Research On Alloying Of NiMnGa High Temperature Shape Memory Alloys

Ni-Mn-Ga alloys have attracted much attention because their excellent performance with high martensitic transformation temperatures and sensitivity to composition,good thermal stability at high temperature,large shape memory effect and low cost for preparation.However,the polycrystalline Ni-Mn-Ga alloys are extremely brittle which limits their applications.In the present work,alloying elements were added to NiMnGa alloy to improve the mechanical properties.Polycrystalline alloys with the nominal composition of Ni56Mn25Ga19-xVx(x=0,1,2,4,6 at.%).Ni56Mn25-xGa19Vx(x=0,1,2,4)?Ni54Mn25Ga2i-xLax(x=0,0.2,0.5,1,2)and Ni54Mn25Ga21-xNdx(x=0,0.5,1,2)were prepared.Based on X-ray diffraction,optical microscope,scanning electron microscope,transmission electron microscope,energy dispersive spectroscopy,differential scanning calorimetry and compression testing,the influence of V or rare earth La,Nd substitution on the microstructure,martensitic transformation behavior,mechanical properties and shape memory characteristtics were studied systemically.The results show that a single martensitic phase with non-modulated tetragonal structure is observed in Ni56Mn25Ga19 and Ni54Mn25Ga2i alloys at room temperature.Ni56Mn25Gai9-xVx(x=0-6)and Ni56Mn25-xGa19Vx(r=0-4)alloys still show single martensitic phase when x?1,while dual phases containing martensite and face-centered-cubic y phase are observed in alloys with x>1.Moreover,the volume fraction of y phase increases with increasing V content.La/Nd-rich hexagonal second phase precipitates in alloys when 0.2-2at.%La or 0.5-2at.%Nd are added.Adding V,La and Nd do not change the structure of martensitic phase.At the same time,the grain is remarkably refined by adding rare earth La or Nd.According to martensitic transformation test,Ni-Mn-Ga-X(X=V La,Nd)alloys undergo one-step thermoelastic martensitic transformation during the process of heating and cooling.The transformation temperatures decrease with the increase of V or La content.However,Nd addition has little effect in the transformation temperature.These results are mainly attributed to the electron concentrations and tetragonality of martensite.Compression test results show that adding V,La and Nd can significantly improve the mechanical properties of Ni-Mn-Ga alloys because of fine grain strengthening and precipitation strengthening.Both the compression strength and strain have a strong dependence upon the V content.With the increase of La or Nd content,the strength increase gradually,and the strain first increase and then decrease.The introduction of second phase hinders martensitic transformation,which decrease the shape memory characteristics of alloys.Concequently,addition of alloying with appropriate amount elements can enhance the ductility of the alloys and retain a certain fration of shape memory property.