Research On The Diffusion-mechanical Coupling Process And Models

Due to the excellent mechanical properties,titanium,nickle and their alloys were widely employed in some highly-advanced industrial areas,such as aircraft engines and heavy duty gas turbines.However,they were limited from a wider application and a longer serving life by the thermal oxidation resisitance.For alloys serving under aggressive environment,the research on their oxidation behavior and mechanical properties are of great significance for the analysis of material properties,the prediction of serving life,as well as the optimal design of surface structure.The present research was carried out through modeling and experimental approaches,to clarify the coupled relationships among oxidation beahvior,diffusion process and mechanical properties of metallic materials.Detailed research were as follows,(1)Based upon Clarke's theory,a comprehensive mechanical-chemical model was developed to predict the variation of stress,strain and scale thickness along with time.The growth strain,elastic strain,creep strain and external loads were taken into consideration.Due to the thermal oxidation,severe growth strain and growth stress were generated within the oxide layer,affecting the mechanical equilibrium in the oxide/substrate system.The growth strain was proven to be linearly dependent on the scale thickness.Meanwhile,according to the equation of Helmholtz free energy,the diffusion process of elements would be influenced by the stress state.Consequently,the stress state showed an obvious effect on the oxidation kinetics of metallic materials.(2)By considering the gradient distributions of stress,strain and oxygen concentration,the present mechanical-chemical model was further modified,leading to a wider application and a higher accuracy.In order to describe the gradient structure of oxide layer,the macro oxide scale was divided into several micro layers by using disretization method.In virtue of linear algebra,the distributions of stress,strain and oxygen concentration were solved through a coupling approach.The modified linear-parabolic law was employed to describe the oxidation kinetics,which was proven to be the geranal form of classical Wangner's law.Besides,the effects of different external loads on the oxidation process and mechanical behaviors were examined,including the uniaxial stress loads and constant strain loads.(3)Experiments were conducted to examine the oxidation mechanism of Ti-6Al-4V alloys and the stress evolution during oxidation process.The employed experimental approaches included X-ray diffraction(XRD),scanning electron microscope(SEM),energy dispersive spectrometer(EDS),optical microscope(OM)and nanoidentation.The oxidation experiements revealed that the application of bending loads would affect the formation of oxide layer and oxygen diffusion zone.The calculation results agreed well with the experimental data,indicating a high accuracy and realiability of the present model.(4)Based upon the knowledge of micromechanics,the mechanical homogenization method was generalized to the research area of chemical diffusion.In virtue of Fourior transformation,the diffusional Eshelby tensor was solved in an analytical way.Consequently,we have obtained the effective diffusional diffusivity tensor of inhomogeneous composites.(5)On the basis of the micromechanics,a mechanical-chemical model was developed to describe the relationship between micro-and macro-strains during the diffusion of alloying elements.Consiquently,mechanical equilibriums were established,where the dependence of effective mechanical properties on concentration was taken into consideration.Meanwhile,the effects of different mechanical loads on the stress distribution and diffusion process were discussed.By considering the interaction among chemical potential,effective diffusivity,effective mechanical properties and stress state,the coupled mechanical-diffusion model was developed.Through a coupling approach,the distributions of stress and concentration were solved in macro-and micro-scales.This present model was regarded as the effective modification of classical Fick's law,leading to a higer accuracy during the prediction of stress state and concentration distribution.