Effect Of Defect Doping On Martensite Transformation And Thermodynamic Properties Of NiTi-based Alloys

In recent years,the influence of defects doping on the martensitic transformation of NiTi shape memory alloys has attracted much attention,but the main research has focused on point defect doping,and there are still few about the common doping of surface defects(grain boundary),dislocations and heterogeneous defects research reports.In this paper,NiTi alloys doped with various defects were designed to study the effect of defects on martensitic transformation.In order to study the influence of Fe element,grain boundary and dislocation defect doping on martensite transformation,this paper uses vacuum arc melting,forging,wire drawing and annealing to prepare equiatomic ratios of different grain sizes(15 nm?10?m)NiTi alloy wire,Ti₅₀Ni_50-xFe_x(2?x?4.5)memory alloy wire,and dislocation-doped Ti₅₀Ni₄₇Fe₃ alloy wire.The research shows that with the increasing of the grain boundary density(the grain size decreasing),the B2?R transformation temperature of nanocrystalline equiatomic ratios NiTi alloy.When the grain size is 14nm,it exhibits complete superelastic characteristics at room temperature,the temperature range of superelasticity is wider than that of coarse-grained NiTi alloy,and its temperature dependence of superelastic stress is smaller than that of coarse-grained NiTi alloy.When the Fe element and the grain boundary(nanocrystalline)are co-doped,there is no obvious exothermic effect during the cooling process with the increasing of total defect density;while the resistance shows a continuous abnormal increasing without temperature lag,and the initial temperature of the abnormal increasing was determined by the Fe content,which is similar to the phenomenon of"strained glass".For Ti₅₀Ni_50-xFe_x(2?x?4.5)alloy,when X=3,if the average grain size is 15 nm,the"strain glass"-like phenomenon appears.When X=4.5,if the grain size does not exceed 30 nm,then the"strain glass"-like phenomenon appears.The research also shows that the yield strength,superelastic stress and temperature dependence of Ti₅₀Ni_50-xFe_x(2?x?4.5)alloy are determined by grain boundary,Fe element and dislocation doping.By controlling these three types of doping defects,the yield strength and superelastic stress of the alloy are improved,and the temperature dependence of the superelastic stress is reduced.The NiTi-based memory alloy exhibits high superelastic stress at different temperature ranges which achieves the requirement of having high superelastic stress at wide temperature range.