Study On Strengthening Mechanism Of Al-30wt.%Mg₂Si Composite Based On First-principle Calculation

During the last decades,considerable efforts have been devoted to the development of novel,lightweight materials in the fields of aerospace,automobile and rail transit,especially,because of the increasingly tense of global energy.Mg₂Si intermetallics have a high melting point,high hardness,great elasticity modulus,low density,and so on.But,it has a serious brittle at room temperature.In order to improve the mechanical properties and reduce the brittleness of Mg₂Si intermetallics,Mg₂Si is combined with Al matrix to obtain Al-Mg₂Si composites.Al-Mg₂Si composites are prepared by in situ,which is a simple prepare process and low cost.However,the primary Mg₂Si phases in normal in situ Al-Mg₂Si composites are usually very coarse and lots of sharp corners,and thus,lead to poor properties to these composites.In this study,rare earth?RE?,which is regarded as inoculation,is added to solution.It can refine the morphology and distribution of Mg₂Si particles,and reduce the crystal size,accordingly,improve the strength and hardness.However,with being confined to experimental and testing method,the strengthening mechanism of RE inoculation exits great controversy.Therefore,the strengthening mechanism can been forecasted by both theoretical calculation and experimental method.The significance obviously is there for studying strengthening mechanism.With the development of first-principles calculations independent of any empirical parameters,thermophysical properties of materials can be predicted which are important to set up a materials database.In order to accelerate the study on effect of RE inoculation on Al-Mg₂Si composites,the evolution of some phases were studied by first-principles calculations based on density functional theory.Especially,the thermodynamic and mechanical properties of the binary phase in Al-Mg-Si system,the binary phase in Al-RE system and the Mg₂Si-RE doping phase.Firstly,the lattice constant,formation enthalpy,coheisve energy,density of state,Mulliken population,difference charge density and elasticity modulus of the binary in Al-Mg-Si alloy were calculated by means of first-principle in this study.Meanwhile,the thermodynamic stabilities and mechanical properties can be forecasted.The results show that strengthening phase Mg₂Si exhibits higher alloying and higher stability,that it is stabilized at room temperature and atmosphere pressure.And then,the detective method of SEM,EDS,qualitative and quantitative analysis by XRD were carried out to confirm the phase composition and content.The change of phase and physical and chemical properties of theoretical calculation are same to the experiment result.Secondly,the detective method of SEM,EDS,TEM and XRD were carried out to test the constituent,morphology,distribute and crystal size of the microstructure in Al-30wt.%Mg₂Si composite.And the mechanical properties of composite effected by REs?RE=La,Ce,Pr,Y and Sc?inoculation were investigate.The results show that all these REs can refine the microstructure and improve the mechanical properties of Al-30wt.%Mg₂Si.And then,the relationship between tensile strength or hardness and crystal size were compared and fitted;and the refined crystalline strengthening can be determined.The results show that the relationship between tensile strength and crystal size can be fitted by Hall-Petch,and the equation is=?_b98.22+819.30*d^-1/2.In addition,the lattice misfit between low exponential surface of Mg₂Si phase and RE element is calculated,combining with the testing results,refinement strengthening mechanism can be analyzed by both heterogeneous nucleation and toxic effect.The results show that only La,Ce,Pr and Y can meet the effect of heterogeneous nucleation.The efficiency of strengthening on tensile strength is Pr>Ce>La>Y>Sc;and the efficiency of strengthening on hardness is Ce>Pr>La>Y>Sc.Thirdly,the different occupied positions of the REs?RE=La,Ce,Pr,Y and Sc?element doped Mg₂Si were investigated by the first-principles plane-wave pseudopotential method.RE replace the element Mg or Si forming the substitutional solid solution(Mg₁₅Si₈RE or Mg₁₆Si₇RE),which doping concentration is 4.167 at.%.Or RE occupy the interstitial site forming the interstitial solid solution(Mg₁₆Si₈RE),which doping concentration is 4.000 at.%.And then the formation enthalpy,coheisve energy,lattice distortion,elasticity modulus and mechanical properties of the above-mentioned doping concentration were calculated.Camparing to reinforced phase Mg₂Si,the solution strengthening mechanism of these solid solution can be detemined.The results show that the easiest forming phase is substitutional solid solution with Mg,the interstitial solid solution is the next,and the least is substitutional solid solution with Si.The effect of solution strengthening on strength of strengthening phase is active,the strength of Al-30wt.%Mg₂Si must be improved.The efficiency of strengthening is La>Ce>Pr>Y>Sc.The REs?RE=La,Ce and Pr?can improve the strength and tenacity at the same time.In addition,the effect of different doping concentration on thermodynamic and mechanical stabilities of solid solution were investigate;and the ease and complexity of forming solid solution in different doping concentration can be confirmed.The results show that the solid solubility of substitutional solid solution with Mg is the largest,the olid solubility of interstitial solid solution is the next,and the least is substitutional solid solution with Si.Finally,thermodynamic and mechanical stabilities of 24 intermetallic compounds in the Al-RE binary system were calculated by means of first-principle.Ground state structure and precipitation sequence of the studied phases were forecasted.Meanwhile,elastic properties,which can forecast mechanical properties exactly,were calculated.Camparing to reinforced phase Mg₂Si,the precipitated-phase strengthening mechanism of these intermetallic compounds can be analyzed.The results show that the stable binary phases in Al-RE system are Al₁₁La₃,Al₁₁Ce₃,Al₃Pr,Al₃Y and Al₃Sc,which emerge in solution of Al-Mg₂Si composites after refining by REs?RE=La,Ce,Pr,Y and Sc?.The efficiency of strengthening is Al₃Sc>Al₃Y>Al₁₁La₃>Al₁₁Ce₃>Al₃Pr.Thereinto,Al₃Sc and Al₃Pr can improve the strength and tenacity at the same time.However,the Al₁₁La₃,Al₁₁Ce₃ and Al₃Y will deduce the tenacity.