Research On Crystallization Kinetics And Martensitic Transformation Of Free-Standing Ni-Mn-Sn Thin Films

Ni-Mn-Sn magnetic shape memory alloys thin films have attracted considerable attention due to their high response frequency,high output stress and obvious magnetic thermal effect.However,the high crystallization temperature and substrate constraints of Ni-Mn-Sn thin flmes greatly limit their development and application in MEMS.In this thesis,the free-standing Ni-Mn-Sn thin flims have been prepared by magnetron sputtering technique,and the effect of substrate on film binding are solved.The crystallization temperature of free-standing thin films is calculated by crystallization kinetics.The parameters of Modified Embedded Atom Method?MEAM?potential function of Ni-Mn-Sn thin films are obtained.It provides a reference for the study of solid phase transition of thin films.The results clarify the effect of sputtering parameters on surface morphology and chemical compositions of free-standing Ni-Mn-Sn thin films,revealing the physical mechanism of composition variations.At the same time,crystallization behavior,martensitic transformation and magnetic performance of thin films have been systematically investigated by mean of XRD,AFM,DSC and VSM,respectively.The effects of Co doping on phase composition and crystallization behavior of thin films are investigated.The potential function of Ni-Mn-Sn thin films is constructed by first principles and is modified by Modified Embedded Atom Method?MEAM?.Based on the potential function of MEAM,the effect of heating rate on the crystallization behavior of free-standing Ni-Mn-Sn alloy is studied,revealing the mechanism of crystallization and nucleation of Ni-Mn-Sn thin films.It is found that process parameters of magnetron sputtering technique have remarkable influence on surface roughness and chemical compositions of free-standing Ni-Mn-Sn thin films.The random columnar particles are observed on the surface of the sputtered films,and the density of columnar particles increases with the increasing Ar working pressure and sputtering power.Neighboring columnar particles are gradually merged,and irregular island particles are formed on the surface of the thin films.When the sputtering power is 100 W,the Ni content slightly decreases and the Sn content increases with increasing Ar working pressure,whereas Mn content does not change with the increasing Ar working pressure.When the Ar working pressure is 0.15 pa,the content of Ni and Mn is decreased gradually with the increasing sputtering power,and the Sn content slightly increases with the increasing sputtering power.The results show that the heating rate of non-isothermal crystallization has an important influence on the T_x and T_p for the crystallization of free-standing Ni-Mn-Sn thin films.The T_x and T_p of amorphous thin films increase with the increasing heating rate.Resulting in the fact that the release energy time of the films is decreased in the low temperature zone.The start of nucleation process of thin film decreases with the increasing heating rate.The activation energy of free-standing Ni-Mn-Sn thin films are calculated by Kissinger method and Ozawa method,and the value of activation energy is 195.63±5.2 kJ/mol and 195.66±4.98 kJ/mol,respectively.Two methods of results are obtained basically consistent.The results show that the isothermal temperature of crystallization has an important influence on the nucleation and nucleus growth of free-standing Ni-Mn-Sn thin films.With the increase of crystallization temperature,the incubation time of crystallization of free-standing Ni-Mn-Sn thin films is shortened,and the growth mode of crystallization of thin films is changed with the increasing crystallization temperature.When the crystallization temperature of thin film is 520 K,the value of Avrami exponent is 1.The crystallization process of thin film is pre-existing nuclei growth by diffusion-controlled.When the crystallization temperature of thin film is higher than 520 K,the value of Avrami exponent is 1.5.The value of n ranges from 1 to 1.5 for the thin films indicating that the crystallization process are two dimension diffusion-controlled growths.It is found that the effect of Co doping on refining grain of free-standing Ni-Mn-Sn thin films.When the thin films are crystallized,the number of grain boundary of thin films increases with the increasing Co content.The grain growth of thin films is affected by interfacial energy.Thus,the crystal activation energy,T_x and T_p of free-standing Ni-Mn-Sn-Co thin films increase with the increasing Co content.Meanwhile,the lattice distortion of thin films increases with the increasing Co content.The growth of the free-standing Ni-Mn-Sn thin films is controlled by the interface energy,and the film grows in a two-dimensional diffusion mode.The simulation results that the potential function of MEAM of Ni-Mn-Sn thin films is carried out by first-principles and LAMMPS simulation accordingly.The potential function of MEAM can be reflected the interaction between atoms in Ni-Mn-Sn films.When the heating rate is 0.1k/ps,the curves of DSC have a single exothermic peak related to the crystallization process in the range of300⁶00 K.The T_p is 523 K,indicating that the simulation results are consistent with the experimental results.This phenomenon indicates that the crystallization behaviors in a marked kinetic nature.The experimental results indicate that crystallization temperature has influence on the crystal structure,martensitic transformation and magnetic properties of free-standing Ni-Mn-Sn thin films.As demonstrated by the XRD profile,the pattern of Ni-Mn-Sn free-standing thin films is a cubic single-phase austenite phase?L2₁?.All the peaks of freestanding alloy thin films increase with the increasing crystallization temperature,and the relative intensity of peaks A?220?and A?422?enhance gradually.The grain size,martensitic transformation temperature and magnetic energy increase with the crystallization temperature increasing for thin films.Upon temperature variation,undergo a change of magnetization typical for the first order martensitic transition and at higher temperatures a second order ferromagnetic-paramagnetic transition at Curie temperature of austenite.