Study On Microstructure And Superelasticity Of Nanocrystalline NiTi Alloy At Low Temperature And In Wide Temperature Range

NiTi shape memory alloy（SMA）is one of most widely used smart metal materials in industrial applications due to excellent superelasticity.Upon loading,it goes through B2→B19’transformation,and reverse B19’→B2 transformation upon unloading,generating a large recovery strain of 6-8%.However,the strain is non-linear strain,which is different from elasticity.The stress-strain curve does not satisfy Hooke’s law.It has been found that NiTi SMA can show quasi-linear superelasticity after cold deformation treatment.The tensile curve has no stress platform,and the highest linear elastic strain can reach 4%.The most remarkable advantage of linear superelasticity is that it can realize the precise control of the stress and strain of components.Claperon-Clausius equation has clarified that the superelastic upper and lower stresses decrease linearly with the temperature decreasing.For example,the temperature-dependence（dNiTi/d T）of superelastic stress（critical stress for martensitic transformation）of the equiatomic NiTi alloy with mean grain size of 150 nm is 6 MPa/℃.In addition,the yield strength of coarse-grained NiTi alloy（the mean grain size is larger than 150 nm）is relatively low.When the testing temperature increases,the yield stress decreases and becomes far below the superelastic stress,the NiTi alloy goes through plastic deformation rather than superelastic deformation.These two factors greatly limit the superelastic temperature range of the NiTi SMA.Different from coarse-grained counterparts,nanocrystalline NiTi alloys show different superelastic phenomena and laws due to a large number of grain boundaries or other defects inside.For example,it has been reported that dNiTi/d T decreases significantly with the decrease of grain size,and when the grain size is reduced to 15 nm,dNiTi/d T can be reduced to 4.7 MPa/°C.At the same time,the superelastic stress and yield strength of nanocrystalline NiTi alloy are also significantly higher than those of its coarse-grained counterpart.However,the thermal induced B2→R phase transition occurs during the cooling process of nanocrystalline NiTi SMA,and the complete superelastic cannot be achieved at low temperature,resulting in a narrow superelastic temperature range of about 50℃.In order to meet the needs of special fields such as deep space exploration,this paper aims to obtain nanocrystalline NiTi alloys with high superelasticity in low temperature and at wide temperature range by large deformation cold drawing followed by crystallization annealing.For one hand,the internal stress is introduced by pretreatment,and the synergistic effect of twin deformation and elastic deformation is realized,which facilitates the precise control of the stress and strain of the component.We prepared an equiatomic nanocrystalline NiTi alloy with large linear elastic strain in low temperature and at wide temperature range.On the other hand,it is the first time to prepare the nanocrystalline Ni₅₂Ti₄₈（at.%）alloy by the method of“aging precipitation+mechanical resolution+crystallization annealing”,which can exhibit high superelastic stress in a low temperature and at wide temperature range.The superelastic properties of nanocrystalline Ni₅₂Ti₄₈ alloys prepared by traditional methods and fast electric pluse annealing are systematically studied,and the influence of coupling effect in the two-phase regions of rich and poor Ni on their macroscopic properties is revealed.Furthermore,by controlling Ni atoms distribution,the superelastic stress of nanocrystalline Ni₅₂Ti₄₈ alloy is improved,and the superelastic temperature range is widened.In this paper,the relationship between the microstructure and superelastic properties of nanocrystalline equiatomic NiTi and Ni₅₂Ti₄₈ alloy wire samples is systematically studied by means of universal tensile testing machine,electron microscope,synchrotron high-energy X-ray diffraction（HE-XRD）and atomic probe,and the main research contents and results are as follows:（1）The obtained nanocrystalline NiTi alloy is treated with 14%tensile pre-deformation,and the HE-XRD results show that the sample after unloading is composed of B19’martensitic,in which some grains are subjected to tensile stress and other grains are under compressive stress.The pre-deformed nanocrystalline NiTi alloy exhibits an ultra-large linear elastic strain of up to 5.1%and an ultra-high yield strength of 2.16 GPa in the temperature range of 270°C（-197°C to 70°C）.The HE-XRD results show that the grains subjected to tensile stress do not undergo elastic deformation during the subsequent tensile loading and unloading process,but go through the twin-detwinning deformation.The grains subjected to compressive stress first undergo elastic deformation and then twin-detwinning deformation.The grains of with different stress states undergo different deformation,that is,twinning deformation and elastic deformation occur together,so that the samples can exhibit large linear elastic deformation over a wide temperature range.（2）Based on the poor workability of Ni₅₂Ti₄₈ alloys,which makes it difficult to achieve amorphization by severe cold deformation,the following ideas are proposed in this project:The Ni content in the matrix is reduced by aging pretreatment and the processing plasticity is improved.Then the severe cold drawing deformation treatment is carried out.As a result,it is achieved that the matrix is amorphous,the precipitated phase generated during the aging treatment can be redissolved into the matrix.Finally,the nanocrystalline Ni₅₂Ti₄₈ alloy is prepared by subsequent crystallization annealing.（3）The three-dimensional atomic probe results show that an obvious Ni atoms segregation to the grain boundary in the nanocrystalline Ni₅₂Ti₄₈ alloy,that is,there are two phases of rich-Ni region and poor-Ni region.This unique microstructure creates a coupling effect of phase transformation and elastic deformation,and the prepared nanocrystalline Ni₅₂Ti₄₈ alloy can exhibit high superelastic stress of 730-1600 MPa in the temperature range of-100°C to 120°C.（4）The superelastic characteristics of nanocrystalline Ni₅₂Ti₄₈ alloy prepared by alternating current and direct current electric annealing at different temperatures are studied.The Ni atoms segregation is suppressed by shortening the annealing time.It is found that the superelastic stress of the nanocrystalline Ni₅₂Ti₄₈ alloy prepared by rapid electric pulse annealing is slightly lower than that of the samples prepared by the traditional method.It is supposed that the superelastic stress of nanocrystalline Ni₅₂Ti₄₈alloy can’t be increased by inhibiting the Ni atoms segregation.But if the microstructure characteristics of rich and poor Ni regions are formed inside the nanocrystalline Ni₅₂Ti₄₈alloy,the coupling effect of these two regions during tensile loading will enhance its superelastic stress.（5）The nanocrystalline Ni₅₂Ti₄₈ alloy prepared by electric pules annealing is treated by low-temperature aging method,and the effect of annealing time on the superelasticity of the sample is studied.It is found that the superelastic stress is increased after short-term aging treatment,and if the annealing time is further extended,and the superelastic stress will gradually decrease.It is believed that it is related to the redistribution of Ni atoms.In this study,the prepared nanocrystalline NiTi alloy can show high superelastic stress（higher than all the reported memory alloys）,at low temperature（down to-197℃）and wide temperature range（wider than 200℃）.The equiatomic nanocrystalline NiTi alloy with low temperature,wide temperature range and large linear strain is obtained,by introducing internal stress,making the elastic deformation and twinning-detwinning deformation going in parallel.The nanocrystalline Ni₅₂Ti₄₈ alloy with low temperature,wide temperature range and high superelastic stress is obtained due to the Ni segregation,leading to the poor/rich Ni region,thus the elastic deformation and martensite deformation go simultaneously.This study provides an idea for preparing wide temperature range nanocrystalline NiTi alloy and expands its application range.