Microstructure And Shape Memory Effect Of Thermomechanically Treated Ti-16Nb High Temperature Shape Memory Alloy

Compared with other high temperature shape memory alloys,Ti-Nb alloys have excellent hot and cold workability,martensitic transformation temperature of up to300^?,and has broad application prospects in driving and connection in high temperature applications.However,the shape memory effect of the Ti-Nb alloys is poor and the fully recoverable strain is less than 2%,which has hindered their further popularization and application.In this paper,the effects of thermomechanical treatment on microstructure and interface structure,martensitic transformation and shape memory effect of Ti-16Nb high temperature shape memory alloy were studied by means of transmission electron microscopy?TEM?,high resolution transmission electron microscopy?HRTEM?,differential scanning calorimetry?DSC?and tensile test.The microstructure evolution and deformation mechanism of Ti-16Nb alloy during tension at room temperature were revealed and the mechanism of thermomechanical treatment to improve the shape memory effect of Ti-16Nb alloy was proposed.TEM observation shows that the???martensite in the solution treated Ti-16Nb alloy is in the V-shaped self-accommdated form mainly.The martensite variants forming the V-shaped cluster have the{1 1 1}type I or the?2 1 1?type II twin relationship.The twin interface has good coherence.Except for the twin interfaces,it is observed that????2 0?_CV?i?^{1 3 1???}_CV?j?or????2 0?_CV?i?^{???0 2}_CV?j?lattice plane acts as a junction plane to form a semi-coherent interface,when the side of the V-shaped martensite is in contact with the ends of other variants.After proper thermomechanical treatment of the Ti-16Nb alloy,the dislocations introduced by cold working are partially eliminated and rearranged during annealing to form a local internal stress field,which makes the martensite variants occurs the preferred orientation forms a single-oriented martensite lath.Increasing the annealing temperature or prolonging the annealing time,the number of preferred orientation martensite laths decreases,and the amount of martensites with the V-shape configuration increases.Thermomechanical treatment has a significant effect on the martensitic transformation of Ti-16Nb alloy.When the cold rolling/cold drawing deformation is60%,the reverse martensitic transformation peak temperature,A_p,increases as the annealing temperature increases or the annealing time increases.When the annealing temperature is raised,the volumn fraction of?phase is decreased and the Nb content of the??martensite is decreased accordingly,resulting in an increase in A_p.As the annealing time prolongs,the volumn fraction of?changes little and the residual dislocation density gradually decreases.And its promotion of martensite nucleation is weakened,resulting in an increase in A_p.When the annealing parameters are constant,A_p decreases as the degree of cold rolling/cold drawing deformation increases.Appropriate residual dislocations do not significantly hinder the martensitic transformation,but also favor the nucleation of martensite,so the phase transformation temperature decreases.When the solution treated Ti-16Nb alloy is deformed at room temperature,a lot of fine{1 1 1}I type and?2 1 1?II type twin bands form inside the coarse martensite variant.When the deformation variable exceeds 2%,the dislocation slip occurs simultaneously with the martensite twin.When the strain was increased to10%,a small amount of{0 1 1}compound twin formation was also observed.Thermomechanical treatment changes the microscopic mechanism of deformation of Ti-16Nb alloy.When the tensile deformation is less than 5%,the martensite is mainly combined and reoriented in the Ti-16 Nb alloy after appropriate thermomechanical treatment.When the tensile strain is greater than 5%,dislocation slip begins to appear,resulting in a shape that cannot be fully recovered.When the solution treated Ti-16Nb alloy is deformed under tension,the{1 1 1}stacking faults can act as the nucleation sites of the{1 1 1}type I and the?2 1 1?type II twin.Stacking faults themselves are possible nucleation sites for the{1 1 1}type I twins.If the Shockley partial dislocation with a Burgers vector of?0.3053 0.1463????glides continuously on the{1 1 1}lattice plane for 5 atomic layers,the{1 1 1}type I twin forms.While for the?2 1 1?type II twins,they may form at the end of a series of{1 1 1}stacking faults,where Shockley partial dislocations exit.The?0 0 2?_m plane is approximately parallel to the?0 ???0?_t plane for the two crystals in?2 1 1?type II twin related orientation.The d-spacing of?0 0 2?_m lattice planes increase,resulting from the Shockley partial dislocations nearby and the?2 1 1?type II twin forms after tiny modification of the?0 0 2?_m lattice planes.When deforming is occuring,the movement of the{1 1 1}type I twin interface is achieved by the slip of the twinning dislocation on the twin plane.When the tensile strain is 5%,there are unequal steps on the twin interface,and a small number of{1 1 1}stacking faults are observed near the partial twin interface.When the strain is increased to 10%,a distortion layer is generated near the twin interface.During the deformation process,the?2 1 1?type II twins realize the movement of the twin plane????5 ????by the shearing of multiple?1 ???1?crystal planes.Thermomechanical treatment significantly improved the shape memory effect of Ti-16Nb alloy.When the cold rolling/cold drawing deformation is 60%and the annealing time is 0.5h,the recoverable strain of Ti-16Nb alloy increases with the increase of annealing temperature,and reaches the maximum when the annealing temperature is 700 ^?.When the rolling/cold drawing deformation is 60%and the annealing temperature is 700 ^?,the recoverable strain decreases as the annealing time increases.The optimal thermomechanical treatment parameters for Ti-16Nb alloy are 60%cold rolling/cold drawing,annealing at 700 ^? for 0.5 h.In such cases,5%fully recoverable strain can be obtained.The thermal stress of Ti-16Nb alloy after forming a preferred orientation of lath martensite has low reorientation critical stress,and good interface mobility is the main reason for the improvement of shape memory effect.