In Situ Electron Microscopic Study Of Amorphous Nanocrystalline NiTi Shape Memory Alloy

NiTi shape memory alloys are widely used in aerospace,life medicine,civil and other fields because of their biocompatibility,corrosion resistance,hyper elasticity and shape memory effect.Metallic glass(amorphous metal)NiTi has high hardness,but the damage caused by its internal inelastic deformation is mainly concentrated in the local lattice,so that its maximum elastic strain is usually less than 2%.Most known metallic glass materials hardly have elastic strain.Therefore,it is of great significance to improve its hyperelasticity and preserve the original hardness.In amorphous nanocrystalline NiTi shape memory alloys,the self-limiting martensitic transformation of nanocrystalline helps to balance the internal stress,reduce the local damage of lattice caused by stress concentration of metallic glass,and enhance the overall macro mechanical properties.This way of controlling crystal strain makes amorphous nanocrystalline NiTi shape memory alloy achieve excellent hyperelasticity in macro size.This design can provide a new idea for the development of highperformance bulk materials and the elastic strain engineering of metallic glass.In this thesis,nanocrystalline NiTi capable of martensitic transformation is used as the strain loading medium to enhance the elastic strain of amorphous NiTi alloy block,and the crystallization behavior and martensitic transformation behavior of nanocrystalline are studied by means of mechanical and thermal holders in-situ electron microscopy.The main research contents and conclusions are as follows:1.Preparation of amorphous nanocrystalline NiTi shape memory alloyBulk metallic glass NiTi alloy was prepared by arc melting and cold drawing techniques.Nano crystals were recrystallized internally through heat treatment.Bulk amorphous nanocrystalline NiTi shape memory alloy with macro elastic strain of 7%,fracture strength greater than 2 GPa and yield strength of 1.7 GPA was obtained,and its macro mechanical properties reached ideal strength.2.In situ thermal electron microscopic study of amorphous NiTi alloyThe amorphous NiTi alloy was heated in situ high-resolution transmission electron microscopy.It was found that the internal crystal structure of the amorphous NiTi alloy was combed at high temperature to make it nucleate and grow into nanocrystalline structure.The nucleation rate of nanocrystalline was measured to be about 0.1 ?M/s,the growth rate was about 0.012 ± 0.002 nm/s.The analysis of crystallization behavior provided a certain experimental basis for the crystal size of nanocrystals and the proportion of nanocrystals in amorphous alloys.3.In situ mechanical electron microscopic study of amorphous nanocrystalline NiTi shape memory alloyBased on the in-situ mechanical technique of transmission electron microscopy,the transformation behavior of nanocrystals in NiTi shape memory alloy under external stress was observed and analyzed in real time.It was found that the reduction of grain size did not inhibit the martensitic transformation behavior.At the same time,the interface effect between nanocrystalline and amorphous NiTi alloy was analyzed,It was also found that the synergistic effect between nanocrystalline and amorphous phases makes NiTi shape memory alloy has a better elastic strain.The research results should provide an important guidance for the design and development of metallic glass with excellent macro elastic strain properties.