Study On Microstructure And Superelasticity Of Porous NiTiNb Alloy Prepared By Eutectic Reaction

Porous NiTi-based shape memory alloys（SMAs）have attracted much attention in the fields of automatic control,aerospace and biomedicine due to their excellent superelasticity,high strength and biocompatibility.Nb element is added to the NiTialloy as the third element,which can not only improve the biocompatibility of the material,but also improve the phase transition wide hysteresis effect.Among them,NiTiNb memory alloys,which have low phase transition temperature and high reverse phase transition temperature,thus exhibiting shape transition alloy hysteresis（As-Ms）,are expected to become high-strength intelligent damping materials with great application potential.In order to study the distribution mechanism and superelasticity of the precipitated phase produced by the martensitic transformation before and after deformation of the truss porous NiTiNb alloy material with a three-dimensional space structure.In this paper,nickel-titanium powder,niobium powder and titanium wire are used as raw materials to prepare a three-dimensional porous NiTiNb SMAs by vacuum sintering.The effects and mechanisms of the microstructure,phase,mechanical properties and superelastic behavior of the alloy were systematically studied.The results show that at 1180℃ vacuum sintering temperature,different reaction zones appear in the microstructure of the three-dimensional porous NiTiNb SMAs,that is,the eutectic region and the pre-eutectic region are included in the middle of the titanium wire.In the eutectic region,as the content of Nb element increases in the alloy,the solid solubility of the Nb element increases,and the eutectic structure andβ-Nb phase continue to increase and coarsen.The recoverable strain is porou NiTiNb alloy tend to decrease first and then increase.They are typical wide hysteresis memory alloys,which show superior linear superelasticity（about 8%）.At a vacuum sintering temperature of 1180℃,the precipitated phases in the eutectic zone of porous NiTiNb alloy include B2 phase,Ti₂Ni phase,andβ-Nb phase,and the rod-shaped R phase and layered L phase are precipitated in the pre-eutectic zone.In order to further compare the micromorphology and mechanical properties of the eutectic region and NiTimatrix region in NiTiNb SMAs,three alloys of Ni₅₁Ti₄₄Nb₅,Ni₄₆Ti₄₇Nb₇,and Ni₃₃Ti₅₁Nb₁₆ were prepared by vacuum melting.Using SEM,TEM,tensile and compression,micro-pillar compression,and simulation methods,the martensitic transformation of the alloy with an annular eutectic structure during compression has been studied to cause the microstructure change.The research shows that the grain size changes of Ni₄₆Ti₄₇Nb₇ alloy during cyclic loading were observed during the simulation.When Ni is 46at%,the Nb content in the eutectic region is significantly larger than that in the NiTiregion and the solubility of Nb is significantly increased.This phenomenon can be attributed to the large number ofβ-Nb particles precipitated and uniformly distributed in these eutectic regions.The results of cyclic compression and micro-pillar compression show that the Ni₄₆Ti₄₇Nb₇ alloy with annular eutectic microstructure is more susceptible to martensitic transformation and exhibits better plasticity during compression.Compared with NiTiNb alloys with different compositions,it was found that the alloys can significantly improve the mechanical properties and superelasticity of porous NiTiNb SMAs after solution treatment and aging treatment.