Study On The Growth Kinetics And Diffusion Model Of The Boronized Layer On The Surface Of Pure Titanium In The ??? Phase Transition Temperature Region

Titanium and titanium alloys are widely used in aerospace,automobile,ship,biomedicine and sporting goods due to their excellent properties such as high specific strength,low density and wide temperature range.However,due to its poor friction resistance and low hardness,the application of titanium and titanium alloys in friction and wear and related fields is limited.At present,solid boronizing is an effective surface modification technology.The boronizing layer formed by solid boronizing can improve the surface properties of titanium and titanium alloys.Titanium has two isomorphic forms,?and?,and the change of crystal structure will inevitably affect the growth of boronized layer.Therefore,it has certain scientific value and academic significance to study the effect of phase transformation of pure titanium on Boronized layer.In this paper,TA2 industrial pure titanium,which is easy to process,is selected as the base material and surface modification is carried out by solid boronizing method.The phase transition temperature of pure titanium is 882?.so the boronizing temperature is 860?⁹50?,and the boronizing time is 1²0 h.The samples after boronizing were analyzed and tested by means of scanning electron microscopy,X-ray diffraction,electron probe,electrochemical workstation and friction and wear tester.The composition and comprehensive properties of the boronized layer were discussed.The results show that the boronizing layer of TA2 industrial pure titanium is double-layer structure,the surface layer is continuous and compact TiB ₂ layer,and the sub-surface layer is TiB whisker.The thickness of boronized layer is between 4and 30?m.When the boronizing temperature is lower than the phase transition temperature,the thickness of the boronizing layer increases slowly with the increase of the boronizing temperature,and the growth of the boronizing layer accelerates near the phase transition temperature of alpha to beta.The thickness of boronized layer is the largest under the condition of boronizing at 920?-20h.TA2 boronizing layer can improve the corrosion resistance of matrix in acidic and saline solutions.In addition,the surface friction coefficient of TA2 boronized layer is 0.28⁰.41,which is smaller than that of TA2 matrix 0.43.Two diffusion models,d²=Dt and d=kt^0.5,were used to simulate the growth kinetics of boronized layer.The accuracy of the two diffusion models was qualitatively analyzed by using three parameters:correlation coefficient R,mean absolute relative error MARE and RMSE.It is found that the diffusion model d²=Dt has high accuracy in predicting the thickness of boronized layer.The diffusion activation energies of boron in TiB₂ and TiB layers are 207.85 kJ mol^-1 and 278.49 kJ mol^-1,respectively,calculated by Arrhenius equation.The interstitial diffusion behavior and mechanism of B atoms in?-Ti and?-Ti have been studied by first-principles calculation.The results show that the octahedral gap in?-Ti and the tetrahedral gap in?-Ti are the preferential positions of B atoms.The preferred diffusion paths of B atoms in?-Ti are O-O,and the diffusion energy barrier is 0.8403 eV.The preferential diffusion paths of B atoms in?-Ti are T-T,and the diffusion energy barrier is 0.7751 eV.The electronic structure shows that B atom obtains electrons from Ti atom in the diffusion process,thus forming B-Ti covalent bond.With the increase of temperature,the diffusion coefficients of B atoms in?-Ti and?-Ti increase.At the same time,the diffusion coefficient of B atom in?-Ti is always less than that of?-Ti,which indicates that?-Ti is the main channel of B atom migration and diffusion.