Martensitic Transformation Behavior And Damping Property Of Compositionally Graded TiNi Alloys

In present work, the preparation method, martensitic transformation behavior and damping properties of compositionally graded TiNi alloys were investigated. Firstly, a thin layer of Ni was deposited by electroplating on a homogeneous TiNi alloy. Then the compositional gradient was created by diffusion annealing. The effect of annealing process on compositional gradient was investigated by using scanning electron microscope (SEM)and energy dispersive spectroscopy (EDS). The relationships between annealing,transformation temperature and transformation interval were investigated by using differential scanning calorimetry (DSC). The effect of thermal cycling and aging on martensitic transformation behavior of compositionally graded TiNi alloys were also investigated. The effect of frequency and strain amplitude on the damping properties of compositionally graded TiNi alloy were investigated by the dynamic mechanical analy zer(DMA).The results show that with increasing electroplating time, the thickness of deposition layer increases with a parabolic law. When the deposition time is longer than 20 min, the adhersion between the layer and matrix becomes worse. The compositionally graded TiNi alloys can be obtained by the following diffusion annealing. With increasing diffustion temperature or time, the diffusion distance of Ni increases.Upon cooling and heating, a single step B2(?)B19’ martensitic transformation and reverse transformation occurs in the compositionally graded TiNi alloys. The annealing process has little influence on Ms and Af temperatures at low annealing temperature and short duration. However, the temperatures of Mf and As decrease with increasing annealing temperature or duration. When the annealing temperature is high,the Ms、Af、Mf and As temperatures decrease with increasing annealing temperature or duration. The forward transformation temperature interval increases with increasing annealing temperature or duration. The maximum value can reach up to 64 ℃.After aging at 300 ℃ for 30 min,a single-step transformation occurs upon cooling and heating. With increasing aging temperature to 500 ℃,a two-step reverse transformation is observed during heating and a single-step transformation upon cooling.When aging 400 ℃ for a duration less than 10 h, the sample annealed at 950 0C for 2 h shows a single-step transformation. While, the sample annealed at 950 ℃ for 4 h shows multi-stage transformation behavior.The damping capacity peak resulting from the transformation of compositionally graded TiNi alloys is reduced as compared to homogeneous alloy. When the frequency is less than 5 Hz, the peaks of homogeneous and compositionally graded alloys decreases with increasing frequency. When the frequency is larger than 5 Hz, the damping behavior of compositionally graded alloy does not show any obvious transformation peak. When the frequency is less than 5 Hz and the strain amplitude is 3×10-5, the damping capacity of martensite in the sample annealed at 950 ℃ for 2 h increases with decreasing temperature.The temperature range with a dampling capacity larger than 0.04 is larger than 100 ℃.