The effect of Ti substitution for Mn on the transformation temperature of the alloy is more significant than that of Ti substitution for Ga.The precipitation of y phase can significantly increase the compressive fracture strength and stress of the alloy,but the maximum recoverable strain reduces.When the content of Ti is 2at%,it has high phase transition temperature-good toughness and shape memory properties.The TEM experimental results show that martensitic phase with non-modulated tetragonal structure is observed in Ni56Mn25Ga11Fe8,Ni56Mn25Ga1--xVx(x=0.4,6)alloys at room temperature.Large stripe-like martensitic variants and sharp interface between two variants can be seen clearly.Each variant contains many inner micro-twins.And the second phase is face-centered-cubic structure.The martensitic phase of Ni54Mn25Ga19Nd2 is also a non-modulated tetragonal structure,but its second phase is hexagonal structure.