Study Of The Mechanical Behavior And Microstructure Evolution Of Martensitic TiNi Alloys Under Compressive Loading

TiNi shape memory alloys have been widely used in biomedical engineering,aerospace and civil engineering owing to their unique shape memory effect,super-elasticity and excellent mechanical properties.With the development of functional devices of electronic products in the direction of miniaturization and precision,higher requirements are put forward for mechanical properties of the devices under low stress load service conditions.Martensitic TiNi alloy has the advantages of low elastic modulus and small deformation driving force,so it is especially suitable as the key material for the preparation of precision execution and driving devices of micro-electro-mechanical systems(MEMS).This thesis work mainly studies the compressive mechanical behavior and microstructure evolution of martensitic TiNi alloy,so as to provide reference for designing high-performance TiNi alloy components and solving the problems of functional stability in practical devices.In this study,a series of as-cast martensitic TiNi alloys with nominal composition Ti_100-xNi_x(x=36.0?50.0)were successfully prepared by non-consumable vacuum melting furnace.The compressive mechanical behavior and strain rate effect of Ti-rich TiNi alloy were investigated.As the Ti content increases,the volume fraction of the high modulus Ti₂Ni phase gradually increases,so the stress of the alloy increases rapidly with the strain during the loading process.When TiNi alloy has a near-equiatomic ratio of Ti to Ni,two yielding phenomena will occur during the alloy deformation process.The strain rate change has little effect on the first loading mechanical behavior of martensitic TiNi alloy,and the critical stress value of the first stress yield platform of some alloys increases to a certain extent;but for non-first loading deformation,as the strain rate increases,the work hardening effect becomes more significant and the anelastic effect gradually disappears during deformation.In this work,the transmission electron microscope(TEM)was also used to characterize the microstructure evolution process of the martensitic TiNi alloy under different compressive strains.The compression deformation process of martensitic TiNi alloy can be divided into three stages:(1)Elastic deformation of martensite and inelastic-accommodation process of martensite variants;(2)reorientation and detwinning process of martensite variants;(3)the plastic deformation process of de-twinned martensite.In this study,it was also found for the first time that there are two modes of unloading stress plateaus in the deformation of martensitic TiNi alloys;the main reason for the stress plateaus is that the martensitic deformation undergoes a reorientation recovery process(reverse process)during the unloading process.That is,the martensite variant that has undergone reorientation spontaneously undergoes orientation recovery under the combined action of internal and external stress during the unloading process.This study also explored the effect of rapid solidification texture on the mechanical behavior of martensitic TiNi alloys.The results show that the solidification texture has little effect on mechanical properties of the alloy under a small amount of deformation;when the amount of deformation exceeds 4%,the presence of the texture will introduce greater internal stress and increase the reorientation of the martensite modification.At the same time,the critical driving force of de-twinning process and plastic deformation of the alloy in different mechanical loading directions also show obvious differences.