Research On The Stress Relaxation Law And Characterization Technology Of NiTiNb Shape Memory Alloy

NiTiNb shape memory alloy which both has excellent shape memory performance and mechanical properties is widely used as pipeline for connecting and sealing.Under such conditions,the magnitude and change of the recovery stress namely the stress relaxation behavior is a key factor influencing the usage life of the products.The stress relaxation is now characterized mainly by the tension method with standard specimen.It may last several years to conduct a room temperature stress relaxation test by this method,and it can only simulate the macro-scale stress relaxation of the material simply.So it is not too fit for the systematic research.So as to the stress relaxation of the NiTiNb shape memory alloy,only a few work of the stress relaxation under elevated temperature have been done,researches which reflect the actual use and relevant influences still can not be found.Aims at the urgent needs of the characterization of the room temperature stress relaxation of the Ni47Ti44Nb9 shape memory alloy new characterize methods and the room temperature stress relaxation behaviors were systematically researched.Firstly,the neutron diffraction device with temperature and force controlling functions was involved to study the phase transformation and its relation with the recovery stress in-situ during the recovering process.Based on the results obtained and the phase transform rule of the Ni47Ti44Nb9 shape memory alloy,the mechanism of this phenomenon was analyzed.This work may boost the understand of the change of the recovery stress during the recovering process.Secondly,a new fast and economic characterization method was established based on the nano-indentation equipment.The influences including the upload speed and max-load were studied using this method,and the phenomena observed was analyzed from the point of dislocation theory.Experimentation was designed to study the stress relaxation of samples recovered under different temperature,and stress relaxation risk was compared with the common used alloys.Thirdly,a new method which can characterize the stress relaxation of the Ni47Ti44Nb9 shape memory alloy products in-situ was established base on the X-Ray diffraction device.The feasibility to characterize the recovery stress through the surface stress and the impact of the temperature on the recovery stress was studied using this method.Lastly,the stress relaxation of the pipeline in-use was tested in 5 months.In this paper,new methods which were fast economic nondestructive and in-situ were established and the stress relaxation behavior of the NiTiNb shape memory alloy was studied.The results may boost the study of the room temperature stress relaxation of the NiTiNb shape memory and guidance the preparation and optimization of the NiTiNb shape memory products.