Research On Mechanism Of Combustion Synthesis Of Porous NiTi Shape-memory Alloys

In this paper, porous NiTi shape-memory alloys are prepared by combustion synthesis. A relatively general research on the weak exothermic reaction system of Ni-Ti has been made from thermodynamics and kinetics. The curves of standard reaction free enthalpies have been drawn by thermodynamic calculation based on thermodynamic principle. The relationships, among the preheat temperature, adiabatic temperature and the melt rate of Ni-Ti are definite through the calculation of adiabatic temperature under ideal state. Kinetic parameters of combustion synthesis have been obtained by non-isothermal thermal analysis method, and the kinetics equation could be established.The SHS process used for producing NiTi alloys with high porosity, and the porous NiTi with three-dimensionally interconnected pores has the isotropic pore structure. The combustion wave propagate through spiral combustion and the resultant product has laminar structure. The effect of different technical parameters in preparation for NiTi during reaction mode on the phase compose and morphology of product has been studied, these parameters include pressure,Ni contents, the size of Ti etc.The NiTi alloys have been successfully prepared by combustion front quenching. Phase transformation and microstructural evolution of the quenched samples were observed by scanning electron microscopy (SEM),X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS), the mechanism of reaction process and structure forming process were discussed. The results shows that the combustion proceeded by a diffusion-dissolution-precipitation mechanism. â‘ The combustion reaction between Ni and Ti particles started with the solid-state diffusion reaction at lower temperature, which formed NiTi₂ phase.â‘¡The melting of NiTi₂ turn out with the increasing of temperature, which led to the dissolving of the (Ni, Ti) NiTii eutectic.â‘¢The NiTi precipitate out and grow from droplet (Ni,Ti)NiTi₂, which the Ni and Ti contents are saturated.On the base of diffusion-dissolution-precipitation mechanism, the activation energy of process is lower than solid diffusion only, so the precipitate of NiTi is easy to occupy. And the mechanism attention above are happened simultaneous in the combustion synthesis. At the combustion temperature higher than the liquid temperature, the singal phase NiTi increase by the dissolution-precipitation mechanism.Combustion dynamical model of SHS process was drawn according the diffusion-dissolution-precipitation mechanism and it could describe the structural evolution of combustion synthesis NiTi.