Microstructure And Strain Recovery Characteristic Of Proton Irradiated NiTi Alloys

NiTi shape memory alloys as functional materials are widely used in intelligent structures of aerospace fields.Proton irradiation is an important factor affecting the reliability of shape memory materials and device in aerospace applications.The objective of this study is based on the requirement of the long-life and high stability for the new generation of the spacecraft.Proton irradiation researches are carried out on NiTi alloys with different phase states?martensite and austenite phases?by selecting characteristic energy?low energy:60-120 keV,high energy:3 MeV?distributed in the space environment.In the study,radiation effect on the microstructure,martensitic transformation,mechanical behavior and strain recovery characteristics of NiTi alloy and their mechanisms were investigated systematically by the TEM,PAT,DSC,and DMA.The study found that the irradiation parameters have a significant effect on the microstructure of NiTi alloy.The austenitic Ni-48.5at.%Ti alloy thin film has a bbc B2 phase and a Ti₃Ni₄ phase at room temperature.The structural phase states don't change after 120 keV proton irradiation,but the size of Ti₃Ni₄ particles in the irradiated layer increases a little.The unirradiated Ni-51.4at.%Ti alloy film has a monoclinic structure B19?martensite at room temperature.After 120 keV proton irradiation,when the fluence below 5.0?10¹⁵ p/cm²,two new twin structures are formed in the matrix:?001?compound twins and?111?type I twins;when the fluence increasing to 2.0?10¹⁶ p/cm²,the irradiated layer undergoes reverse martensitic transformation to form the B2 structural parent phase,and the nano-scale Ti-riched GP zone is dispersed in the B2 phase.At the same time,proton irradiation induces many kinds of crystal defects in alloys:vacancy-type point defects under low fluence irradiation are dominant,and dislocation,amorphous and precipitates with the increase of irradiation fluence are the main defects.The martensitic Ni-50.0at.%Ti alloy is irradiated by 3 MeV protons,and the irradiation layer forms a complex multilayer structure comprising TiH₂,Ti₂Ni,austenite,austenite-martensite layers and martensite layer along the irradiation direction with increasing distance,and the thickness of each layer increases with the increase of fluence.The irradiation layer has high-density dislocations and a small amount of amorphous micro-areas.Irradiation-induced multilayer structure was shown to be closely related to the preferential sputtering effect and the inverse Kirkendall effect.As a result of preferential sputtering,Ti is abundant at the outermost surface compared with Ni;the radiation-induced high-density vacancy-type point defect forms a defect gradient distribution,which induces the inverse Kirkendall effect,leads to the migration of Ti atoms toward the surface and the migration of Ni atoms toward the interior;under the combined action of the two factors,the redistribution of alloy atoms results in the appearance of multilayer structure in the alloy surface.The Ni-48.5at.%Ti alloy film was irradiated by low-energy protons,Rs did not change and As decreased slightly with the increase of energy,and the R phase had better anti-irradiation stability.The Ni-51.4at.%Ti film was irradiated by low-energy120 keV proton.Transformation temperature of the irradiated layer gradually decreased with increasing fluence,and the transformation temperature interval and transformation hysteresis gradually increased.In the low fluence of 1.0?10¹⁵ p/cm²,it is a single-step phase transition:B2?B19?phase transition;when the irradiation fluence is increased to 2.0?10¹⁶ p/cm²,the alloy exhibits a two-step phase transition,resulting in phase separation of the irradiation layer and the substrate layer.The Ni-50.0at.%Ti alloy was irradiated by high-energy 3 MeV protons.The martensitic transformation temperature decreased significantly with increasing fluence,and the phase transition temperature interval and phase transition hysteresis increased.After irradiation of NiTi alloy,the martensitic transformation temperature is significantly decreased,which is caused by the irradiation defects such as high-density dislocations,the redistribution of components in the alloy and the constraint of the irradiation-induced multilayer structure.After irradiation of martensitic NiTi alloy,the hardness and elastic modulus increase remarkably with the increase of irradiation fluence/energy,which is related to the occurrence of radiation-induced austenite phase and accumulation of crystal defects.At 120 keV proton irradiation,the critical stress of re-orientation of martensitic Ni-51.4at.%Ti alloy martensite is monotonously increased with the increase of fluence.When the fluence is 2.0?10¹⁶ p/cm²,the reorientation critical stress of the martensite reorientation is increased by about 25%compared to the unirradiated alloy.With the increase of irradiation fluence/energy,the fracture strength of Ni-51.4at.%Ti alloy increased and the elongation decreased slightly.After irradiation,with the increase of irradiation fluence/energy the recoverable strain of Ni-51.4at.%Ti alloy decreases from 4.8%to 3.6%.With the increase of irradiation fluence/energy,the internal friction of Ni-51.4at.%Ti alloy decreases in martensite transformation,and the intrinsic martensite internal friction increases from 0.045before irradiation to 0.075,which is increased by about 60%.Based on the above research,the intrinsic relationship between the irradiation parameters-microstructure-martensitic transformation-macrofunction is established:proton irradiation introduces multiple types of irradiation defects such as high-density vacancy-type point defects,dislocations,nano-precipitation and amorphous micro-region which induces the alloy to form a new type of twins and undergoing phase structure transformation;the high-density defects in the irradiation layer,the redistribution of components and the mutual restriction between the multi-layer structures,the three factors work together to reduce the phase transformation temperature,the phase transition of the irradiated layer and substrate layer was carried out stepwise and transformation temperature interval widening;the hardness and elastic module of the alloy are combined by the defect strengthening and the irradiation-induced B2 phase structure,high-density dislocations in the alloy hinder the slip of twin interface and phase interface,which reduces the mobility of the interface and reduces the strain recovery.The high-density crystal defects and the interface mobility decrease lead to the intrinsic internal friction increased.