Phase Transformation Behavior And Recovery Properties Of Ti-Ni-Nb-Co Alloys

Ti-Ni-Nb shape memory alloys can obtain wide hysteresis by proper deformation at specific temperature.The pipe joint with Ti-Ni-Nb alloys can restore at room temperature after expanding,which bring more convinent to applications.However,recovery ratio decreases significantly after excessive deformation.Therefore,improving recovery properties while simultaneously maintaining wide transformation hysteresis is the crucial challenge for pipe joint with excellent performance in acrospace applications.In this thesis,the addition of Co element and thermalmechanical treatment was used to achieve uniformly nanoscaled?Ti,Nb?₂Co precipitates and construct high-density low angle boundaries,which could strengthen parent phase and improve recovery properties.The effect of Co contents,annealing and thermalmechanical treatment on the precipitation,martensitic transformation,mechanical behavior and recovery properties of Ti-Ni-Nb-Co alloys were systematically investigated by X-ray diffraction,scanning electron microscopy,transmission electron microscopy,three-dimensional atom probe,differential scanning calorimetry and mechanical testing machine.It was revealed that nano-scale?Ti,Nb?₂Co precipitates were found in the annealed Ti_44.5Ni_44.5Nb₉Co₂ alloy ranging from 550 ? to 750 ?.The precipitation process was Ti-clustersàGP zonesà?Ti,Nb?₂Co precipitates.With annealing temperature increasing,the size of?Ti,Nb?₂Co precipitate was remarkably increased and concurrently the distribution characteristics of the?Ti,Nb?₂Co precipitates changed.As the annealing temperature was lower than 750 ?,the?Ti,Nb?₂Co precipitates were mainly dispersed at the edge region of TiNi matrix;while?Ti,Nb?₂Co phase was only distributed at the edge region of TiNi matrix when annealing at 850 ?.The thermalmechanical treatment also had significant impact on the precipitation and microstructure.After 40% cold rolling and annealing process with 550?750?,?Ti,Nb?₂Co precipitates were uniformly dispersed on the TiNi matrix and the size of precipitates increased with the annealing temperature increasing.When the annealing temperature was 650 ?,high-density low-angle grain boundaries and submicron-sized sub-crystals formed.With annealing temperature increasing,recrystallization began and the large angle grain boundaries were dominated.In the as-cast and annealed Ti_44.5Ni_44.5Nb₉Co₂ alloy with 450 ? and 850 ?,only one-step martensitic transformation occured during cooling process.It was worthy noticable that the two-step martensitic transformation occured in the cooling process of the annealed Ti_44.5Ni_44.5Nb₉Co₂ alloy ranging from 550 ? to 750 ?.The in-situ transmission electron microscopy results confirmed two-step B2àB19' martensitic transformation.It showed that the martensite without substructure first nucleated in the core region with less precipitation.The second step martensitic transformation took place in the edge region with the dispersed precipitates.The substructure of martensite variant was mainly type ?(11 1and type ? <011>.The main reason for the occurrence of two-step martensitic transformation was the difference in the number of?Ti,Nb?₂Co phase within the edge and core regions of the matrix,which resulted in the obvious difference of Ni content and strain field,leading to different martensitic transformation temperature.The study indicated that the yield strength,critical stress for induced-martensite,recovery ratio and recovery stress increased first and then decreased with the increase of annealing temperature and time.For the Ti_44.5Ni_44.5Nb₉Co₂ alloy with 40% cold rolling ratio and different annealing temperatures,yield strength decreased with annealing temperature increasing,while the recovery properties first increased and then decreased.When the pre-deformation strain was 16%,the maximum recovery ratio and stress could reach 84.5% and 584 MPa,respectively.Concurrently the phase transformation hysteresis after deformation remained 142 ?.The improvement of reocovery properties was mainly due to dispersed precipitates and high-density low angle grain boundaries.Therefore,Ti-Ni-Nb alloy with wide phase transformation hysteresis,large recovery ratio and high recovery stress had been achieved,which could lay the foundation for shape memory alloys pipe couplings in engineering applications.