The AFM Study On Microstructure Of TiNiNb Shape Memory Alloy

Superelastic shape memory alloys are widely used because of their recoverable strain of 6-8%,but the superelastic stress is generally low and the temperature window is narrow.However,the existing alloys can not meet the demand under the limit service condition.Previous studies of our group have found that nanocrystalline Ti Ni Nb shape memory alloy exhibits GPa hypersonic stress and a series of singular physical properties in the temperature domain of 77-333 K.It is preliminarily speculated that the above mechanical and physical properties are related to the nano-scale component non-uniformity in the alloy.In this paper,multiple imaging modes of AFM are used to study the size and distribution characteristics of the component non-uniform structure in Ti Ni Nb alloy,so as to provide theoretical basis for understanding the excellent properties of Ti Ni Nb alloy.In tapping mode,AFM probe contact with the sample surface intermittently,and this mode can respond to the different performance of micro area.The phase diagram USES the phase lag imaging of the signal after the probe contacts the sample to show the non-uniform Ti Ni Nb alloy components,and different parts of the phase lag may be different,we suspect AFM phase diagram is the representation of non-uniform Ti Ni Nb alloy components.In this paper,through the surface state comparison experiment including the water film,cleanliness and fluctuation on the test results was eliminated,and it was found that the phase lag of the homogeneous material was below 3.AFM test of Ti₄₃Ni₅₁Nb₆ and Ti_46.5Ni_51.5Nb₂ was carried out with the best sample surface state and equipment parameters.It is found that the class domain of size 5-10 nm exists in the phase diagram of Ti₄₃Ni₅₁Nb₆.In the phase diagram of Ti_46.5Ni_51.5Nb₂,a class domain with a size of 10-15 nm exists.The class domain size of Ti₄₃Ni₅₁Nb₆ is similar to that of the nano domain measured by atomic probe.By controlling the amplitude of AFM to change the force between the probe and the sample and observing the phase diagram,it was found that the amplitude increased and a high phase difference region appeared in the phase diagram of Ti₄₃Ni₅₁Nb₆ alloy,with the size gradually increasing from 5 nm to 15 nm.The size of the high phase difference region of Ti_46.5Ni_51.5Nb₂ alloy gradually increased from 10 nm to about 20nm,and it was preliminarily speculated that AFM phase diagram could characterize the phase transition ability of different regions with non-uniform composition of Ti Ni Nb alloy.In addition,AFM tests were performed on Ti₄₃Ni₅₁Nb₆ alloy after annealing for 20minutes for 435?and 600?,and Ti_46.5Ni_51.5Nb₂ alloy after annealing for 10 minutes for 400?and 500?.By observing the phase diagram,it was found that the class domain size of the annealing treatment Ti₄₃Ni₅₁Nb₆ with 435?nm was 5-10 nm,and that the class domain size of the annealing treatment Ti₄₃Ni₅₁Nb₆ with 600?nm was6-8 nm.The class domain size of 400?annealing treatment Ti_46.5Ni_51.5Nb₂ was 10-15nm,and that of 500?annealing treatment Ti_46.5Ni_51.5Nb₂ was 13-15 nm.With the increase of annealing temperature,the phase hysteresis of the alloy decreases.Preliminary guess heat treatment temperature affects composition nonuniformity.