Study On The Phase Transformation Characteristics And Mechanical Properties Of TiNiCoNb Shape Memory Alloys

Superelastic Ni Ti shape memory alloys have been widely used in aerospace,energy and power,architecture,biomedicine and other fields.In recent years,some advanced aerospace countries（such as China,the United States and Europe）have proposed a new idea of using super elastic shape memory alloy to make shock-absorbing tires for deep space detectors,which requires that super elastic shape memory alloy exhibit high phase transformation stress at low temperature and wide temperature range.However,the current commercial Ni Ti memory alloys do not meet the demand.In view of this problem,a series of TiNiCoNb shape memory alloy wires were prepared by doping Ni Ti with Nb and Co elements,vacuum melting,hot rolling,cold drawing,large deformation and low-temperature annealing;By means of resistance,DSC,DMA,TEM,tensile machine and synchrotron radiation high-energy XRD,the microstructure,thermal/stress-induced phase transformation and superelasticity of the alloy were systematically studied.A TiNiCoNb memory alloy with high elastic stress in low temperature and wide temperature range was obtained,and the mechanism of its superior superelastic properties was preliminarily discussed.The main research results are as follows:（1）The effect of Co on the phase transformation behavior and superelastic properties of Ti₅₀Ni_50-a-xCo_a+x（x=3,5,9）alloys was studied.The results show that the starting temperature rs of parent phase to R phase transition decreases with the increase of Co content.When the content of Co is 9 at.%,Ti₅₀Ni_41-aCo_a+9 alloy shows hyperelasticity in the temperature range of-196℃～-40℃;The superelastic stress is up to 1200 MPa at-40℃ and is 967 MPa at-196℃.（2）The effect of Nb on the phase transition behavior and superelastic properties of Ti_50-b-yN_i47-aCo_a+3Nb_b+y（y=0,0.25,0.5,1,3,5）alloys was studied.The results show that the starting temperature rs of parent phase to R phase transition decreases with the increase of Nb content.When the Nb content increases to 3～5 at.%,the B2→R transition is suppressed and the superelastic stress is significantly increaⁱsed.Ti_45-bNi_47-aCo_a+3Nb_b+5alloy shows excellent superelasticity in a wide temperature range（276℃）.The superelastic stress is 1770 MPa at 80℃ and is 1230 MPa at-196℃.（3）Ti_46-bNi_46-aCo_a+5Nb_b+3 shape memory alloy with an average grain size of 15 nm was successfully prepared.Martensitic pre transformation occurred during the cooling process,and the superelastic stress appeared in the wide temperature range from-196℃ to 25℃.Among them,the maximum superelastic stress was about 1.95 GPa at 25℃,the minimum superelastic stress was 1.28 GPa at-140℃.The average superelastic stress was 1.53 GPa in the whole temperature range,it is more than twice that of Fe based shape memory alloys reported in science journals in 2020.（4）The temperature coefficient of elastic modulus of Ti_50-b-yNi_41-aCo_a+9Nb_b+y（y=0,0.2,0.4,0.7,1.5）alloy was systematically explored.Above 304℃,the temperature coefficient of elastic modulus gradually approaches zero with the increase of Nb content;Ti_48.5-bNi_41-aCo_a+9Nb_b+1.5 sample presents Elinvar effect in a wide temperature range from 35℃ to 304℃.