He Ion Irradiation Induced Martensitic Transformation And Functional Properties Of TiNi Shape Memory Alloy

As a driving,damping and elastocaloric material,TiNi shape memory alloy has been successfully used in the intelligent structure of spacecraft.He ion irradiation is the important factor in the performance damage of spacecraft materials and devices during in-orbit service.This paper aims at the reliability requirements of long-life spacecraft TiNi alloy components.The paper systematically studys the change law of microstructure,martensitic transformation behavior and functional properties of TiNi alloy irradiated by He ions,and reveal the microscopic mechanism of He ion irradiation effect of TiNi alloy.The reliability evaluation of TiNi alloy and its components in long-term space services provides theoretical guidance and experimental basis.According to positron annihilation spectrum analysis,a variety of irradiation defects are formed in TiNi alloy.The single-vacancy defects form at the initial stage of irradiation by 3 MeV He ion,and the single-vacancy concentration increases with the increase of irradiation dose.When the irradiation dose exceeds 1×10¹⁶ ions/cm²,the single-vacancy defects form vacancy clusters through migration and aggregation,and the concentration of vacancy clusters increases with the further increase of irradiation dose.Meanwhile,the helium bubble in the alloy is produced by the irradiation of He ion,and the size of the helium bubble increases with the increase of irradiation dose.When the irradiation dose is 1×10¹⁷ ions/cm²,a large number of dislocations occurs in the alloy.The martensite TiNi alloy forms a multilayer structure of Ti₂Ni,B2 parent phase and martensite phase along the incident direction of He ions.The reason is that the Ni-poor layer is produced by the preferential sputtering effect on the surface when the He ions are incident.High-density vacancy-type defects,accompanied by defect beam diffusion,induces Ti atoms to migrate to the surface of the alloy and Ni atoms to migrate to the interior of the alloy through inverse Kirkendall effect,resulting in the appearance of Ni-rich and Ti-poor layer in the irradiated layer,causing a drop in the phase transformation temperature,and a parent layer is formed.He ion irradiation does not change the type of martensitic twinning,and<011>Ⅱtype twinning dominated the martensitic modification before and after He ion irradiation.However,the irradiation-induced defects,such as vacancy clusters,helium bubbles and dislocations,tend to aggregate at the martenstite<011>Ⅱtwinning interface,resulting in the destruction of the coherence of the<011>Ⅱtwinning interface,and the formation of certain lattice in the twinning variant.At the same time,a large amount of stacking faults along the（11（?））crystallographic plane are found near the local<011>Ⅱtwinning boundary.And forming a distortion layer with an atomic scale of 5 nm.In-situ TEM showed a typical discontinuous movement of the parent/martensite interface during the stress-induced martensite transformation.The destruction of the interface structure and the introduction of defects such as vacancy clusters,helium bubbles and dislocation during irradiation reduce the mobility of the interface between the parent phase/martensite.A decrease of phase transformation temperature,and increased phase transformation hysteresis and an improvement of thermal cycle stability of transformation temperature is observed in TiNi alloy with the increase of He ion irradiation dose.When the irradiation dose is lower than 5×10¹⁶ ions/cm²,the martensite TiNi alloy exhibits a one-step B2（?）B19’transformation.As the irradiation dose reaches 1×10¹⁷ ions/cm²,the two endothermic/exothermic peaks appeared on the DSC curves,an in-situ heating XRD analysis showed that these two endothermic peaks corresponding to the B2（?）B19’martensite transformation.The reason for the two-step martensitic transformation is that the uneven distribution of Ni atoms caused by the He ion irradiation.Along the incident direction,an irradiated layer composed of Ti₂Ni,Ni-rich and Ti-poor layer is formed on the surface of the alloy.The phase transformation temperature of the irradiated layer is significantly decreased,resulting in the phase change separation from the matrix.With the increase of irradiation dose,the surface hardness of TiNi alloy increase,and the recoverable strain decreases from 6.9%to 5.3%.The elastocaloric properties of TiNi alloy increase first and then decrease with the increase of irradiation dose.The isothermal entropy of TiNi alloy at room temperature increases from 38 J/kg·K to 68 J/kg·K at 1×10¹⁶ ions/cm².The adiabatic temperature variation increased from2.9 K to 6.5 K.The reason is that the introduction of vacancy-type point defects caused lattice distortion,and increase the volume change of the unit cell during stress-induced martensite transformation,resulting in the increase of entropy change of transformation.He ion irradiation could significantly improve the damping properties of TiNi alloy.With the increase of irradiation dose,the damping value of TiNi alloy first increase and then decrease.The peak value of phase transformation damping is 0.035,the martensite damping is 0.032,and the relaxation type damping is 0.052,after irradiation with 5×10¹⁶ ions/cm².A wide temperature range and high damping properties with a damping value higher than 0.03 are obtained in the 200 K temperature range.The high damping properties is due to the retardation of twinning boundary/phase boundary movement by helium interstitial atoms and vacancy defects introduced by He ion irradiation and the synergistic effect of（001）compound twinning produced by irradiation.