Microstructure And Properties Of NiTi(Nb) Alloys Through Surface Nano-crystallization

NiTi（Nb）alloys with a gradient structure have a large phase transition stress range and good cycling stability,and have great application potential in temperature-controlled cycling drives,such as micropackers and bionic robotic hands.Previously,some scholars used magnetron sputtering,laser local heat treatment and other methods to prepare NiTi alloy with gradient structure parallel to the direction of force.In this paper,a NiTi（Nb）alloy with a gradient structure perpendicular to the force direction was prepared by surface mechanical grinding and shot peening.The microstructure,phase transformation behavior and mechanical properties of the gradient structure NiTi（Nb）alloy were studied.The effects of low temperature aging treatment on the phase transformation behavior and mechanical properties of NiTi（Nb）alloys were investigated.The average grain size of Ni₄₇Ti₄₈Nb₅alloy is 43μm,and the average grain size of the surface after grinding with 350 N is 31 nm.The average grain size of Ni₅₀Ti₅₀alloy is 49μm,and the average grain size of the surface after shot peening is 32 nm.The average grain size of Ni_50.8Ti_49.2alloy is 102μm,the average grain size of the surface after shot peening is 29 nm,and the average grain size of Ni_51.5Ti_48.5alloy is 104μm after grinding with 350 N.,the average grain size of the surface after shot peening is 60nm,and it is 22 nm after grinding with 350 N.The surface of NiTi（Nb）alloy is B2 parent phase after surface nano-treatment,and B19’martensitic phase is formed at a depth of 60-70μm from the surface.This is due to the increase of the surface temperature of the sample during the process of surface nanometerization,which causes the stress-induced B19’martensite phase to undergo reverse transformation and transform into the parent phase.After the surface of NiTi（Nb）alloy is nano-treated,the phase transition peak is broadened,which is due to the existence of gradient structure and uneven grain size.After aging treatment,the phase transition peak becomes narrower,because the grain size tends to be uniform.The single-pass shape memory strain of Ni₄₇Ti₄₈Nb₅alloy decreased from 4.3%to1.3%after grinding,and the shape memory strain became smaller;the absolute value of phase transformation rate decreased from-0.414 to-0.023,which improved the controllability of deformation drive.Ni5_0.8Ti_49.2alloy was shot peened and then aged at 250℃for 8 h,the hyperelastic strain increased from 3.8%to 7.4%.