The Study On The Microstructure Of NiTiNb Shape Memory Alloys With High Superelastic Stress In Wide Temperature Range

Due to the low strength of parent phase and limitation of thermodynamics,it is difficult for shape memory alloys to exhibit high superelastic stress in wide temperature range.Recently,Ni_x Ti_yNb_z shape memory alloys developed by our research group exhibit high superelastic stress in low temperature and wide temperature range?-196? to 60??,which triples that of commercial NiTi alloys and is much higher than the reported superelastic stress of SMA.To explore the deformation mechanism of Ni_xTi_yNb_z shape memory alloys with high superelastic stress,we investigated the microstructure of Ni_xTi_yNb_z shape memory alloys by scanning electron microscope?SEM?,transmission electron microscopy?TEM?,STEM and EDS in this paper.The SEM results show that there is inhomogenous composition microstructure with bright and dark regions in Ni_xTi_yNb_z ingots and warm-draw wires;however,this phenomenon does not appear in cold-draw wires.The cold-draw wires contain parent phase with B2 structure.STEM graphs consist of bright and dark regions in the matrix of Ni_xTi_yNb_z cold-draw wires,but there are no differences in elemental distribution of the bright and dark regions.We also prepared Ni_x+0.5Ti_y+1.5Nb_z-2 warm-draw wires.The results of TEM and STEM show that there are also bright and dark regions in Ni_x+0.5Ti_y+1.5Nb_z-2 warm-draw wires.The dark regions belong to B2 parent phase,which contain more Ni,Nb and less Ti compared with those of bright regions.Bright regions are distributed in strip microstructure,and we found second phase particles in these regions.Besides,this dissertation also investigated phase transformation behavior and microstructure evolution of NiTiNb alloys in the cooling process by resistivity-temperature tests,HEXRD and TEM.The results show that NiTiNb alloys keep a“B2”structure,but the resistivity increases in the cooling process and the SAEDs show some additional,faint,diffuse superlattice reflections appear.With temperature lowering,these reflections become sharper.These abnormal performances are similar with characteristics of premartensitic transformation and“strain glass”transition.However,the cause of these phenomena in the cooling process needs further study.