Phase Transformation Behavior And Mechanical Properties Of Ni-Mn-Ga High Temperature Shape Memory Alloys

In recent years,Ni-Mn-Ga high-temperature shape memory alloys(HTSMAs)have attracted extensive attentions and show great potentials to become a new generation of intelligent driving materials due to their excellent thermal stability and high characteristic temperatures for martensite phase transformation.However,the brittleness of Ni-Mn-Ga alloy limits their practical applications till date.In this thesis,two alloy systems,i.e.Ni54+xMn25Ga21-x and Ni54Mn25Ga21-xDyx were designed via partially repacing Ga with Ni and Dy in the Ni-Mn-Ga base alloy,respectively.Their microstructures,lattice parameters,martensitic transformation behavior,mechanical properties under compressive condition and shape memory effect were comparatively investigated.Moreover,the high temperature superelastic deformation of Ni54Mn25Ga20.9Dy0.1was also studied.The main results are summarized as follows:A single martensite phase with tetragonal crystal structure for x<3 was obtained whereas the alloy with x≥3 exhibited dual phases of martensite and face-centered cubic γ phase.Dy addition refined the grain size significantly.EBSD analysis showed that there are four kinds of martensite plates with different orientations in an original austenite grain.Type Ⅰ and type Ⅱ twin relationship was found between the four kinds of martensite,but there was no twinning relationship among the martensite variants at the transverse interface.Grains with only one orientation of martensite variants possessing the strong texture were obtained after unloading and compession of NissMn25Ga20 alloy sample upto 4%.One-step thermoelastic martensite phase transition was observed in the two systems.Martensitic transformation temperatures were improved dramatically with increasing Ni content,which was attributed to the combinative effects of increasing electron concentration and tetragonality of martensite.Dy element reduced the martensite phase transformation temperature from 247℃ of Nis4Mn25Ga21 alloy to 103 ℃ of Ni54Mn25Ga21-xDyx alloy.Higher Ni addition led to the formation of y phase,resulting in significant improvement in both compressive strength and ductility.In contrast,the shape memory effect and recovery ratio for the alloy containing higher y phase were reduced sharply.With the doped Dy content,the mechanical properties of Ni-Mn-Ga-Dy were improved significantly and shape memory effect decreased.Fracture strength and strain reached the maximum value with 0.3 at%Dy content.Ni54Mn25Ga20.9Dy0 1 alloy exhibited good superelasticity at 230 C,the larger the difference between the experimental temperature and A f,the smaller superelasticity is.